A review on covalent organic frameworks: exploration of their growing potential as porous materials in photocatalytic applications

Abstract

Photocatalysis powered by unlimited solar energy is an effective strategy to resolve energy and environmental issues. To achieve an efficient photocatalytic system, photocatalysts need to be highly crystalline and porous with excellent photostability under extreme conditions. In this case, covalent organic frameworks (COFs) have shown immense potential for photocatalytic applications owing to their unique structure as well as electronic and photophysical characteristics. COFs possess a crystalline porous network with light absorption capabilities and excellent stability. Furthermore, functionalized COFs can be developed through organic unit variation to obtain broader absorption, narrow bandgap, effective charge separation, and transportation. Furthermore, high photocatalytic efficiency can be achieved via the formation of heterostructures through anchoring or post-synthetic modification. Our review is focused on the recent advancements in COFs as photocatalysts for various photocatalytic applications. Initially, we emphasize the topological design, linkage chemistry, and functionalization of COFs, underscoring the principles and requirements for high photocatalytic efficiency. This provides deep insights into the capabilities of COFs in different photocatalytic applications, covering areas such as hydrogen and oxygen evolution, carbon dioxide reduction, organic transformation, and organic pollutant degradation. Finally, we summarize the pivotal points that need urgent attention and outline future avenues, offering fresh perspectives and contributing to revolutionary innovations in this rapidly evolving field.

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Kamal Prakash

Kamal Prakash received his PhD degree in Chemistry from the Indian Institute of Technology (IIT) Roorkee, India in May 2018. Following his PhD, he worked as a Postdoctoral Fellow at the University of Houston, Texas, USA (2018–2019) and Indian Institute of Technology Ropar (2021–2022). He currently works as an NPDF fellow at IIT Indore since 2022. His current research interests focus on the design and synthesis of novel covalent organic frameworks (COFs) and their material applications.

Rakesh Deka

Rakesh Deka was born in 1994 in Assam, India. He received his Master's Degree (2018) in Chemistry (Hons.) from Gauhati University. He is a Research Scholar in the Department of Chemistry, IIT Indore. His research work is related to the fields of metal–organic frameworks/covalent organic frameworks and their energy related applications.

Shaikh M. Mobin

Shaikh Mobin obtained his Bachelor's and Master's from Wilson College, University of Mumbai with a major in Chemistry and PhD from Mumbai University in Chemistry. In 2012, he joined IIT Indore and is now working as a Professor in the Department of Chemistry. He has developed his multi-disciplinary research group working in a wide area of research including solid-state structural transformations, design and synthesis of a newer class of MOFs and their applications in energy storage, conversion and biomedical devices, exploring metal nano-oxide materials for energy storage, conversion, optical and electro-chemical sensing, metal nano-oxide materials derived by employing metal complexes/MOFs as single-source molecular precursors as catalysts in organic transformations and developing greener c-dots for bioimaging and biomarkers. Moreover, his research group designs and synthesizes small molecules as cellular organelles target and docking.

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1. Introduction

From the last decade, fossil fuels have been the primary source of energy playing an important role in enhancing the quality of human life, starting from the laboratory to the industrial scale. However, the high and rapid consumption of fossil fuels not only poses the threat of an energy crisis in the near future but also affects the environment.¹ Furthermore, due to pollution and global warming, fossil fuel consumption has become a concern for the global environment.^2,3 Thus, it is necessary to replace fossil fuels with greener and eco-friendly energy technology to fulfil the world's energy demand while maintaining a safe environment.^4–6 In our Solar System, the sun is an unlimited source of energy for our planet, and solar energy is considered the best green alternative to fossil fuels. However, producing solar energy as fuels on a large scale is challenging owing to the low solar energy conversion efficiency.⁷ Accordingly, researchers are applying different techniques to utilize solar energy with a high conversion rate, and among them, photocatalysis has emerged as a promising technology to convert solar energy with high efficiency.^8,9 In natural systems, photosynthesis in plants is an efficient photocatalysis process, where the green leaves of plants utilize sunlight to form oxygen and carbohydrates from water and carbon dioxide.¹⁰ Chlorophyll, a porphyrin derivative in leaves, is responsible for the photosynthesis process, which converts solar energy into chemical energy for various chemical reactions.¹¹ Inspired by this, researchers have focused on developing efficient photocatalysts that can mimic the photosynthetic mechanism and utilize sunlight for various applications such as hydrogen or oxygen evolution, CO₂ reduction, and organic transformation reactions.^12–15 The understanding of the mechanism of a particular photocatalytic conversion reaction is key to developing an efficient photocatalyst. Generally, a photocatalyst should have light absorption ability, chemical or thermal stability, and high solar-to-power conversion efficiency to achieve efficient photocatalytic performance.^16,17

Fujishima and Honda first reported a type of photocatalyst for photocatalytic water splitting, where TiO₂, an inorganic semiconductor, produced hydrogen under light illumination.¹⁸ Their works inspired researchers in this field to develop efficient photocatalysts for light-driven applications. Subsequently, many inorganic photocatalysts such as oxides (ZnO, TiO₂, and SnO₂), sulfides (CdS, ZnS and MoS₂), and nitrides have been explored as photocatalysts due to their high abundance, appropriate band structure, high stability, and anticorrosive properties.^19–21 However, these materials cannot utilize solar energy effectively because of their lower absorption capability in the visible region, which hinders their large-scale applicability. Similarly, carbon-based materials having high π–π conjugated and layered structures have also been investigated for photocatalysis but their fabrication and functionalization are expensive and difficult.^22–24

Over the last decade, porous organic materials such as covalent organic frameworks (COF), metal–organic frameworks (MOF), and conjugated organic polymers (COP) have emerged as new crystalline materials consisting of reticular structures and porous networks.^25–28 Among the porous materials explored for photocatalytic applications because of their distinctive properties, covalent organic frameworks (COF) have attracted wide recognition considering their regularly ordered network, highly porous surface, and excellent stability.^29,30 COFs are highly crystalline materials with strong covalent bonding between two or more organic moieties. In contrast to MOFs, which consist of metal centres in their frameworks, COFs are generally metal-free networks and have a lower density due to the presence of lightweight elements such as C, H, B, O, and N in their structure.³¹ The first COF was synthesized by Yaghi and his group in 2005 using the solvothermal method.³² Their pioneering work led the way for the synthesis of COFs, with hundreds of new COFs developed within the next 10–15 years for different applications such as energy storage, catalysis, sensing, optoelectronics and biomedical.^33,34

In recent years, COFs have attracted significant attention for various photocatalytic applications such as hydrogen evolution reaction (HER), CO₂ reduction/conversion, organic transformation, oxygen evolution reaction (OER), and pollutant degradation³⁵ due to their unique structure and photophysical properties. Jiang and his group first reported a photo-responsive COF in 2008 that behaves as a semiconductor upon light irradiation. Lotsch and co-workers prepared a triazine COF with hydrazone linkage for photocatalytic HER application in 2014, opening the way for the utilization of COFs in photocatalytic applications.³⁶ Later, photoactive COFs were developed as photocatalysts for organic transformations and decomposition of organic pollutants.^37,38 In 2018, a rhenium (Re) complex was incorporated on the surface of a 2D COF for visible-light-driven CO₂ conversion into CO, which showed a higher photocatalytic performance compared to the pristine Re catalyst.³⁹ Similarly, covalent triazine frameworks, a subclass of COFs, were investigated for photocatalytic water splitting to produce H₂ and O₂, simultaneously.⁴⁰ COFs possess a well-defined polymer network with permanent porosity, strong light absorption, and high stability, which boost their performances as photocatalysts for particular photocatalytic applications. Especially, their π-conjugated network arranged directionally contributes to facile charge migration and transportation in photocatalytic processes.⁴¹ Thus, COFs have gained popularity as promising materials for energy and environmental applications.⁴²

As the field of COFs is expanding at a rapid pace, particularly in the case of their photocatalytic application, many reviews have been published on COFs, discussing their application in the field of photocatalysis. However, most reviews only focused on COFs for energy-related applications such as hydrogen evolution, CO₂ reduction, and water splitting,^43–46 while a few comprehensive reviews thoroughly covered these applications.^35,47 Thus, a detailed review concerning the design and structural manipulation of COFs for the development of photocatalysts for various photocatalytic applications is still lacking. Considering this, our review presents a brief introduction to the design, functionalization and desirable properties of COFs as efficient photocatalysts, and further systematically summarizes the recent development of COFs for different photocatalytic applications, focusing on hydrogen evolution, CO₂ reduction, oxygen evolution, organic transformation, and organic pollutant degradation (Fig. 1). Also, the development of hybrid COFs as highly efficient photocatalysts is briefly discussed in each section. Finally, future challenges and perspectives are discussed to develop efficient COF photocatalysts. Thus, this review will provide a systematic study on COFs in photocatalytic applications to promote their development as highly efficient photocatalysts.

Fig. 1 Schematic representation of COFs as photocatalysts for various applications.

2. Topological design, linkage chemistry, and functionalization of COFs

COFs are composed of a crystalline layered network of organic building blocks known as knots and linkers and the geometry of these organic units determines the topology of a COF system. The main advantage of COFs is their topological design and structural tunability using different organic building blocks.³⁴ By varying the linkers and knots based on different organic moieties, the topology of COFs can be defined into one-dimensional to three-dimensional (3D) networks with a variety of pore channels. Presently, the formation of a 3D network is not very popular given that optimization of a 3D layered crystalline structure is quite challenging. In contrast, the development of two-dimensional (2D) COFs is well-known and has been mostly explored for photocatalytic applications. Generally, 2D COFs can be formed from organic building blocks that have various symmetrical geometries known as C₂, C₃, C₄, and C₆ geometries. Most 2D COFs are formed by the combination of [C₃ + C₂], [C₃ + C₃], [C₄ + C₂], [C₄ + C₄], and [C₆ + C₂] building blocks.^48,49Fig. 2 represents the different topologies according to the symmetry combinations. The [C₂ + C₃] or [C₃ + C₃] combination forms the hexagonal hcb topology, whereas the [C₄ + C₂] or [C₄ + C₄] combination results in the tetragonal sql topology.^50–52 The topology of COFs dictates their pore network with a uniform pore size and shape, but it has been observed that the pore size and shape vary for the same type of topology originating from different combinations. [C₆ + C₂] and [C₆ + C₃] construct a rhombic topology with dual pore sizes.^53,54 One of the fascinating combinations is [C₂ + C₂], which gives two different topologies, kgm and sql, with different pore structures depending on the reaction conditions and building block functionality.^55,56 Another way to develop a structural skeleton in COFs is multicomponent COF synthesis, for example, designing a COF with one knot and two different linkers provides two different topological structures in one COF.⁵⁷ This can open the way for the synthesis of complex COFs with unique properties by the integration of two different COFs. The important topological designs for 2D COFs are trigonal, tetragonal, rhombic, and hexagonal forms. Thus, based on the topology of COFs, their pore structure, surface area, active sites, and interlayer structure can be varied, which can have a great effect on their photophysical properties. In this case, the appropriate selection of organic moieties enables precise control of the pore network in COFs to be achieved. Topology has a significant impact on the formation of COFs with different structures and is crucial in the reticular chemistry of COFs.

Fig. 2 Representative topological design of 2D COFs.

Generally, COFs are formed by two or more monomers consisting of different reactive groups. The organic moieties are coordinated via a covalent linkage, which has a great impact on the effective π-delocalization and stability of the reticular COF network. Different types of linkages have been explored to synthesize COFs such as boron-ester, imine, azine, hydrazone, vinyl, β-ketoenamine, and amide linkages (Fig. 3).⁵⁸ The boronic ester linkage is formed via the reaction between the boronic acid and hydroxy group, while the imine linkage results from the Schiff base reaction between amine and formyl groups. This indicates that appropriate functional groups are necessary to acquire COFs with a particular linkage. To perform efficient photocatalytic reactions, COFs should possess excellent chemical stability in aqueous and non-aqueous solutions, different pH, and harsh environments.⁵⁹ Boronic-ester-linked COFs are highly sensitive toward water and transform into their parent organic units, inhibiting their application in aqueous solution. Hence, imine-linked COFs have been largely explored for photocatalytic applications. The first imine-based COF was introduced as COF-300 formed by the Schiff base reaction of tetra(p-aminophenyl)methane amino and terephthalaldehyde, which opened the way for developing numerous imine-linked COFs using different building blocks.⁶⁰ To date, β-ketoenamine COFs are the most explored COFs for different applications because of their keto–enol tautomerization, which improves their stability.⁶¹ Imine, azine, and olefinic-linked COFs have better crystallinity, higher stability, and are easier to design compared to hydrazone, hydrazine, dioxine, and ester-linked COFs.⁵⁸ Imine and β-ketoenamine-linked COFs can undergo protonation or bind with metal atoms, metal nanoparticles, and inorganic compounds to form hybrid composites and have been further explored in photocatalytic applications.^62,63

Fig. 3 Important linkages for COFs as photocatalysts.

Functionalization is an important factor in modulating the properties of COFs. As discussed previously, various building blocks can have different types of symmetries to develop specific topological designs, affecting the structural properties of COFs. The functionalization of COFs can be developed by modulating their organic moieties as knots and linkers, post-synthetic modification, or in situ formation of hybrid materials.⁶⁴ Consequently, COFs have become more advantageous compared to semiconductors and noble metals, where their electronic and photophysical properties can easily be tuned using different organic moieties as building units for their synthesis. Based on the linkage chemistry, several organic moieties have been explored to develop COFs for photocatalysis (Fig. 4). The absorption region of COFs can be expanded by applying organic moieties having a broad absorption in the visible region. The inclusion of chromophore moieties offers broader absorption with extended conjugation in the 2D network. Pyrene or porphyrin moieties provide strong absorption in the range of 400 to 800 nm upon integration in COFs, thus increasing their light-harvesting ability.^65,66 A porphyrin-containing 2D COF was reported for the light-activated transformation of amines into imines, where the generation of light-activated electron–hole pairs was responsible for the oxidation reaction.⁶⁶ COF structures with the triazine moiety possess high crystallinity and porous surface, given that the rigid and planar triazine units stacked to form orderly arranged COF layers.⁶⁷ Also, the band structure such as band positions or band gap of COFs can be optimized by varying the organic moiety for their formation. A series of vinylene-linked 2D COFs displayed different band gap values by modifying their building block with a benzobisthiazole linker, affecting their photocatalytic activity.⁶⁸ Extended conjugation in the organic moiety can be very effective in enhancing the photocatalytic activity of COFs. Kuo et al. enhanced the HER performance of a carbazole-based COF by changing the degree of conjugation in its organic linkers.⁶⁹ Further, the introduction of donor and acceptor groups as functional groups in COF networks is very effective to induce efficient charge separation and migration during photo-excitation.⁷⁰ Thus, a library of COFs can be developed by modulating their organic moieties, topologies, and linkages, which can have diverse applications based on their properties and functionalization.

Fig. 4 Representative structures of different types of building units (knots and linkers) of COF photocatalysts.

Further, post-synthetic modification of COFs and the formation of hybrid COFs are alternative ways to introduce functional groups, active sites, and heterogeneous structures in the COF network.⁴³ Post-synthetic modification can provide new active sites by attaching metal and metal nanoparticles to the COF periphery or transforming the functional groups into active groups in the precursor COF.^48,71,72 To anchor metal atoms or metal nanoparticles (NPs) on the surface of COFs, the pore wall of COFs should have anchoring sites, where the metal or metal NPs can coordinate. COFs with bipyridine groups or nitrogen heteroatoms (imine or β-ketoenamine linkage) are capable of binding metal atoms or metal NPs effectively, while maintaining the integrity of the original COFs.^73,74 The surface area of COFs is reduced upon the introduction of new active sites but these active sites improve their uptake capacity for reactant molecules and lower the energy barrier for photocatalytic reactions, resulting in better charge separation and charge migration for electron–hole pairs, subsequently improving their photocatalytic activity.⁷⁵ Hybrid COFs can be developed by incorporating metal oxides, metal sulfides, inorganic compounds, metal–organic frameworks, or carbon-based materials in COFs via in situ solvothermal reaction.^64,71,76 The combination of two different materials generates a synergetic effect, constructing a Z-scheme or S-scheme-based heterojunction system, improving the charge separation and charge transfer capacity of COFs for photocatalytic activity.

3. Basic principle of photocatalytic reaction

Photocatalysis is a light-driven process, where solar energy transforms into chemical energy to carry out a chemical reaction.⁷⁷ Photocatalysis has three main steps, as follows (Fig. 5): (i) absorption of visible light for photoexcitation of electrons, (ii) excited electrons move from the valence band (VB) to conduction band (CB), leaving holes behind and generating electron–hole pairs; and (iii) the photogenerated charges further reach to the surface of the photocatalyst to assist in the corresponding reaction. Photocatalytic reactions will be feasible when the VB is more positive or the CB is more negative than the redox potential of the corresponding half-reactions. Thus, the band positions are very important for photocatalytic reactions to proceed, where a smaller band gap is more favourable for increasing the light absorption, subsequently producing a high efficiency. Light-activated processes such as HER, CO₂ reduction, and organic transformations generally proceed through the capture of photo-excited electrons, whereas the oxygen evolution reaction involves the reaction of photogenerated holes with water molecules.^78–80 Many COF photocatalysts exhibit overall water splitting activity, where electrons and holes simultaneously yield hydrogen and oxygen as products, respectively.

Fig. 5 Representative photocatalytic overall water-splitting mechanism.

Generally, COF-based photocatalytic systems carry out photocatalysis in the presence of co-catalysts and sacrificial donors.⁸¹ In HER or CO₂ photoreduction, electrons are received by a co-catalyst, and then transferred to H⁺ and CO₂ for H₂ evolution and CO/CH₄ production, respectively. A sacrificial donor is utilized as an electron donor to replace the oxygen evolution reaction and regenerate the photosensitizer. Mostly, the noble metal platinum is used as a co-catalyst given that it forms an efficient heterojunction system with the photocatalyst, while triethylamine, triethanolamine, and ascorbic acid are used as sacrificial donors.⁸² The use of co-catalysts and sacrificial donors also inhibits photo corrosion during photocatalysis and limits the OER process.

The major factor causing low photocatalytic activity is the charge recombination process, where excited electrons can combine with holes again, reducing the number of photogenerated electrons for chemical reactions. Thus, the recombination process must be reduced and effective charge separation and efficient charge transfer promoted to achieve the maximum efficiency. Therefore, the rational design of COFs as photocatalysts with appropriate band edges, narrow bandgap, and high charge separation efficiency can make them efficient for photocatalytic applications. Further, COFs can be integrated with other materials such as metals, metal complexes, and carbon-based entities to develop hybrid COF photocatalysts, which can form different heterojunction structures such as type-II-heterojunction and Z- and S-scheme heterojunction systems via the synergetic effect between COFs and other materials.^83,84 Due to the formation of these heterojunction systems, the charge separation efficiency and charge carrier transportation are greatly enhanced, resulting in high photocatalytic performances.

4. Required properties of COFs as photocatalysts

To act as an efficient photocatalyst, COFs must possess a crystalline structure, light-harvesting ability, narrow bandgap, large surface area, excellent stability and enriched π-conjugation for feasible charge separation and migration. It is necessary to satisfy many factors to realize the crystalline nature of photocatalysts with fewer defects given that this significantly reduces the charge recombination process, provides longer lifetimes, and enhances the charge transfer efficiency, eventually contributing to the photocatalytic efficiency. The primary requirement of a photocatalyst is that it should have high absorption in the visible region of 400 to 800 nm.⁸⁵ Organic moieties with large conjugation aromatic systems are capable of harnessing light in the visible region due to their effective π–π delocalization. Thus, to consider this aspect, many building blocks such as triphenylamine, anthracene, pyrene, and porphyrin have been employed for the development of COFs.^86–89

Further, a crystalline structure provides a long-range ordered arrangement of the organic precursor employed for the formation of COFs. Amorphous polymers have short-range ordered structures, which limit the delocalization of π-conjugated electrons, and suffer from weak charge dynamics and high charge recombination rate. In contrast, the ordered network of crystalline COFs minimizes the defect sites and provides efficient charge separation and charge mobility by inhibiting the charge recombination process.⁹⁰ Dong Jiang reported the use of porphyrin-based COFs as efficient photocatalysts for HER. It was found that high-crystalline ZnP-PZ-PEO-COF produced an excellent HER rate of 11 mmol g⁻¹ h⁻¹ with a high apparent quantum yield (AQY) efficiency, which was five-times higher than its amorphous counterpart ZnP-PZ-PEO-POP.⁹¹ Crystalline COFs also exhibit a uniform distribution of pore channels, increasing their surface area compared to amorphous polymers. ZnP-PZ-PEO-COF possessed a large porous surface area of 557 m² g⁻¹, while ZnP-PZ-PEO-POP showed a smaller surface area of 474 m² g⁻¹, respectively. It is evident that highly ordered crystalline COFs achieve wide absorption in the visible region, increased porosity with the maximum number of accessible active sites, and effective electron–hole pair generation with efficient charge transfer, leading to high photocatalytic performances.⁹² A reduced crystallinity not only results in diminished properties, but the surface area is also effectively reduced, leading to a lower HER performance.⁹³

The most established method to achieve high crystalline COFs is the solvothermal method, where reactions are performed under vacuum-pressure at a fixed temperature and pressure.³² However, the solvothermal process has many limitations such as small-scale reactions and longer reaction times. Several other methods such as microwave synthesis, mechanochemical synthesis, and atmospheric solution method have been reported with fast reaction and mild conditions.⁹⁴ However, the research on these methods is still in its infancy and they result in structures with low crystallinity and reduced surface area, restraining their application in photocatalysis.

Another important factor is the stability of COFs, which should have excellent thermal and chemical stability under extreme conditions to work as efficient photocatalysts.^95–97 Most photocatalytic applications are carried out in different aqueous solutions or at different pH solutions. It is very well known in the literature that boronic ester-linked COFs are unstable in water and can easily decompose into precursor monomers due to their sensitive B–O bonds.⁹⁸ Thereon, COFs with different linkages such as β-ketoenamine, azine, imine, olefin, and cyanovinyl have been explored for photocatalytic applications, which show high stability in acid, base, and neutral media.⁹⁹ COFs with long-term stability and recyclability can be considered highly efficient photocatalysts by producing a high yield of products without degrading after use in multiple cycles. A pyrene-based COF with amide linkage showed remarkable recyclability for visible-light-driven uranyl reduction for up to 5 cycles without degradation.¹⁰⁰ It is also necessary that COFs should be stable after post-synthetic treatment to incorporate active sites or functional groups and should not lose their structural properties. Many imine and β-ketoenamine COFs have been anchored with metal atoms or metal nanoparticles, exhibiting enhanced photocatalytic performances with excellent stability.¹⁰¹ A palladium-decorated sp²-carbon-linked COF, PD-COF, showed extreme stability after 8 cycles, exhibiting remarkable photodegradation and HER activity.¹⁰² The excellent structural stability of COFs during photocatalytic experiments is a necessity for their large-scale practical application.

Porosity is a significant property of COFs for catalytic activity, where highly porous COFs have more accessible sites for catalytic reactions.¹⁰³ A well-designed pore network with a uniform pore size and shape is an ideal platform for gas adsorption, charge transportation, and functionalization.^104,105 Mostly, the pore size of COFs depends on the topological design of their monomers. The size and length of their building units regulate their pore size, i.e. larger or longer building units will form larger pore sizes. Pang et al. synthesized two COFs with different pores by simply utilizing one C₄ monomer with two C₂ monomers having different lengths.¹⁰⁶ COFs can have microporous and mesoporous structures according to their pore size.^107,108 It was observed that hexagonal, tetragonal, and rhombic types of pores produce mesoporous COFs, while trigonal pores form microporous COFs. Microporous COFs possess a large surface area with high stability but mesoporous COF structures are more prominent given that π-organic building units generally have large molecular sizes.¹⁰⁹ Mesoporous COFs are also more compatible with pore surface engineering compared to microporous COFs because larger pores can easily adsorb large molecules.⁴⁸ A series of mesoporous and microporous COFs has been developed by modulating the size and length of the monomers, where small organic moieties formed microporous COFs, while large organic units with longer linkers produced microporous COFs.¹¹⁰ The size and distribution of pores are essential factors in photocatalysts, and modifying the surface of these pores plays a pivotal role in enhancing the accessibility of the reactants to the active surfaces.⁴⁹ The development of COFs with large pores is very important to realize high adsorption and surface area for photocatalytic activity. Further, the tailored construction of the pore wall interface can greatly improve the capability of functional design and properties.

The high efficiency of a photocatalyst toward photocatalytic activity is dependent on its absorption of photon energy from sunlight, which generates electron–hole pairs to participate in catalytic reactions on its surface. The photoexcitation of an electron is related to the band gap energy. If the band gap is narrower, photon absorption increases, and subsequently more electrons can undergo photo-excitation and easily transfer from the VB to CB.¹¹¹ Thus, by varying the band positions or band edges of a photocatalyst, its band gap can be optimized for a particular catalytic reaction. Large conjugation systems can elevate the HOMO (highest occupied molecular orbital) level, resulting in a reduction in the HOMO–LUMO (lowest unoccupied molecular orbital) gap, which is commonly known as the band gap. Diacetylene-functionalized COFs possess a comparatively smaller band gap than that of acetylene-functionalized COFs due to their extended conjugation, resulting in a high HER performance.¹¹² Another effective strategy to tune the band gap is to append functional groups to the building blocks. The addition of various functional groups such as amino, nitro, hydroxyl, and halogen to organic linkers has been observed to alter their electronic densities. This modification results in a smaller band gap and a redshift in the light absorption edge.^113,114 One of the simple strategies for rearranging the band structure of COF photocatalysts is to anchor metal atoms, metal complexes, or metal nanoparticles on their porous surface. Metal ions can shift the band positions due to ligand-to-metal charge transfer, improving the light absorption and increasing the photocatalytic activity. Guo and co-workers appended nickel (Ni) atoms on a bipyridine-containing COF to achieve high photocatalytic efficiency. The coordination of Ni negatively shifted the conduction band, increasing the reducing ability for H₂ evolution and allowing metal-to-ligand charge transfer, which led to broader absorption and high charge mobility, thus leading to a high HER performance.¹¹⁵

The separation of electron–hole pairs and charge transfer are the decisive effects of photocatalytic reactions. The excited electrons tend to recombine with holes, which is known as the charge recombination process, and decrease the number of electrons reaching the surface of the catalyst for chemical reactions, thus decreasing the photocatalytic activity. To enhance the separation of electron–hole pairs and their transfer rate, some strategies can be applied in COFs, as follows: (i) extending their π-conjugation system, (ii) preparation of highly conjugated aromatic systems, (iii) functional group modification, and (iv) use of donor–acceptor (D–A) system. The introduction of extended conjugation in the COF network can be very effective to facilitate π-electron delocalization and charge carrier mobility. Two metal-free sp²c-COFs were investigated for CO₂ photoreduction by modulating intrinsic the π-conjugation in their structure. The ethylene moiety in the COF skeleton significantly enhanced CO production due to its extensive π-conjugation, facilitating charge separation and migration.¹¹⁶ The use of highly π-conjugated aromatic systems such as triphenylamine, pyrene, porphyrin, and carbazole moieties as building blocks for COFs can enhance the charge transfer for photocatalysis.^{61,65,117,118} Additionally, it has also been found that the presence of functional groups such as methoxy, hydroxyl, and halogen groups is very effective in improving photocatalytic performance. Fan and his group prepared β-ketoenamine-linked COFs using different functional groups to analyse their effect on photocatalytic CO₂ reduction. The presence of electron-donor groups in COFs has a positive impact on CO₂ photoreduction compared to electron-acceptor groups, resulting in enhanced photocatalytic activity.¹¹⁹ Chen and co-workers prepared a series of pyrene-based COFs consisting of substituted benzothiadiazole units as linkers and studied their HER performance. The fluorine-substituted benzothiadiazole-linked COF achieved high HER activity due to the electron-withdrawing effect of the fluorine moiety, inducing effective charge separation, which increased the charge migration rate for excited electrons.¹²⁰

The photocatalytic performance of COFs can be greatly enhanced by developing donor–acceptor-based (D–A) COFs as photocatalysts.¹²¹ Many benzothiadiazole, thiazole, and other electron-withdrawing organic units have been utilized to develop donor–acceptor COF systems to enhance their photocatalytic performances.^28,122 By introducing donor and acceptor groups, donor–acceptor COF systems can be achieved, which suppress the charge recombination process and generate effective charge separation with charge species possessing a longer lifetime. The photogenerated electrons readily move from the donor to acceptor moiety through the π-conjugation system and their spatial arrangement in the COF allows the flow of holes and electrons in opposite directions, and thus a stable charge separation system with reduced charge recombination exists for a longer period, enabling smooth charge transfer for the photocatalytic process.

Further, other materials can be incorporated in COFs such as metal oxides, inorganic complexes, carbon nitride, and metal–organic frameworks to prepare COF hybrids as photocatalysts to enhance their photocatalytic activity. These hybrid COF composites such as MOF–COF hybrids, COF–CNT hybrids, and COFs often construct Z-scheme or S-scheme heterojunction systems, which enhance the charge separation efficiency and charge migration rate, leading to a high photocatalytic performance.

5. Photocatalytic applications of COFs

5.1 Photocatalytic hydrogen evolution reaction

In recent times, the energy crisis has become one of the main issues to address for economic development. Thus, hydrogen energy from water splitting can be a promising source of clean and sustainable energy. However, the light-driven water splitting process to produce H₂ is a challenging target to achieve and needs an effective photocatalyst. The energy barrier for light-driven water splitting is 1.23 eV given that the redox potential for hydrogen and oxygen evolution is zero and 1.23 eV against NHE (normal hydrogen electrode), respectively. Photocatalysts need to overcome the energy barrier to perform the water-splitting process. They must possess a band gap of 1.5–3.2 eV and their conduction band and valence band should be more negative and more positive, corresponding to their redox potential, respectively. The structural diversity of COFs through skeleton modulation allows them to exhibit light absorption in the visible region, optimized band structure, and large porous surface, which can be advantageous for photocatalytic activity. Owing to these properties, COFs have been largely explored for solar-driven hydrogen generation with excellent stability and recyclability.

5.1.1 Free-standing COFs. The first COF for H₂ evolution was reported in 2014 by Lotsch and his group, which was synthesized by using 2,5-diethoxy-terephthalohydrazide and 2,3,5-tris(4-formyl-phenyl)triazine (TFPT) as organic moieties. TFPT was a high-crystalline COF with a BET surface area of 1603 m² g⁻¹ and band gap value of 2.8 eV, enabling its application in HER. TFPT-COF exhibited an H₂ evolution rate of 230 μmol h⁻¹ g⁻¹ using sodium ascorbate as a sacrificial agent and platinum as a co-catalyst. Furthermore, the HER rate reached upto 1970 μmol h⁻¹ g⁻¹ when triethanolamine (TEOA) was used as a sacrificial donor. TFPT-COF became amorphous during photocatalysis and showed low crystallinity with a comparatively reduced surface area, but its crystalline nature could be regained by applying the initial reaction conditions. This work demonstrated a bottom-up approach to developing COFs for photocatalytic hydrogen evolution.¹²³ In continuation to their early work, the same group further developed four two-dimensional (2D) azine-linked N_x-COFs by varying the primary building block of triphenylarylaldehydes. The number of nitrogen atoms at the central phenyl varied from 0 to 3, effectively modulating the planarity of the triphenylarene moiety, which is critical for efficient charge migration. N_x-COFs (x = 3, 2, 1, and 0) produced hydrogen at the rate of 1703, 438, 90, and 23 μmol h⁻¹ g⁻¹, respectively, with a four-fold enhancement in the HER rate upon the addition of a nitrogen atom in the central phenyl ring. Among them, N₃-COF exhibited the best HER rate due to the stabilization of the anion radical by the nitrogen-rich triazine group, enhancing the charge separation and migration to the co-catalyst. Thus, the structural engineering of building blocks in COFs can be an effective strategy to increase their photocatalytic activity.¹²⁴

Extended conjugation can be very effective in improving photocatalytic efficiency. Thomas and co-workers synthesized two β-ketoenamine COFs, TP-BDDA and TP-EDDA, consisting of a diacetylene and acetylene functionality, respectively, and utilized them for HER. Notably, TP-BDDA COF achieved an HER rate of 324 μmol h⁻¹ g⁻¹, which was 10 times greater than that of TP-EDDA COF. The extended conjugation due to diacetylene functionalization provided a narrow band gap for photoexcitation and enhanced charge mobility for photogenerated electrons, resulting in an improved hydrogen evolution rate.¹²⁵ Likewise, three novel pyrene-based COFs, A-TEXPY-COF, were reported with extended alkynes, which were modulated by peripheral phenyl units with nitrogen atoms. Interestingly, A-TEBPY-COF without nitrogen at the peripheral phenyl ring exhibited the highest HER rate of 98 μmol h⁻¹ g⁻¹ among the A-TEXPY-COFs. This trend of HER rate for A-TEXPY-COF suggested that the photocatalytic HER process proceeded via a radical cation mechanism, which is opposite to the radical anion mechanism by the N_x-COF series. The high electron-rich A-TEBPY-COF greatly stabilized the radical cation, resulting in high photocatalytic activity.¹²⁶

The crystalline and hydrophilic nature of COFs can drastically affect the photocatalysis process. Two sulfone-based COFs, FS-COF and S-COF, were prepared by incorporating two different sulfone moieties to achieve higher photocatalytic performance than N₃-COF. Owing to the parallel C–N bonds in the FS-COF linker and their effective π–π stacking, FS-COF possessed a highly crystalline and stable COF structure compared to S-COF and TP-COF (without the sulfone moiety). FS-COF and S-COF exhibited HER rates of 16.3 and 4.4 mmol h⁻¹ g⁻¹ and outperformed N₃-COF by exhibiting 22 times higher HER rates, respectively. Sulfone moieties are hydrophilic and increase the wettability of COFs in aqueous solution, which results in excellent HER performances. Additionally, FS-COF exhibited long-term stability under light irradiation and HER activity for up to 50 h.¹²⁷

A donor–acceptor (D–A)-based COF system can be very advantageous for HER application. Effective intermolecular charge transfer in the COF system can generate effective charge separation and further enhance the charge transport ability. Many donor organic units such as porphyrin, pyrene, carbazole, and triphenylamine have been employed to form D–A COFs using electron-withdrawing moieties such as thiazole, benzothiadiazole, and substituted phenylenediamine as linkers. Dong et al. developed two donor–acceptor COFs, BT-TAPT-COF and BDF-TAPT-COF, using the triazine moiety as the acceptor unit, whereas benzothiadiazole and benzodifurane moieties as donor units. Both COFs possessed a highly crystalline structure with strong light absorption and excellent stability, exhibiting impressive HER rates of 949 μmol h⁻¹ g⁻¹ and 1390 μmol h⁻¹ g⁻¹, respectively.^128,129

L. Chen and his team also reported a set of D–A-based COFs featuring electron-donating pyrene as the main core with different substituted benzothiadiazole moieties as linkers. To enhance the charge separation efficiency, chloro- or fluoro-groups were attached to the benzothiadiazole organic units for the formation of COFs, where X represents the substituted group. The average lifetimes of excited electrons of the halogenated COFs were drastically improved compared to the non-substituted COFs, representing the high charge separation of excitons and inhibition of charge-recombination phenomena. It also impacted the HER efficiency of COFs, where Py-FTP-BT-COF and Py-ClTP-BT-COF produced hydrogen at the rate of 177 μmol h⁻¹ g⁻¹ and 57 μmol h⁻¹ g⁻¹, which are comparatively higher than that of Py-HTP-BT-COFs (21 μmol h⁻¹ g⁻¹).¹³⁰ Similarly, Li and his group constructed PyTz-COF by employing thiazole (Tz) as an acceptor moiety with pyrene as the donor. PyTz-COF exhibited a highly crystalline structure with an ordered arrangement of thiazole and pyrene groups, which subsequently generated fast intermolecular charge transfer within the COF and effectively extended its π-conjugation. Consequently, PyTz-COF displayed remarkable optoelectronic properties such as broad absorption, photogenerated electrons with a long lifetime, and a high cathodic photocurrent, reaching up to 100 μA cm⁻². Accordingly, PyTz-COF demonstrated an impressive HER rate of 2072 μmol h⁻¹ g⁻¹, outperforming many reported COFs.¹³¹

Lee et al. introduced 1H-phenanthro[9,10-d]imidazole-5,10-diamine (PIDA) as a linker with triformylbenzene and triformylphloroglucinol moieties to develop two new highly crystalline COFs, PIm-COF1 and PIm-COF2, which produced H₂ evolution rates of 528.5 and 7417.5 μmol g⁻¹ h⁻¹, respectively. The photocatalytic performance of PIm-COF2 surpassed that of PIm-COF1 due to the donor–acceptor characteristics facilitated by the strong conjugation between the PIDA moiety and β-ketoenamine linkage. This resulted in increased light absorption, a suitable bandgap, and effective charge separation, leading to high HER activity.¹³²

Further, band gap engineering via structural modification significantly changes the photophysical properties of COFs. To understand this, Y. Chen and group synthesized four different β-ketoenamine-linked COFs, represented as COF-OH-n (n = 0–3), using the triazine moiety as the core with four trialdehyde-benzene units with a varying number of hydroxyl groups (Fig. 6). The presence of the –OH group determined the proton tautomerism in COFs given that COF-OH-0 was unable to undergo tautomerization, whereas COF-OH-1 and COF-OH-2 exhibited reversible keto–enol tautomerization (Fig. 6a). COF-OH-n exhibited proton tautomerism, influencing its band edge positions, band structure, and light absorption, which are critical factors in water splitting. Notably, the HER rate of COF-OH-3 was found to be 9.89 mmol g⁻¹ h⁻¹, which is the highest among the COFs (Fig. 6c). COF-OH-1 did not exhibit any HER activity under similar photocatalytic reaction conditions. COF-OH-3 exhibiting irreversible keto–enol tautomerisation featured moderate visible absorption, high stability, and a suitable optical bandgap of 2.28 eV (Fig. 6b), and therefore exhibited the highest photocatalytic efficiency among them.¹³³ Similar types of results were obtained for Tz-COF-X developed by the same group using a benzothiazole-diamine (Tz) derivative as the donor unit with similar hydroxyl-substituted triarylaldehydes. As the number of –OH groups increased, the donor–acceptor interaction was enhanced for Tz-COF-3, triggering exciton dissociation for efficient charge separation with a longer lifetime, enforcing better photocatalytic activity. Thus, Tz-COF-3 produced an excellent HER rate of 43.2 mmol g⁻¹ h⁻¹ with a quantum efficiency of 6.9% at 420 nm.¹³⁴

Fig. 6 (a) Keto–enol tautomerism in salicylidene anilines and synthesis and structures of all COFs; (b) band structure of COFs vs. the standard hydrogen electrode (SHE) and (c) plot of H₂ evolution versus time for all COFs under solar simulator irradiation. Reproduced from ref. 133 with permission from the Royal Society of Chemistry.

Vinylene-linked COFs have been explored for photocatalytic hydrogen evolution because their organic units are linked via C=C linkages, providing a highly stable and fully π-conjugated structure. Imine, azine, and hydrazone-linked COFs possess partial π-conjugation between building blocks, which limits extended conjugation in the COF system, while vinylene-linkage forms a fully conjugated system, which leads to strong absorption, effective charge transfer, and charge migration, benefitting their photocatalytic activity. Jiang and his group first reported the synthesis of vinylene-linked sp²c-COF using pyrene and phenylenediacetonitrile (PDAN) as building units. Surprisingly, sp²c-COF exhibited a highly crystalline, porous network with excellent chemical stability under ambient conditions. sp²c-COF further transformed sp²c-COF_ERDN by anchoring the electron-deficient ERDN group (3-ethylrhodanine) on its edges. This generated a push–pull effect, developing a heterojunction structure to facilitate charge separation of excitons to increase H₂ generation. Thus, sp²c-COF_ERDN produced hydrogen at the rate of 2120 μmol g⁻¹ h⁻¹ and showed enhanced photocatalysis compared to sp²c-COF (1360 μmol g⁻¹ h⁻¹).¹³⁵ Similarly, X. Liu and co-workers prepared a highly crystalline, porous, and durable sp²-carbon-linked COF, COF-JLU100, using triazine as the core moiety. The vinylene-linked triazine moieties formed a large π-conjugated structure, promoting charge separation and limiting charge recombination, which resulted in bulk conductivity and high charge mobility. Additionally, COF-JLU100 achieved high surface hydrophilicity, which increased its affinity towards water molecules, thus improving photocatalytic water reduction into hydrogen. Combining these features, COF-JLU100 achieved a high photocatalytic performance, achieving an HER rate of over 100 000 μmol g⁻¹ h⁻¹.^136,137

To highlight the importance of the vinylene linkage, COF-JLU35 and COF-JLU36, representing sp² carbon and imine linkages were synthesized via a multicomponent synthetic method using benzotrithiophene and triazine moieties as electron-deficient and electron-rich sites, respectively (Fig. 7a). Despite their similar COF frameworks, COF-JLU35, as the sp²-carbon-linked COF, displayed broader absorption in the visible region, higher charge separation efficiency, and efficient charge transfer than the imine-linked COF. Thus, COF-JLU35 demonstrated a better photocatalytic performance with an HER rate of 70.8 mmol g⁻¹ h⁻¹ than COF-JLU36 and many reported COFs (Fig. 7b). The HER rate of COF-JLU36 was effectively maintained for five cycles without losing its initial properties (Fig. 7c).¹³⁷ To signify the impact of the cyano group in the olefin linkage, Cai and his group designed a series of COFs (TTI-COF, TTV-COF, and TTAN-COF) with identical frameworks but different linkages. Among them, TTAN-COF displayed a significant enhancement in photoluminescence and photocatalytic properties such as a strong emission, improved photo-response, and high conductance of charged species. Consequently, the HER rates of these COF were found to follow the order of TTI-COF (0.46 mmol g⁻¹ h⁻¹) < TTV-COF (5.50 mmol g⁻¹ h⁻¹) < TTAN-COF (11.94 mmol g⁻¹ h⁻¹), where the TTAN-COF consisting of a vinylene linkage with cyano group displayed the highest photocatalytic performance. This work established that cyano functionalization in vinylene-linked COFs increases their chemical and photostability and enhances their optoelectronic properties.¹³⁸

Fig. 7 (a) Synthetic route for COF-JLU35 and COF-JLU36. (b) Photocatalytic H₂ evolution by COF-JLU36 under light irradiation (λ > 420 nm) and (c) hydrogen evolution curve as a function of time for COF-JLU36. Reproduced with permission from ref. 137. Copyright 2023, the American Chemical Society.

Later, Zhang et al. developed two novel vinylene-linked COFs, g-C₁₈N₃-COF and g-C₃₃N₃-COF, via the Knoevenagel condensation of a triazine derivative with 1,3,5-tris(4-formylphenyl)benzene (TFPB) and 1,4-diformylbenzene (DFB) for sunlight driven H₂ evolution, respectively. Although both COFs possessed strong light absorption ability, g-C₁₈N₃-COF displayed a superior HER rate of 14.6 μmol h⁻¹ compared to g-C₃₃N₃-COF (14.6 μmol h⁻¹). The longer conjugated backbone of g-C₁₈N₃-COF effectively suppressed the charge recombination and enhanced the charge separation and transfer ability for photocatalysis, which was also reflected in the longer lifetimes of its excitons and higher photocurrent value compared to g-C₃₃N₃-COF.¹³⁹ Similarly, g-C₃₁N₃-COF, g-C₃₇N₃-COF, and g-C₄₀N₃-COF, with –C=C– linkages were prepared using dicyano-pyridine derivatives with different aldehydic organic moieties. Among them, g-C₄₀N₃-COF with the longest polyphenylene chain possessed a highly crystalline structure and porosity with large pore size, demonstrating an HER rate of 153 μmol h⁻¹ with an apparent quantum yield of 4.84% (±0.27%) under 420 nm. Notably, the HER rates of g-C₃₁N₃-COF (27.1 μmol h⁻¹) and g-C₃₇N₃-COF (19.8 μmol h⁻¹) were relatively lower than g-C₄₀N₃-COF, suggesting that the crystalline nature of these COFs play an important role in enhancing their photocatalytic efficiency.¹⁴⁰

Porphyrins are macrocyclic compounds with highly conjugated π-electron systems, which possess strong visible absorption, redox properties, and high thermal stability. Further, the porphyrin core can be metalated with different metal centres for the appropriate application. Porphyrin as the building block for COF networks can be beneficial for photoactivity given that it will enhance the absorption of light and develop unique photophysical and optoelectronic properties for COFs. R. Chen reported the synthesis of a series of porphyrin-based COFs, MPor-DETH-COF (M = 2H, Co, Ni, and Zn) for application in HER for the first time. MPor-DETH-COF was constructed via the Adler–Longo reaction between metalloporphyrinic aldehydes, p-MPor-CHO, and 2,5-diethoxyterephthalohydrazide (DETH). All the porphyrinic-based COFs exhibited a highly crystalline porous structure, as evidenced by the sharp peaks in their PXRD patterns and large BET surface. The HER performances of MPor-DETH-COF were observed to follow the order of CoPor-DETH-COF (25 μmol g⁻¹ h⁻¹) < H₂Por-DETH-COF (80 μmol g⁻¹ h⁻¹) < NiPorDETH-COF (211 μmol g⁻¹ h⁻¹) < ZnPor-DETH-COF (413 μmol g⁻¹ h⁻¹) as the highest HER rate for a Zn-containing COF system. During photocatalytic hydrogen evolution, photoexcited electrons transfer through metal-on-metal channels to the catalytic surface for chemical reaction, while holes migrate through macrocycle-on-macrocycle channels. However, cobalt ions have a great tendency to undergo the ligand-to-metal transfer (LMCT) process, restraining hole migration, resulting in weak photocatalytic activity. The Ni ion showed a partial LMCT process, while the free base displayed charge transfer via macrocycle-on-macrocycle channels, producing hydrogen with lower HER rates. The LMCT process is strictly forbidden for Zn ions, which allows the free migration of holes and electrons, resulting in the best photocatalytic performance.¹⁴¹

5.1.2 Hybrid COFs. The HER performance of COFs can be increased by forming hybrid COFs via elemental doping, metal complexes, inorganic compounds, carbon-based materials, and other porous structures. Hybrid COFs form heterojunction structures with the doped material, which optimize the band gap position for photocatalysis or hampers the charge recombination process with effective charge separation and migration, resulting in a higher photocatalytic efficiency. Many hybrid COFs have been reported to exhibit HER activity. In 2014, R. Banerjee and co-workers hybridized a crystalline and stable TaPa-2-COF with CdS nanoparticles and studied their application in photocatalytic HER. The CdS material is highly photoactive with a wide bandgap, and thus is utilized to increase photocatalytic properties via doping. Consequently, CdS–COF (90 : 10) achieved an increased HER rate of 3678 μmol g⁻¹ h⁻¹, exceeding that of the pristine COF and CdS materials.¹⁴² Recently, Liu and his group fabricated T-COF with CdS to prepare a T-COF@CdS hybrid via an in situ reaction. T-COF was obtained using 2,4,6-tris(4-aminophenyl)-1,3,5-triazine as a building block with 1,3,5-benzenetricarboxaldehyde as the linker. CdS and T-COF formed a shell–core structure, where T-COF protected the CdS nanoparticles from deactivation and photo-corrosion during photocatalysis. T-COF@COF developed a Z-scheme heterojunction through strong interfacial interaction and gained a suitable band structure, which enhanced its charge separation and transfer capability. Thus, T-COF@CdS-3 achieved an AQY of 37.8% at 365 nm and exhibited a higher photocatalytic performance than CdS and T-COF.¹⁴³ Further, Zhang et al. integrated MoS₂ into a model TpPa-2 COF to develop a noble metal-free MoS₂/TpPa-1 COF composite for photocatalytic HER application given that MoS₂ can be a more effective co-catalyst than platinum. Among the different loadings of MoS₂, MoS₂-3%/TpPa-1-COF showed the highest HER activity, producing hydrogen at a rate of 55.85 μmol h⁻¹. Notably, its HER performance is approximately similar to Pt/TpPa-1-COF (54.79 μmol h⁻¹) with Pt loading {3 wt% (weight percentage)}, which can inspire the development of noble metal-free COFs as photocatalysts for HER. The photocurrent studies revealed that MoS₂ behaves as a co-catalyst and enhances the charge transport capacity and reduces the charge recombination process, and thus the maximum electrons participate in H₂ evolution.¹⁴⁴ Likewise, D. Shang prepared an SnS₂/TpPa-1-COF hybrid to produce a maximum HER rate of 37.11 μmol h⁻¹ without the addition of a co-catalyst (Fig. 8a and b). SnS₂/TpPa-1-COF formed a 2D–2D heterojunction, where the CB potential of TpPa-1-COF is more negative than that of SnS₂ and the conduction band of SnS₂ is higher compared to the potential for H₂ generation (Fig. 8c). Thus, SnS₂ displayed not only a broader absorption up to 600 nm, maximizing the photon absorption but also sped up the charge separation and migration, which resulted in a higher HER performance.¹⁴⁵

Fig. 8 (a) Schematic synthesis of the 2D–2D SnS₂/TpPa-1-COF hybrid, (b) photocatalytic H₂ evolution of TpPa-1-COF, SnS₂, S/C (3 : 7), and mixed (3 : 7) and (c) plausible mechanism of photocatalytic H₂ evolution for 2D–2D SnS₂/TpPa-1-COF. Reproduced with permission from ref. 145. Copyright 2021, the American Chemical Society.

Zhang and his co-workers incorporated hematite (α-Fe₂O₃) on the surface of a COF to form a Z-scheme-based hybrid, α-Fe₂O₃/TpPa-2 COF, for effective water splitting. The α-Fe₂O₃/TpPa-2 COF hybrid was synthesized via an in situ method under solvothermal conditions by varying the ratio of α-Fe₂O₃ and COF. The highest HER activity was found to be an HER rate of 3.77 mmol h⁻¹g⁻¹ by the α-Fe₂O₃/TpPa-2 COF (3 : 7 ratio). The excited electrons of α-Fe₂O₃ transfer to the VB of the COF and recombine with the photo-generated holes, inducing efficient charge separation on the COF surface. Thus, the photogenerated electrons in the COF at its CB are effectively involved in the reduction of protons to hydrogen.¹⁴⁶ Similarly, SA Bao and his group constructed a core–shell structure of TpBD-COF@ZnIn₂S₄ (ZIS) nanosheets via the in situ growth of ZIS on the COF surface. Firstly, TpBD-COF was synthesized via the solvothermal reaction of triformylphloroglucinol (Tp) and benzidine (BD) and its hydrothermal treatment with ZIS formed the TpBD-COF@ZIS composite. The mass ratio of COF and ZIS was varied in the H₂ evolution studies, where TpBD-COF@ZIS-10 with a mass ratio of 1 : 10 exhibited an excellent performance under visible light, showing an HER rate of 2304 μmol g⁻¹ h⁻¹ with an AQE of 5.02% at 420 nm. The H₂ evolution mechanism involves an S-scheme charge transfer, in which the charged electrons on the CB of COF transfer to VB of ZIS and are accompanied by holes, which provide efficient charge separation on the ZIS surface and retarded charge recombination.¹⁴⁷

MOF–COF hybrids have attracted significant attention for various applications given that they provide an enhanced ordered structure with high porosity and unique optoelectronic properties. In this regard, Lan and his group reported the synthesis of an MOF–COF hybrid, NH₂-UiO-66/TpPa-1-COF, by appending NH₂-UiO-66 MOF on the surface of the TpPa-1 COF via in situ solvothermal reaction. The MOF–COF hybrid possessed a crystalline porous network of both MOF and COF with a higher surface area than its parent COF. Notably, the CB position of TpPa-1-COF was found to be more negative than MOF, developing an n-type heterojunction system, where photogenerated electrons transfer from the CB of COF to the CB of MOF, increasing the charge separation efficiency on the COF surface. The photocatalytic performance of the NH₂-UiO-66/TpPa-1 COF was greatly improved compared to its parent COF, producing hydrogen at an HER rate of 23.41 mmol g⁻¹ h⁻¹, which was 20 times higher than that of TpPa-1 COF under the same conditions.¹⁴⁸ Likewise, Yan and his co-workers fabricated NH₂-UiO-66 with TAPT-TP-COF to prepare the NH₂-UiO-66/TAPT-TP-COF-X composite, where x represents the weight ratio of COF. NH₂-UiO-66/TAPT-TP-COF-50 achieved an H₂ evolution rate of 8.44 mmol h⁻¹ g⁻¹ under visible light irradiation using triethanolamine as a sacrificial agent.¹⁴⁹ Additionally, a conventional Zr-MOF, MOF-808, was modified with an –NH₂ ligand, resulting in the development of a core–shell MOF–COF hybrid, MOF-808@TpPa-1-COF, through an in situ solvothermal reaction with suitable COF organic units. The HER rate of MOF-808@TpPa-1-COF reached 11.88 mmol h⁻¹ g⁻¹, a notable increase of 5.6 times compared to its parent COF. Notably, the CB position of TpPa-1-COF was observed to be more negative than that of MOF, establishing an n-type heterojunction system. In this system, photogenerated electrons transfer from the CB of COF to the CB of MOF, enhancing the charge separation efficiency at the COF surface. The increased photocatalytic activity of the MOF/COF hybrid materials was observed due to their exceptional stability, effective charge separation/migration, and efficient inhibition of the charge recombination process for excitons.¹⁵⁰

Graphitic carbon nitride (g-C₃N₄) is an emerging material as a metal-free photocatalyst due to its facile synthesis, light absorption capability, and optimized band structure for catalytic reactions. However, its high charge recombination of excited electrons and holes rapidly inhibits its photocatalytic activity. Therefore, the construction of COF hybrid materials with carbon nitride can improve its photocatalytic efficiency. H. Yan and his group first reported a g-C₃N₄-modified CN–COF hybrid via the in situ solvothermal reaction of g-C₃N₄, triformylphloroglucinol (TP) and triazine moieties. The ratio of COF was varied to prepare different COF : CN composites for the HER study. The photocatalytic water splitting experiment showed that CN–COF with 0.3 wt% COF produced the best HER activity with an HER rate of 10.1 mmol g⁻¹ h⁻¹, which was higher than that of g-C₃N₄ (1.05) and COF (0.16) under similar conditions.¹⁵¹ Further, Li and his group synthesized COF/g-C₃N₄ composites, represented as COF–CN, by varying the mass ratio of COF and g-C₃N₄ (1 : x; x represents the mass ratio of COF, x = 2.5, 5, 10, 15, 20). COF/g-C₃N₄ developed a 1D/2D heterojunction system, where 1D COF extended its absorption area to 560 nm. Interestingly, COF–CN (1 : 10) exhibited a hydrogen evolution rate of 12.8 mmol g⁻¹ h⁻¹ with an AQY of 15.09% under 500 nm light illumination.¹⁵² In this context, g-C₃N₄ enhances the light absorption in the visible spectrum, facilitating efficient charge carrier transfer from g-C₃N₄ to COF through the imine linkage. This reduction in charge recombination of electron–hole pairs resulted in significantly improved charge separation and transfer. The substantial and close interface contact between the materials played a critical role in this, ultimately contributing to the outstanding visible photocatalytic performance exhibited by these composites.

Metalation of covalent organic frameworks (COFs) is another strategy to develop hybrid COFs for photocatalytic hydrogen evolution. COFs can have many functional groups that can bind with a single metal effectively without losing their crystallinity such as 2,2-bipyridine, phenanthroline, porphyrin, β-ketoenamine, and phosphine. In this regard, P. Dong et al. anchored single-atom platinum on the surface of β-ketoenamine-linked TpPa-1-COF to enhance its HER performance. Single-atom platinum atoms were highly dispersed on the COF surface and formed a six-coordinated species, C₃N–Pt–Cl₂, to gain stability. 3% Pt1@TpPa-1 displayed the optimal hydrogen evolution with a rate of 99.86 mmol g_Pt⁻¹ h⁻¹, which was found to be 3.9 and 48-times greater than that of 3% Pt NPs/TpPa-1 and pristine COF under similar conditions, respectively. The coordinated Pt single atom acts as the photocatalytic active site and reduces the energy barrier for the formation of H*, which becomes a key factor in the improved catalytic activity.¹⁵³ Recently, Guo and co-workers prepared a nickel-metalated COF, TpBpy-NiX, by binding Ni(II) on a bipyridine-containing COF via a room temperature and solvothermal method (Fig. 9a). The solvothermally prepared metalated TpBpy-NiX (X = 1%, 2%, 3%, 10%, 20%) showed stable hydrogen evolution under visible light, and the HER rate increased as the content of Ni(II) ions increased. Among them TpBpy-Ni2% exhibited high photocatalytic activity, producing the highest HER rate of 51300 μmol h⁻¹ g⁻¹ (Fig. 9c).¹⁵⁴ The emission studies revealed that the interaction between the nickel ion and bipyridine decreased the bandgap between the HOMO and LUMO compared to the metal-free COF (Fig. 9b). This prolonged the lifetimes of the photoexcited electrons and improved the electron–hole separation, which enhanced the photocatalytic performance.

Fig. 9 (a) Synthesis of TpBpy-NiX COF by traditional and solvothermal methods. (b) Comparison of the band gap of TpBpy and TpBpy-Ni COFs and (c) H₂ evolution rates for TpBpy-Ni2% with other COFs. Reproduced with permission from ref. 154. Copyright 2023, Wiley Online Library.

5.2 Photocatalytic CO₂ reduction reaction

Presently, global warming and climate change have become the main environmental problems due to the high concentration of carbon-dioxide in the atmosphere.¹⁵⁵ In this case, although many CO₂ capture/conversion technologies have been developed, the photocatalytic conversion of CO₂ into valuable chemicals has emerged as green technology to address environmental issues.¹⁵⁶ The breaking of the C=O bond in CO₂ requires high energy, making CO₂ conversion a kinetic and thermodynamically unfavourable process.¹⁵⁷ The conversion of CO₂ into CO occurs at −0.53 V vs. NHE, while other products such as formic acid, methane, and methanol can also be produced at different potentials depending on the nature of the photocatalyst. Hence, efficient photocatalysts are being employed to reduce CO₂ by utilizing sunlight as an energy source.¹⁵⁸ Among them, COFs have attracted worldwide attention as promising candidates for the photocatalytic reduction of CO₂. In this case, the COFs should possess high CO₂ adsorption ability, light-harvesting capacity, and appropriate band structure for CO₂ photoreduction.¹⁵⁹ Although metal-free COFs have been applied as photocatalysts, metal-supported COFs are highly effective for CO₂ reduction given that CO₂ molecules can coordinate with the metal site, enhancing the CO₂ adsorption capacity and lowering the activation energy for CO formation.

5.2.1 Free-standing COFs. The earliest metal-free COF as a photocatalyst for CO₂ photoreduction was reported in 2018. Two COFs, A-COF and N3-COF, were developed using two different building blocks, 1,3,5-triformylbenzene (TFB) and 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (N₃-Ald), respectively, with hydrazine hydrate as the linker and investigated for CO₂ conversion. Both COFs converted CO₂ into methanol under visible light illumination. However, N₃-COF showed higher photocatalytic activity then A-COF, producing methanol at a rate of 13.7 μmol g⁻¹ in 24 h. The high crystallinity, porous network, and high nitrogen content of N₃-COF enhanced its photocatalytic performance.¹⁶⁰

Later, Lie et al. designed a 2D-COF, CT-COF, which produced CO with 98% selectivity upon CO₂ photoreduction without the use of any sacrificial donor or co-catalyst. CT-COF, having carbazole and triazine moieties, formed a donor–acceptor COF, which displayed strong light absorption, optimal band gap, and high nitrogen content. The donor–acceptor COF system generated effective charge separation and charge transfer of electron–hole pairs, which resulted in good photocatalytic activity. CT-COF achieved a CO production rate of 102.7 μmol g⁻¹ h⁻¹ with high selectivity.¹⁶¹

Wang et al. proposed that the modulation of the band structure of COFs can provide effective CO₂ conversion upon light irradiation. They modified a porphyrin-based COF, COF-366, into TAPBB-COF by introducing a bromo group. Consequently, the valence band position of TAPBB-COF shifted to 1.10 V compared to COF-366 (0.86 V), making its band energy gap more suitable for CO₂ photoreduction. TAPBB-COF achieved a CO production rate of 295.2 μmol g⁻¹ h⁻¹, which was significantly higher compared to that of COF-366 (3.9 μmol g⁻¹ h⁻¹). Notably, the porphyrin ring activated the CO₂ molecules and the bromo group increased the adsorption of water molecules, thus achieving high CO₂ conversion into CO.¹⁶² Similarly, Gao and co-workers utilized porphyrin-based covalent organic nanosheets for visible light-driven CO₂ reduction. Two porphyrin-based COFs, 2,3-DhaTph, and 2,3-DmaTph, were first exfoliated into CONs (covalent organic nanosheets) with a high yield via a simple and efficient method. The photocatalytic experiments revealed that the CO yield of CONs was 132.2 and 115.5 μmol g⁻¹ h⁻¹, which are much higher than that of their parent COFs, 2,3-DhaTph (56.6 μmol g⁻¹ h⁻¹) and 2,3-DmaTph (115.5 μmol g⁻¹ h⁻¹) under similar conditions, respectively. The formation of CONs produced more active sites for CO₂ uptake, exhibiting excellent electron–hole separation and high charge transfer during photocatalysis. Overall, CONs exhibited a superior CO production rate compared to bulk COF photocatalysts, providing a new direction to develop novel photocatalysts for CO₂ reduction.¹⁶³

To demonstrate the effect of functionalization on the photocatalytic activity of COFs, four different COFs, TpBD-H₂, TpBD-(CH₃)₂, TpBD-(OCH₃)₂, and TpBD-(NO₂)₂, were developed by varying different functionalized diamines with 1,3,5-triformylphloroglucinol as the building block and further explored for CO₂ reduction. All TpBD-X COFs transformed CO₂ into formic acid under light illumination without any side products. Among them, TpBD-(OCH₃)₂ and TpBD-(CH₃)₂ produced formic acid at a rate of 108.3 and 86.3 μmol g⁻¹ h⁻¹, respectively, which are comparatively higher than that of TpBD-H₂ and TpBD-(NO₂)₂. The electron-donating effect of methoxy and methyl groups exhibits strong conjugation within the COFs, leading to the maximum light absorption, rapid charge transfer, and enhanced charge separation compared to electron-withdrawing groups, and thus a high photocatalytic performance was observed. The results demonstrated that the photophysical properties of COFs can be tuned by the functionalization of their organic units to achieve high photocatalytic performances.¹⁶⁴ Islam and his group designed a novel 2D triphenylamine-triazine COF, Tta-TFPA COF, via a hydrothermal reaction between triazine and triphenylamine organic moieties. Tta-TFPA COF exhibited a highly crystalline structure with wide light absorption and produced solar fuels such as formic acid and methanol upon photocatalytic CO₂ reduction. It formed HCOOH at a rate of 48 mol g⁻¹ h⁻¹ using TEOA as a sacrificial donor and a blue LED (445 nm) as the light source. It also converted CO₂ into MeOH at a rate of 8.3 mmol g⁻¹ h⁻¹ using triethylamine (TEA) as a sacrificial agent under similar conditions (Fig. 10). Notably, Tta-TFPA COF was also capable of performing CO₂ photoreduction under sunlight irradiation, producing HCOOH and MeOH at the rate of 41 mol g⁻¹ h⁻¹ and 0.73 mmol g⁻¹ h⁻¹, respectively.¹⁶⁵ Likewise, Zhu and co-workers prepared two imine-linked COFs, PDA-TTA and PDA-TAB, using diphenylamine as the linker and 4,4′,4′-(1,3,5-triazin-2,4,6-triyl)triphenylamine (TTA) and 1,3,5-tris(4-aminophenyl)benzene (TAB) as building blocks and explored their activity as photocatalysts for CO₂ reduction. PDA-TTA showed better photocatalytic efficiency than PDA-TAB for the conversion of CO₂ into HCCOH with a rate of 65.7 μmol g⁻¹ h⁻¹ under visible light. PDA-TTA COF exhibited high light absorption and efficient charge transfer due to its planar triazine units, which increased its photocatalytic performances.¹⁶⁶

Fig. 10 (a) Photocatalytic generation of HCOOH and MeOH by Tta-TFPA COFs, (b) HCOOH production rate and (c) MeOH generation rate using the Tta-TFPA COFs under visible light. Reproduced with permission from ref. 165. Copyright 2022, the American Chemical Society.

Likewise, Goa and his group developed a series of COF membranes, XN-COF, using an ionic–liquid interface and mimicked an artificial leaf system for efficient light-driven CO₂ reduction. In the XN-COF membranes, X represents the number (0, 1, and 2) of triazine units present in the parent amine and aldehyde monomers. The 2N-COF membrane exhibited a highly crystalline network and larger surface area compared to 1N-COF and 0N-COF given that its high number of triazine units forms an ordered stacking pattern. Thus, the 2N-COF membrane exhibited excellent visible light CO₂ reduction, giving the highest CO production rate of 310 μmol g⁻¹ h⁻¹ without the use of any photosensitizer or donor. The 2N-COF membrane outperformed the 2N-COF powder as a photocatalyst, which produced CO at a rate of 98 μmol g⁻¹ h⁻¹ under similar conditions. The remarkable photocatalytic activity of the leaf-like COF membrane was based on its triazine–imine–triazine framework, where the triazine moieties functioned as electron reservoirs, the imine bond acted as a photocatalytic centre and the leaf-like structure enhanced the charge/mass transfer. This study suggests that artificial leaf systems based on COFs can be very effective as photocatalysts for CO₂ reduction to CO.¹⁶⁷

Yang and his group reported the synthesis of two novel sp²c-COFs, TFPB-COF and BTE-TBD-COF, for improved light-driven CO₂ reduction to CO. BTE-TBD-COF possessed an ethylene bridge in its COF network, which enhanced the conjugation effect, significantly resulting in improved charge separation of photogenerated electrons and efficient charge migration. This is also supported by the photocurrent responses from both COFs, where the average lifetime of the excited electrons for BTE-TBD-COF (8.3 ns) was much longer compared to TFPB-COF. Further, the band gap of BTE-TBD-COF became more favorable for the CO₂ reduction process, producing CO at a rate of 382.03 μmol h⁻¹ g⁻¹, while TFPB-COF showed a CO production rate of 109.8 μmol h⁻¹ g⁻¹. This demonstrated that the extended conjugation in the COF framework can facilitate charge separation and migration to enhance its photocatalytic activity.¹⁶⁸

Tang and co-workers explored hydrazone-linked COFs for photocatalytic CO₂ reduction and synthesized two new COFS, BTT-Hz-1 and BTT-Hz-2 COFs, using a conjugated aldehyde (benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene-2,5,8-tricarbaldehyde, BTT) with two different hydrazone monomers. BTT-Hz-2 exhibited a CO evolution rate of 774.3 μmol g⁻¹ h⁻¹ with CoCl₂/Bpy as a co-catalyst and TEOA as a sacrificial donor, while BTT-Hz-1 produced CO at a rate of 251.4 μmol g⁻¹ h⁻¹ under similar conditions. To signify the importance of the BTT unit in the COF, TFB-1 and TFB-2 COFs were prepared by applying a benzene-containing aldehyde (1,3,5-triformylbenzene, TFB) monomer with the corresponding hydrazones, and these COFs were unable to produce CO under similar conditions. The conjugated nature of the “benzotrithiophene” moiety in BTT than the “benzene” ring of TFB increased the electron delocalization in the BTT-based COFs and decreased their bandgap, making them suitable for photocatalysis. The better photocatalytic performance of BTT-Hz-2 compared to BTT-Hz-1 can be ascribed to its broader visible-light absorption range due to its extended conjugation, large porous surface, and efficient charge separation within its COF network.¹⁶⁹ Jin and co-workers enhanced the photocatalytic CO₂ reduction by introducing a hydrophilic group in the COF photocatalyst. They developed QL-COF by carboxylquinoline linkages, which possessed hydrophilic carboxylic acid in its porous surface. Due to the presence of carboxy groups, QL-COF exhibited a low binding energy toward water and CO₂ molecules, leading to the high adsorption of water and CO₂ on the surface of the COF. Thus, QL-COF showed a CO production rate of 156 μmol g⁻¹ h⁻¹ with 99.3% selectivity, which is much higher than that of LZU1-COF (25 μmol g⁻¹ h⁻¹) lacking a carboxy group in its structure.¹⁷⁰

5.2.2 Hybrid COFs. The photocatalytic CO₂ conversion efficiency of COF frameworks can be greatly improved by coordinating them with metal complexes or metal atoms. Bipyridine-containing COFs have been utilized for this purpose given that the pyridinic nitrogen possesses coordination ability. Huang et al. designed a rhenium complex-incorporated COF, Re-COF, for photocatalytic CO₂ reduction. Firstly, the parent COF was prepared by Schiff base reaction, and then rhenium complexes were introduced in the COF by stirring with the bipyridine moiety and Re(CO)₅Cl, forming Re-COF as the final product. The PXRD analysis of Re-COF confirmed that possessed a similar pattern as its parent COF, suggesting that its crystalline structure was retained after the incorporation of Re. Re-COF showed a CO production rate of 15 mmol CO per g over a 20 h period. Upon photo-excitation, the photogenerated electrons transport from COF to the Re moiety increasing the charge transfer rate and reducing the charge recombination, and thus a higher number of electrons reach the Re complexes, which transform CO₂ into CO. The CO production rate of Re-COF was much higher than that of the pristine COF and Re(bpy)(CO)₃Cl complex under similar conditions.³⁹ Inspired by this, Cooper and his group incorporated a rhenium complex into a highly crystalline sp²c-COF to achieve a high photocatalytic performance for CO₂ reduction. sp²c-COF, which was prepared by the solvothermal reaction between pyrene and bipyridine moieties, transformed into rhenium-centred Re-Bpy-sp²c-COF using Re(CO)₅Cl. The crystallinity of Re-Bpy-sp²c-COF remained intact after the incorporation of the Re complexes. The photocatalytic CO₂ reduction experiment for Re-Bpy-SP²c-COF exhibited a CO yield of 1400 μmol g⁻¹ h⁻¹ with 86% selectivity. The pyrene and bipyridine units of sp²c-COF form a donor–acceptor system that narrows the band gap by stabilizing the conduction band and enhances the transfer of excited electrons from the pyrene unit to the bipyridine moiety, leading to effective charge separation and migration. The photogenerated electrons are further transported to the active rhenium site for participating in CO₂ conversion, improving the CO₂ transformation.¹⁷¹ Similarly, Chen and his group developed a novel rhenium–bipyridine COF, viCOF-bpy-Re, in which the rhenium–bipyridine complex and triazine unit are connected via a vinylene linkage complex (Fig. 11a and b). The incorporation of Re increased the loading of CO₂ molecules due to the high affinity of CO₂ for the rhenium site. Further, viCOF-bpy-Re displayed high charge mobility and efficient charge separation due to the lower band gap and extended conjugation in the COF structure, which formed two active sites (Fig. 11b). The bipyridine–rhenium complex acts as a reduction site, converting CO₂ molecules into CO, while the triazine moiety serves as an oxidation site, generating oxygen from water oxidation. Thus, viCOF-bpy-Re acted as an artificial photosynthetic system, yielding both CO and O₂ at the optimal rate of 190.6 μmol g⁻¹ h⁻¹ and 90.2 μmol g⁻¹ h⁻¹, respectively (Fig. 11c).¹⁷² Other metal complexes have also been appended on bipyridine-containing COFs to enhance their photocatalytic performance. Kou et al. incorporated Mo(CO)₅ complexes in a TpBpy COF to prepare Mo-COF for photocatalytic CO₂ reduction. Mo-COF produced a variety of valuable products such as CO and C_xH_y. Mo-COF achieved the generation of CO, CH₄, and C₂H₄ with a rate of 6.19, 1.08, and 3.57 μmol g⁻¹ h⁻¹ upon 6 h of light irradiation, respectively. The MoN₂ sites possess higher affinity for CO and utilize it as a reactant molecule for further transformation into CH₄ and C₂H₄.¹⁷³

Fig. 11 (a) Route for the synthesis of viCOF-bpy-Re. (b) Schematic representation of bipyridine and triazine ring units in viCOF-bpy. (The purple balls indicate bipyridine units while yellow balls indicate triazine units and vinyl linkages.) (C) Band gap diagram of viCOF-bpy-Re and viCOF-bpy and (d) photocatalytic CO₂ reduction by Re(CO)₆Cl, viCOF-by and viCOF-by-Re. Reproduced with permission from ref. 172. Copyright 2023, Wiley Online Library.

The incorporation or loading of a single metal atom on the surface of COFs can also be beneficial for solar-driven CO₂ reduction because a metal site can act as an active centre for photocatalytic CO₂ conversion. The coordination of metal sites on COFs effectively enhances the rate of CO₂ reduction with high selectivity for the products. In this regard, Lan et al. reported the modification of a series of COFs by appending transition metals such as cobalt, nickel, and zinc atoms on their porous surface. The DQTP-COF-M (M = Co/Ni/Zn) COFs were prepared via a facile and simple post-treatment method with metal complexes. The metalated COFs showed high crystallinity given that their PXRD patterns were similar to that of the non-metalated DQTP-COF. The photocatalytic experiments were performed in acetonitrile solution using triethanolamine (TEOA) as a sacrificial agent and Ru(bpy)₃Cl₂·6H₂O (bpy = 2,2-bipyridine) as a photosensitizer. Surprisingly, the different metals produced different products with high selectivity. DQTP COF-Co produced CO at a rate of 1.02 × 103 μmol h⁻¹ g⁻¹, while DQTP COF-Zn produced formic acid at a rate of 152.5 μmol h⁻¹ g⁻¹ with 90% selectivity over CO. The adsorbed CO₂ activated upon photocatalysis and formed intermediate CO₂, which can produce the final product via two pathways. Based on the experiments, it was suggested that Co(II) has good π–donor ability and converts CO₂ to CO, while Zn(II) as a poor π–donor produces HCOOH over CO.¹⁷⁴ Additionally, this work demonstrated that metal centres on COFs as active sites can be very effective for CO₂ reduction.

Porphyrin-based COFs have emerged as efficient catalysts for the catalytic reduction of CO₂. A series of single-atom porphyrin-based COFs, TTCOF-M (M = 2H, Zn, Ni, Cu), was synthesized using the tetrathiafulvalene (TTF) metalloporphyrin as the building block, which was utilized as photocatalysts for reducing CO₂ with water. Porphyrin moieties have strong absorption in the visible region, while sulfur-rich TTF moieties possess strong electron-donating nature with fast electron transfer, thus forming a donor–acceptor COF system. Therefore, this drives the effective separation of photogenerated electrons and holes on the porphyrins and TTF sites, respectively. TTCOF-M was developed as an artificial photosynthetic system, where its porphyrin unit participates in CO₂ reduction to CO, while its TTF unit transforms H₂O into oxygen. Among them, TTCOF-Zn showed the best CO production rate of 12.33 μmol over a period of 60 h, which is double the oxygen evolution rate.¹⁷⁵ Similarly, Zhu et al. also applied a nickel-metalated porphyrin-based COF, PD-COF-23-Ni, for the photocatalytic transformation of CO₂ into CO and produced CO at a rate of 40.0 μmol g⁻¹ h⁻¹ without any photosensitizers or noble metal catalyst. Compared to PD-COF-23 without any metal site, PD-COF-23-Ni showed an effective charge separation efficiency and CO₂ reduction process at the Ni(II) centre, enhancing its activity for the photoreduction process.¹⁷⁶ The incorporation of metal nanoparticles (NPs) is the most effective way to overcome the charge recombination process during photocatalytic reactions. Using this approach, Fan and co-workers deposited ruthenium nanoparticles on a bipyridine-containing COF to develop a ruthenium-decorated COF, Ru@TpBpy. Ruthenium nanoparticles were deposited on the COF via the solution infiltration technique with different loading amounts. The photocatalytic experiments revealed that Ru@TpBpy with 0.7 wt% Ru NPs displayed a higher photocatalytic CO₂ conversion compared to TpBpy and yielded formic acid at a rate of 172 mmol g_cat⁻¹ h⁻¹. The ruthenium nanoparticles in the COF widened its absorption region and enhanced the charge separation and migration by reducing the charge recombination process, leading to an increased photocatalytic performance.¹⁷⁷

Z-scheme heterojunction systems can be developed by covalently connecting COFs with inorganic semiconductors, which can be effective for photocatalytic applications. Zhang and co-workers developed a Z-scheme-based heterojunction system of COF-semiconductor composites as photocatalysts. Three different inorganic semiconductors, TiO₂, Bi₂WO₆, and α-Fe₂O₃, were combined with COF-316 and COF-318 and employed for photocatalytic CO₂ reduction. The photocatalytic CO₂-reduction process was carried out under visible light without the use of any photosensitizer, sacrificial agent, or co-catalyst. Among them, the COF-318-TiO₂ heterojunction COF system exhibited the highest photocatalytic performance, achieving a CO production rate of 69.67 μmol g⁻¹ h⁻¹, which was six-times higher than that of the pristine COF-318, TiO₂ powder, and their physical mixture. The covalent linkage in the Z-scheme heterojunction COF-semiconductor facilitates effective charge transfer between the COF and semiconductor, which leads to the accumulation of excited electrons on the COF and holes at the semiconductor for the reduction and oxidation process, respectively, forming an artificial photosynthetic system.¹⁷⁸ Further, Do and co-workers integrated single-atom Co-1T-MoS₂ on a β-ketoenamine-linked 2D COF TpPa-1 COF for the selective reduction of CO₂ into CO under visible light irradiation. The single-atom Co-1T-MoS₂ was prepared via the hydrothermal reaction of a molybdenum complex ((NH₄)₆Mo₇O₂₄·4H₂O) with thiourea and cobalt acetate and incorporated it in a COF via a physical method (Fig. 12a). The TpPa-1/Co-1T-MoS₂ composite exhibited increased CO₂ reduction, giving a CO production rate of 196 μmol g⁻¹ h⁻¹ with 93% selectivity (Fig. 12b). The photocatalytic efficiency of CO generation by the composite was 1.23 and 1.6-times higher than that of the bare COF and Co-1T-MoS₂, respectively. The mechanistic pathway for the composite depicted that the energy band of MoS₂ lies below the conduction band of the COF and the photogenerated electrons in the COF effectively transfer to the CB of MoS₂ under light irradiation, which further reach the surface of the photocatalyst to participate in the catalytic reaction.¹⁷⁹

Fig. 12 (a) Schematic representation of the formation of the TpPa-1/Co-1T-MoS₂ composite. (b) Photocatalyzed CO₂ reduction of (a) MoS₂, (b) Co-1T-MoS₂, (c) TpPa-1 (hollow), and (d) TpPa-1/Co-1T-MoS₂. Reproduced with permission from ref. 179. Copyright 2023, the American Chemical Society.

Graphene as a carbon-based material can increase the photocatalytic activity of COFs for CO₂ photoreduction given that its extended conjugated network hampers the charge recombination of excitons, enhancing the adsorption of CO₂, activation, and stability under visible light. Do and co-workers synthesized a hybrid composite of COF and reduced graphene oxide, rGO@TpPa-1, to improve the CO₂ reduction process under light illumination. The rGO@TpPa-1 composites were synthesized via the in situ reaction of rGO (content of 5%, 10%, 15%, and 20%), triformylphloroglucinol (Tp), and diaminobenzene under solvothermal conditions. The formation of the hybrid composite was confirmed through PXRD analysis based on the appearance of the characteristic peaks for COF and rGO. The CO₂ photoreduction experiments for rGO₁₅@TpPa-1 exhibited a CO yield of 198.97 μmol g⁻¹ h⁻¹ with 89% selectivity over HER, which is higher than that of the parent TpPa-1 COF (126.36 μmol g⁻¹ h⁻¹) and bare rGO (28.51 μmol g⁻¹ h⁻¹). The integration of rGO with the COF improved the light-absorption capacity, inhibited the charge recombination process, and enhanced the charge mobility, consequently increasing its photocatalytic performance.¹⁸⁰ Likewise, Ye and his group developed a 2D/2D heterojunction COF hybrid (g-C₃N₄(NH)/COF) as a photocatalyst by combining g-C₃N₄ with TpTta-COF, which was synthesized using triazine and trifomylphloroglucinol moieties. g-C₃N₄(NH)/CO achieved high photocatalytic CO₂ reduction, yielding CO at a rate of 11.25 μmol h⁻¹ with 90% selectivity. The CO production rate for the COF hybrid was 45-times higher compared to that of the bare g-C₃N₄ (0.25 μmol h⁻¹). The g-C₃N₄(NH)/COF hybrid works as an S-scheme heterojunction system, where the photo-excited electrons in the CB of COF combine with the photogenerated holes in the VB of g-C₃N₄, effectively reducing the charge recombination rate, while the excited electrons from the CB of g-C₃N₄ reaches the photosensitizer for the conversion of adsorbed CO₂ molecules.¹⁸¹

Further, Wu and co-workers proposed a novel MOF/COF hybrid as a photocatalyst using olefin-linked TTCOF and NUZ MOF (NH₂–UiO-66 (Zr)) as precursors to achieve high photocatalytic activity for CO₂ reduction to CO. TTCOF/NUZ hybrids were prepared via the in situ reaction of TTCOF with NH₂-BDC and ZrCl₄ by varying the ratio of TTCOF (x = 5, 10, 15, 20, 30). The optimized TTCOF/NUZ was an S-scheme heterojunction system and displayed a higher photocatalytic performance than the bare COF and MOF. TTCOF/NUZ produced CO at a rate of 6.56 μmol g⁻¹ h⁻¹ in a gas–solid system under visible light. The increased photocatalytic activity can be ascribed to the effective charge carrier separation in the heterojunction system, which enhances the charge transportation and redox properties.¹⁸²

5.3 Photocatalytic oxygen evolution reaction

The photocatalytic oxygen evolution reaction (OER) is a half-reaction of photocatalytic overall water splitting, where photogenerated holes produce oxygen as the product from water molecules. The redox potential for oxygen evolution is up to 1.23 eV, which is a four-electron process consisting of two water molecules.¹⁸³ Thus, the OER process is a thermodynamically and kinetically unfavorable process. In this case, the design of a photocatalyst for OER not only requires strong light absorption and a highly porous surface but also needs an appropriate valence band potential to successfully produce oxygen from water splitting. Thus, very few COFs have been reported as photocatalysts for the OER process.¹⁸⁴

Metal-free COFs can not facilitate the photocatalytic OER process. Thus, COFs have been combined with suitable metals, noble metals or photosensitizers or modified with them to develop hybrid COFs to perform this reaction. These hybrid COFs exhibit activity for overall water splitting, producing oxygen and hydrogen in high yield. In 2018, Yang et al. and co-workers synthesized a cobalt-incorporated bipyridine-containing COF, BpCo-COF, for visible light-driven oxygen evolution. Bp-COF was prepared via the solvothermal reaction between tris(4-aminophenyl)benzene and 2,2-bipyridine-5,5′-dicarboxaldehyde organic moieties and further incorporated cobalt metal through coordination with the bipyridine unit via post-synthetic treatment. The HOMO level of Bp-COF appeared at +1.46 V vs. RHE and positively shifted at 1.54 V for BpCo-COF, providing a feasible oxidation potential for oxygen evolution. Also, BpCo-COF showed a higher water uptake capacity than Bp-COF due to the high affinity of water molecules toward the Co atom, which will be beneficial for ORR activity. BpCo-COF exhibited superior oxygen evolution at a rate of 152 μmol g⁻¹ h⁻¹ under visible light with an AQY value of 0.46%, while Bp-COF was unable to produce oxygen under similar conditions. BpCo-COF was also tested for long-term stability, exhibiting excellent ORR activity over 31 h. It was evidenced that cobalt serves as a co-catalyst in BpCo-COF and increases its photocatalytic activity by lowering the activation energy for redox reactions by facilitating charge separation and migration. This was the first report in which a COF was successfully employed for visible-light-driven oxygen evolution.¹⁸⁵

Likewise, Li et al. incorporated cobalt ions in a triazine-based COF, TAPT-Bpy-COF, to enhance its photocatalytic performance for oxygen evolution. The absorption profile of TAPT-Bpy-COF-Co was red-shifted compared to that of the parent COF, indicating its increased light absorption capability. The wettability of the COF also improved after the integration of cobalt, depicting a high water intake capacity. TAPT-Bpy-COF-Co showed an impressive oxygen evolution rate of 483 μmol g⁻¹ h⁻¹ upon light irradiation. TAPT-Bpy-COF-Co displayed a superior oxygen evolution rate compared to the similar BpCo-COF with an increased AQY of 7.6%. This improved photocatalytic activity of TAPT-Bpy-COF-Co can be ascribed to its high crystallinity, porosity, good wettability, and better charge separation.¹⁸⁶

Further, He and his group designed a novel COF (BtB-COF) with N,S-chelated atomic Co catalytic sites in its pore channel for photocatalytic water splitting. BtB-COF was synthesized via the condensation reaction of benzotrithiophene tricarbaldehyde and benzothiadiazole-diamine building units (Fig. 13a). A strong peak was observed in the EPR study, corresponding to the unpaired electrons, which indicated that the electron-deficient benzothiadiazole and electron-rich benzothiophene groups developed a push–pull effect, increasing the optical absorption with a smaller band gap and enhancing the photoinduced charge separation and transfer (Fig. 13b). The light-driven oxygen evolution by BtB-COF was investigated using Co as co-catalyst and AgNO₃ as an electron acceptor, achieving an oxygen evolution rate of 6.65 μmol h⁻¹ (Fig. 13d). The FT-IR and XPS analyses revealed that the cobalt ions chelated with nitrogen and sulfur atoms under light illumination to form active metal sites for oxygen evolution activity. The chelated cobalt sites adsorbed water molecules through a hydration process, which increased the water dispersibility and lowered the energy barrier for the oxygen evolution process, overall promoting the water oxidation process (Fig. 13c). Under similar photocatalytic experiments, the analogous BtD-COF without a benzothiadiazole unit produced an O₂ evolution rate of 2.2 μmol h⁻¹, which was less than that of BtB-COF, implying the advantages of functionality and structural modulation of COFs for photocatalytic application. Further, a heterostructure CdS/BtB-COF was developed by immobilizing CdS nanoparticles on BtB-COF for overall water splitting. CdS/BtB-COF displayed high photocurrent responses and decreased resistance compared to the bare BtB-COF and CdS due to the effective charge separation and migration in the hybrid composite. Thus, the photocatalytic efficiency of CdS/BtB-COF was greatly improved for water splitting and it achieved an O₂ evolution rate of 212 μmol g⁻¹ h⁻¹, which is comparatively higher than that of many inorganic semiconductor-based photosystems for water splitting.¹⁸⁷

Fig. 13 (a) Synthesis and (b) absorption profile and bandgap of BtB-COFs. (c) Plausible mechanism of light-driven O₂ evolution of BtB-COFs and (d) O₂ evolution over BtB-COFs over a period of 18 h under AM 1.5G light irradiation. Reproduced with permission from ref. 187. Copyright 2023, the American Chemical Society.

Han and his group designed two metal covalent organic frameworks as photocatalysts for the water oxidation process by employing a ruthenium complex as one of the building units for the formation of COF. A solvothermal reaction was performed between a tetradentate aldehyde-functionalized Ru(II) complex, Ru(bpy-CHO)₂Cl₂, and the amino-functionalized organic units ETTA and ETTBA to synthesize novel MCOFs, RuCOF-100 and Ru-COF101, respectively. Both COFs exhibited highly crystalline networks, forming three-dimensional frameworks with high porosity and easily accessible, orderlyarranged Ru sites. The coordinated Cl ligands of the Ru sites were replaced by water molecules, forming Ru-100′ and Ru-101′, which enhanced the hydrophilicity of the COFs and promoted their water dispersibility. The photocatalytic water oxidation reaction of Ru-COFs was investigated using Ru(bpy)₃²⁺ as a photosensitizer. RuCOF-100′ and RuCOF-101′ showed excellent oxygen evolution rates of 2305 and 2830 nmol g⁻¹ s⁻¹, respectively. The plausible mechanism of the water oxidation process involves the oxidation of Ru(II) into high-valent Ru(V)=O species, which is transformed into the hydroperoxo intermediate Ru(III)–O–OH by H₂O coordination. This is further converted to Ru(V)–OO via a proton-coupled electron transfer process, which releases dioxygen as the final product.¹⁸⁸

Similarly, S. Karak synthesized a novel 3D COF, Ru(bda)COF, for improved photocatalytic oxygen evolution via the condensation reaction of aldehyde-functionalized ruthenium complex and tetrakis(4-aminophenyl)methane. The electrochemical studies for Ru(bda)COF showed three oxidation peaks versus NHE at +0.67, +0.89, and +1.08 V, corresponding to the formation of Ru(III), Ru(IV), and Ru(V) in aqueous solution, while oxygen evolution generally proceeds at a lower potential. Upon light-driven water oxidation, Ru(bda)COF exhibited an oxygen evolution rate of 1072 μmol L⁻¹ s⁻¹. The water oxidation catalyst (WOC) mechanism of Ru(bda)COF generally involves second-order rate kinetics, which suggests that the rate-determining step involves two COF particles. However, its amorphous counterpart, Ru(bda)COF, showed much lower activity, exhibiting an O₂ evolution rate of 3.0 μmol L⁻¹ s⁻¹, which demonstrated that the orderly arrangement in the crystalline form is effective for efficient photocatalysis.¹⁸⁹

Xu and his group developed a Z-scheme heterostructure-based piezoelectric material-decorated COF photocatalyst to achieve photocatalytic water splitting. BiFeO₃@TpPa-1-COF was prepared under in situ solvothermal conditions, where the precursor of TpPa-1-COF underwent Schiff base condensation with BiFeO₃ nanosheets that were amino-functionalized by 3-aminopropyltriethoxysilane (APTES) (Fig. 14a). The combination of COF and BiFeO₃ formed a synergistic Z-scheme heterostructure, where electrons accumulated at the COF site, while holes collected at BiFeO₃. Thus, BiFeO₃@TpPa1-COF (BFO@COF20-C) demonstrated the excellent H₂ and O₂ production rates of 1416.4 and 708.2 μmol g⁻¹ h⁻¹ under the combined effect of light and ultrasound, respectively (Fig. 14b). The piezo-photocatalytic effect of the BiFeO₃@TpPa-1-COF heterojunction increased the transfer of photogenerated electrons and holes in the opposite sites, resulting in efficient charge separation and mass transportation, respectively, which subsequently improved the photocatalytic performances (Fig. 14c). The BiFeO₃@TpPa-1-COF hybrid COF showed a superior oxygen evolution rate compared to the reported BaTiO₃ NPs and ZnS nanosheet material, demonstrating the superiority of piezo-catalysis (Fig. 14d).¹⁹⁰

Fig. 14 (a) Schematic synthetic route for the BiFeO₃@TpPa-1-COF hybrid. (b) O₂ evolution of piezo-photocatalysis. (c) Mechanism of O₂ evolution of piezo-photocatalysis for the BiFeO₃@TpPa-1-COF heterojunction and (d) COFs and C₃N₄-based photocatalysts. Reproduced with permission from ref. 190. Copyright 2022, Wiley Online Library.

A series of β-ketoamine COFs with and without a bipyridine moiety, denoted as TpBD-COF, TpBpy-2-COF, and TpBpy-COF, was exfoliated to form COF nanosheets (COF-NS) and their pore channels were encapsulated with Pt nanoparticles for photocatalytic water splitting. The smooth surface of COF-NS increased the dispersion of NPs throughout the pore channel of COF-NS, as evidenced by TEM and AFM studies. Pt@TpBpy-NS displayed excellent photocatalytic water splitting activity, producing H₂ and O₂ evolution at the rate of 9.9 and 4.8 μmol over a period of 5 h, which is comparably higher relative to Pt@TpBpy-2-NS, producing H₂ and O₂ at the evolution rate of 3.1 and 4.1 μmol, respectively. The engineering of the nitrogen site in the COF framework greatly affects the photocatalytic activity of Pt@COF-NS given that bipyridine acts as the binding site for COF. Pt@Tp-BD-COF could not perform the water-splitting process due to the absence of the bipyridine moiety and only exhibited hydrogen evolution, half of the reaction of the water-splitting process. Pt@TpBpy-2-NS possessed differently positioned nitrogen sites, which could not coordinate with Pt NPs, lowering its photocatalytic activity. Pt@TpBpy-NS with adjacent nitrogen positions exhibited the effective accumulation of Pt NPs, which promoted photoinduced charge separation and transfer, thus achieving a high photocatalytic performance. This work demonstrated that the existence and rational position of the nitrogen sites are crucial to inducing charge transfer and reaction potential barriers for water splitting activity.¹⁹¹

Later, Shen and his co-workers constructed a heterojunction system via the in situ deposition of 2D COF on WO₃ nanosheets (Ov-WO₃) to boost the photocatalytic performance for overall water splitting. The COF precursor, triformylphloroglucinol (TP), and 3,7-diaminodibenzo[b,d]thiophene-5,5-dioxide underwent in situ photo-assisted reaction with WO₃ nanosheets and coordinated to the oxygen vacancy of WO₃via a W–O–C covalent bond to form TSCOFW (COF–WO₃) as a hybrid composite. The combination of WO₃ and COF developed a synergistic Z-heterojunction system via the increased built-in electric field of the W–O–C bond, which is beneficial for the effective charge separation of photoinduced electrons and holes and maximizes the utilization efficiency of charge carriers. This led to the high photocatalytic performance of the TSCOFW hybrid for OWS, which exhibited hydrogen and oxygen evolution at the rate of 146 and 68 μmol h⁻¹ g⁻¹, respectively. Under photo-irradiation, the photoinduced electrons of TSCOF and WO₃–O_v reached the CB level, leaving holes in the VB level. The photoexcited electrons in the CB of WO₃–O_v recombine with the holes in the VB of TSCOF due to the internal electrical field. The electrons in the CB of COF participate in HER, while the photogenerated holes in the VB of WO₃ are involved in the water oxidation into oxygen. Thus, the 2D/2D Z-scheme heterojunction of TSCOFW provides a new way for efficient overall water splitting (OWS).¹⁹²

In another report, a fluorenone-based COF, COF-SCAU-2, was modified into ultrathin three-layer nanosheets (UCOF-SCAU-2) for an efficient OWS process. COF-SCAU-2 was synthesized using tris(4-formylphenyl)benzene (TPB) and diamino-fluorenone moieties and further reduced to an ultrathin structure by mechanical delamination treatment. Interestingly, UCOF-SCAU-2 served as an effective photocatalyst for OWS under visible light illumination and produced hydrogen and oxygen at rates of 0.046 and 0.021 mmol h⁻¹ g⁻¹, respectively. The bulk COF-SCAU-2 did not show any activity for photocatalytic OWS under similar conditions. The charge-carrier kinetic calculation and DFT (density functional theory) studies revealed that the ultrathin structure of COF increases the accessibility to the active site, generates a short electron migration distance, enhances the average lifetime of excitons, and lowers the activation energy photocatalytic reaction compared to the parent COF, leading to high photocatalytic activity.¹⁹³

5.4. Photocatalytic organic transformations

COFs with high porosity can be very effective in the field of organic synthesis by acting as photocatalysts in heterogeneous catalysis. Traditional photocatalysts such as transition metal complexes, dyes, and semiconductors have been efficiently utilized as heterogeneous organocatalysts.¹⁹⁴ However, these photocatalysts generally suffer from many problems such as low photostability, high cost, poor recyclability, and laborious separation techniques, which restrict their large-scale application. COFs have a highly conjugated π-network with wide absorption in the visible-region, large surface area, and high chemical stability, which are the primary requirements for photocatalytic organic transformations.¹⁹⁵ Thus, COFs have attracted increasing attention as effective photocatalysts for organic synthesis. The photocatalytic mechanism of organic conversions involves the activation of oxygen into O₂˙⁻ and ¹O₂ intermediates via photogenerated electrons, which perform the reaction with the organic compound. The great advantage of using COFs as photocatalysts is that their photocatalytic properties can be optimized through different types of building blocks according to the corresponding organic reactions. Moreover, hybrid COFs can be developed using metal or metal complexes or other materials to increase photo-activated organic transformation. COFs have been explored in many types of organic transformation reactions such as oxidation, reduction, and coupling.

5.4.1. Free-standing COFs. COFs as photocatalysts have been employed in different types of oxidation reactions. Wang et al.¹⁹⁶ synthesized two vinylene-linked COFs, TC-PT and TC-PB, via the Knoevenagel condensation of 1,3,5-tris(4-cyanomethylphenyl)benzene (TC) with tris(4-formylphenyl)triazine (PT) or tris(4-formylphenyl)benzene (PB) moieties, respectively. TC-PT and TC-PB showed a highly crystalline structure and BET surface area of 604 and 520 m² g⁻¹, respectively. Further, TC-PT afforded a higher photocurrent response than TC-PB due to its donor–acceptor heterostructure. Both COFs were investigated for the aerobic oxidation of phenylboronic acid into phenol under visible light irradiation, where TC-PT achieved a 99% conversion rate, which was comparatively higher than that by TP-PB (78%). The triazine-bearing of TC-PT formed a donor–acceptor system with efficient intramolecular charge transfer (ICT), leading to a better charge separation and charge transfer rate, which enhanced its photocatalytic activity. According to the proposed reaction mechanism, the COFs generated electrons (e⁻) and holes (h⁺) after excitation and formed the superoxide radical anion (O₂˙⁻) from oxygen, which coordinated both atoms, leading to the formation of the product. TC-PT COF also displayed high recyclability and recovery rate after 10 cycles. Similarly, R. Paul and his group designed benzothiazole-linked COFs, BTZ-BCA-COF and BTZ-TPA-COF, for the light-driven transformation of phenylboronic acids to phenols. The COF was synthesized via a one-pot reaction, where elemental sulfur carried out metal-free C–H functionalization, followed by oxidative annulations. BTZ-BCA-COF and BTZ-TPA-COF showed high photocatalytic performances toward the light-induced conversion of phenylboronic acid to phenol with 99% yield. The high efficiency of these COFs can be attributed to their highly crystalline nature, porous surface, effective π-delocalization, and remarkable photophysical properties.¹⁹⁷

Reactive oxygen species (ROS) are another interesting aspect of photocatalytic activity. Employing efficient photocatalysts has emerged as a practical method to induce ROS generation. Wu and his team developed a benzoxazole-linked COF, LZU-191, via photocatalytic polymerization using natural sunlight as the energy source and utilized it for photocatalytic investigations (Fig. 15). Due to the presence of irreversible oxazole linkages in the COF framework, LZU-191 possessed high crystallinity with a BET surface area of 1314 m² g⁻¹ and excellent chemical stability in water, acid, base, and common organic solvents. Furthermore, it demonstrated high photocatalytic activity for the light-driven aerobic oxidation of sulfides to sulfoxides because of its unique morphological properties.¹⁹⁸

Fig. 15 (a) Synthesis of LZU-191 and (b) model photooxidation of methylphenylsulfide. Reproduced with permission from ref. 198. Copyright 2022, the American Chemical Society.

In continuation of studies on the photo-oxidation of sulfides to sulfoxides, Li et al. crafted a photosensitive COF by strategically integrating photoactive triphenylamine moieties into its architecture. The EPR (electron paramagnetic resonance) studies demonstrated the effectiveness of the COF as a photocatalyst for generating ROS, specifically the superoxide radical anion (O₂˙⁻), featuring one unpaired electron.¹⁹⁹ This underscores the potential of COFs as promising photocatalysts for ROS-involved reactions. Trenker and co-workers synthesized FEAx-COF, a groundbreaking photocatalyst inspired by natural flavin cofactors. This work showed the successful incorporation of alloxazine chromophores into the COF backbone, preserving their functionality. This enabled the efficient oxidation of benzylic alcohols to aldehydes under low-energy visible light. It displayed superior versatility across various solvents compared to molecular alloxazines. FEAx-COF can act as a promising and sustainable choice for heterogeneous photocatalysis due to its simple and eco-friendly conversion of organic substrates.²⁰⁰

Lang and co-workers prepared an sp² carbon-conjugated porphyrin-based COF, Por-sp²c-COF, using porphyrin and PDAN as building units. The porphyrin macrocycle in COF forms an extended π-conjugation network, which leads to strong light absorption, while the vinyl linkage ensures the stability of the COF in high concentrations of amine. Por-sp²c-COF with DMPO (5,5-dimethyl-1-pyrroline N-oxide) was explored for the oxidative coupling of amines under light illumination. It readily converted the amine into imine in up to 99% yield within 15–20 min. The cooperation of DMPO with porphyrin established efficient charge separation and migration of photogenerated electrons, resulting in high photocatalytic activity.²⁰¹

Further, COFs were also explored for reduction and coupling reactions. In 2019, Liu and co-workers explored a donor–acceptor COF having pyrene and benzothiadiazole units in its structure for the photocatalytic reductive dehalogenation of phenacyl bromide. COF-JLU22 was synthesized via the Schiff base condensation between amino-functionalized pyrene and formyl-containing benzothiadiazole moieties. It exhibited wide absorption ranging from the visible region to NIR region and possessed a large surface area (954 m² g⁻¹) and pore volume (1.03 cm³ g⁻¹). Upon light irradiation, COF-JLU22 facilitated the effective reductive dehalogenation of phenacyl bromide derivatives, yielding >99% conversion. Its high photocatalytic efficiency can be rationalized in terms of its high crystallinity, porosity, excellent stability, and donor–acceptor structure forming extended π-conjugation, resulting in a broader absorption region and unique photoelectric properties. This work highlights that metal-free COFs can be strategically designed for less feasible reduction reactions.²⁰²

An innovative approach was reported for the synthesis of photoactive COFs as effective heterogeneous photocatalysts for visible-light-driven organic transformations. This method exploited the photocatalytic activity of hydrazone-based 2D-COF-1 in generating molecular oxygen, facilitating the selective aerobic oxidation of several small organic molecules. 2D-COF-1 demonstrated high photo-efficiency and functional group tolerance in the synthesis of various compounds, including heterocyclic compounds, quinolones, sulfoxides, and amides. Noteworthy achievements in large-scale reactions include the selective production of key compounds such as the drug molecule modafinil and an oxidized mustard gas simulant (2-chloroethyl ethyl sulfoxide).²⁰³ Porphyrin-based COFs are highly interesting due to their robust nature and high porosity. A novel porphyrin-based COF (TA-Por-sp²-COF) was developed through a self-polymerization reaction. The porphyrin units with two cyano groups at the opposite peripheral positions formed a highly organized crystalline COF structure. The presence of electron-donating porphyrin and electron-withdrawing triazine units led to efficient photo-induced carrier separation, while the ample porosity of the system facilitated mass transportation. This hyper-conjugated system exhibited outstanding photocatalytic activity toward various organic transformations, particularly excelling in the aerobic coupling of benzylamine and the selective oxidation of thioanisole.²⁰⁴

A donor–acceptor-based COF was developed using the phenothiazine (PTZ) and triazine (TTA) moieties, which served as the donor and acceptor units, respectively. Unlike its triphenylamine (TPA)-based counterpart, which had a smaller contrast in donor–acceptor properties, PTZ–TTA-COF exhibited a reduced exciton binding energy and superior charge separation/transfer. Consequently, it exhibited markedly improved photocatalytic efficacy under light irradiation in the presence of air, specifically in facilitating amine coupling reactions and cyclization of thioamide to 1,2,4-thiadiazole.²⁰⁵ Similarly, Feng et al. synthesized TpTt-COF using triformylphloroglucinol (Tp) and triazine (Tt) moieties to perform the photo-oxidative coupling of amines in the presence of air within 1 h.²⁰⁶ Moreover, Li et al. synthesized a donor–acceptor COF with the benzothiadiazole unit, which could generate super-oxide radical anions under visible-light irradiation. This COF showed the capability to efficiently photocatalyze the oxidative coupling of amines and the cyclization of thioamide, resulting in the formation of 1,2,4-thiadiazole with yields ranging from moderate to high.²⁰⁷

5.4.2 Hybrid COFs. The photocatalytic activity of COFs can be enhanced by appending metals as active sites in their pore wall. Metal-anchored COFs can facilitate charge separation and the transfer of electron–hole pairs, leading to a high photocatalytic performance for organic conversion. Chen et al. reported a green approach for the synthesis of a Cu-doped COF, TPT-COF-Cu, which was further employed for the photocatalysis of H-phosphonates with terminal alkynes at room temperature. The COF material acted as the photocatalyst, while the copper site served as the active site for the C–P cross-coupling reaction. The TPT-COF-Cu catalyst showed excellent catalytic activity with a high recyclable rate and stability.²⁰⁸

In another investigation, Jati et al. introduced a dual metalation strategy in TpBpy COF for photocatalytic C–N cross-coupling reactions. They inserted iridium and nickel metal into the COF structure using [Ir(ppy)₂(CH₃CN)₂]PF₆ [ppy = 2-phenylpyridine] and NiCl₂ as metal precursors, respectively (Fig. 16a). The attachment of iridium units to the COF pore walls resulted in a synergistic enhancement in photosensitization for dual photocatalysis. Further optimization occurred by appending nickel units in the COF structure. The Ni–Ir@TpBpy catalyst showed impressive catalytic activity and stability, substantially boosting its reusability. The simulated study revealed that the intramolecular electron transfer occurred from the Ir-site to Ni-site within the COF system. The developed Ni–Ir@TpBpy photocatalyst efficiently catalyzed coupling reactions involving sulfonamides, amines (aryl, heteroaryl, and alkyl), and carbamides with neutral, electron-rich, and electron-poor (hetero)aryl iodides, respectively, yielding up to 94% isolated yield (Fig. 16b).²⁰⁹

Fig. 16 (a) Schematic of the synthesis of Ni–Ir@Tp-Bpy and coordination of Ni complexes in TpBpy COF and (b) diagram of different substrates utilized for the C–N cross-coupling strategy. Reproduced with permission from ref. 209. Copyright 2022, the American Chemical Society.

In another aspect of synthesizing a hybrid COF structures for photocatalytic organic reactions, Yang and his group reported the synthesized two heterostructures, namely QH-COF@TiO₂ and TiO₂@QH-COF, with a focus on the photocatalytic oxidation of alcohols. This study marks a significant advancement by meticulously orchestrating the spatial arrangement of two semiconductors, QH-COF (tetrahydroquinoline-linked COFs), either in the core or on the shell and thoroughly evaluating their respective photocatalytic performances. In the realm of benzyl alcohol oxidation, QH-COF@TiO₂ exhibited an outstanding threefold increase in activity compared to TiO₂@QH-COF, boasting a reaction rate of 1.19 vs. 0.44 mmol g⁻¹ h⁻¹ (Fig. 17). Interestingly, despite their similar capabilities in light harvesting and charge separation, the superior performance of QH-COF@TiO₂ is attributed to its heightened electron donation to O₂. This pattern continues in the realm of visible-light photocatalytic aerobic cross-dehydrogenative coupling reactions, underscoring the effectiveness of spatially engineered semiconductor heterojunctions, where graphitic carbon nitride (g-C₃N₄) containing a heptazine moiety showed excellent photocatalytic activity.²¹⁰

Fig. 17 Formation of QH-COF@TiO₂ and its utilization in photocatalytic organic conversions. Reproduced from ref. 210 with permission from the Royal Society of Chemistry.

Furthermore, Chen et al. synthesized a heptazine-based COF to increase the photocatalytic activity of the COF due to the presence of the heptazine unit. Two distinct N-rich COFs, namely HEP-TAPT-COF and HEP-TAPB-COF, were obtained by reacting the 4-(methylenediacetate)phenyl substituted heptazine monomer (HEP-OAc) with two C₃-symmetric aromatic amines. HEP-TAPT-COF significantly improved the benzylic C–H oxidation and selective sulfoxidation compared to pristine g-C₃N₄.²¹¹ In another investigation, Li et al. employed interlinked rigid macrocyclic struts, particularly pillar arenes in different proportions featuring electron-rich cavities. This arrangement established a restricted molecular space, facilitating exciton migration and streamlined carrier transport. Consequently, novel interfaces were formed, engaging effectively with photogenerated charge carriers and showing outstanding efficacy in catalyzing the oxidation of amines to imines.²¹²

5.5. Photocatalytic degradation of organic pollutants

Presently, the rapid industrialization has generated many environmental problems. One of the main problems is the increasing amount of industrial waste being released into the environment, which is harmful to human health and the ecosystem. Industrial waste contains organic pollutants, heavy metals, and antibiotics, which have to be eliminated immediately. The major organic pollutants are phenol, methylene blue, methylene orange, tetracycline, rhodamine B, chlorinated biphenyl, etc. Photocatalytic degradation of organic pollutants presents simple, economical, and eco-friendly technology to remove them from the environment. The mechanism for the photocatalytic degradation of pollutants involves the generation of hydroxyl and superoxide radicals via photogenerated electrons, which react with organic compounds and degrade them. In this case, COFs have emerged as potential photocatalysts for the degradation of pollutants and many COFs have been applied in the photocatalytic degradation of pollutants.

5.5.1 Free-standing COFs. Firstly, Giesy et al. developed an enduring and eco-friendly COF material specifically crafted for the photocatalytic degradation of organic pollutants. The synthesis of TpMA COF involved a mechanochemical reaction between triformylphloroglucinol (Tp) and melamine (MA), conducted through the ball-milling technique. The TpMa COF displayed higher photocatalytic properties compared to other COFs, enabling 83.5% and 89% degradation of phenol and methyl orange within 60 min, respectively. The TpMA COF was highly stable after four cycles, showing 87.6% retention in activity compared to the initial COF. The possible photocatalytic mechanism for degradation involves the formation of the O₂ radicals by photogenerated electrons upon visible light irradiation. The O₂ radicals oxidize water molecules to form active OH radicals, which participate in the photodegradation of organic contaminates.²¹³

Further, Qiu and his group synthesized three vinylene-linked COFs, BDA-TMT, EDATMT, and TDA-TMT, for environmental application, which were prepared via the aldol condensation reaction of four different triazine moieties (Fig. 18a). These vinylene-linked COFs possessed highly crystalline structures and large porous surfaces following the order of TDA-TMT < EDA-TMT < BDA-TMT. The valence band (VB) positions of these three COFs appeared at 1.15, 1.37, and 1.45 V, respectively. The lower oxidation potential of BDA-TMT generated a suitable band gap for photocatalytic application. These three COFs were investigated for the photocatalytic degradation of various organic pollutants such as methylene blue, phenol, and norfloxacin. BDA-TMT exhibited excellent photocatalytic activity, resulting in the degradation of organic pollutants up to 96% within 15 min, which was much faster than the other COF (Fig. 18b–d, respectively). The photocatalytic degradation of phenol and norfloxacin reached up to 100 and 96%, respectively, achieving an ultrafast photocatalytic degradation efficiency. Additionally, BMD-TMT COF also possessed higher antibacterial activity under light irradiation. The high photocatalytic performance of BDA-COF can be rationalized in terms of its effective π-conjugation delocalization due to the presence of diacetylene moieties, leading to an optimized band gap and efficient charge separation of photogenerated electrons.²¹⁴

Fig. 18 (a) Method for the synthesis of BDA-TMT, EDA-TMT, and TDA-TMT COFs. (b) Absorption changes in MB aqueous solution upon the addition of BDA-TMT. (c) Reaction kinetic curves of all COFs for the degradation of MB and (d) colour change upon the degradation of MB aqueous solution in the presence of BDA-TMT with time. Reproduced with permission from ref. 214. Copyright 2021, the American Chemical Society.

Similarly, Ma and his group developed five different COFs and tuned their photocatalytic properties by modulating the number and position of nitrogen atoms on p-phenylenediamine as a building block. These five COFs were named COF-1 (without N), COF-PD (one N), COF-PZ (two N at 1,5 position), COF-PMD (two N at 1,6 position), and COF-PDZ (two N at 1,3 position) with respect to p-phenylenediamine. The assessment covered three distinct photocatalytic applications of Cr(VI) photoreduction, Escherichia coli inactivation, and degradation of paracetamol across all COFs subjected to testing. Remarkably, COF-PDZ demonstrated exceptional efficacy in Cr(VI) reduction, achieving an impressive 95% removal within a mere 20-minute timeframe. Conversely, the other COFs exhibited removal rates varying from 49% to 99% over a 120-minute duration. Further, COF-1 and COF-PD did not show any activity for bacterial disinfection, while COF-PZ and COF-PMD exhibited weak capacity, deactivating ∼0.97 log CFU mL⁻¹ of cells within 90 min under visible-light irradiation. In contrast, COF-PDZ showed complete deactivation of 6.2 log CFU mL⁻¹ with the same time, achieving the best antibacterial property. COF-1 and COF-PD also showed low activity for the photocatalytic degradation, resulting in 26% and 68% degradation of paracetamol over 60 min, while the other COFs gave 100% photodegradation rate for paracetamol within 30 min. These results suggest that the number and position of heterocyclic N atoms in the COF structure have a great effect on the photocatalytic properties of COFs. The Cr(VI) reduction mechanism suggests that high adsorption and quantum efficiency play an important role in effective photocatalytic performances. This work rationalized the structure–property relation for the formation of COFs as photocatalysts.²¹⁵

Cui and co-workers designed two pyrene-based covalent organic frameworks, sp²c-COF and Py-NH₂–COF, which were synthesized via the Schiff base condensation of a pyrene precursor with 1,4-phenylene diacetonitrile and 1,4-p-phenylenediamine organic moieties respectively, for the photocatalytic degradation of tetracycline hydrochloride (TC) in water. Different types of linkages can vary the conjugation in the COF structure, which may have an impact on their photocatalytic performances. The photocatalytic experiments for the COFs under visible light evidenced that sp²c-COF was the better photocatalyst compared to Py-NH₂-COF. Sp²c-COF degraded 73.5% of initial TC within 90 min, while Py-NH₂-COF showed the negligible efficiency of 3.1% under visible light. The XPS and absorption data revealed that sp²c-COF showed a narrow band gap of 1.98 eV compared to 2.28 eV of Py-NH₂-COF, resulting in a high charge transport efficiency. Sp²c-COF also had a high photocurrent efficiency, which indicates the efficient charge separation of photogenerated electrons, leading to great photocatalytic performance. Additionally, sp²c-COF possessed good recyclability, exhibiting 70.3% degradation efficiency after a five-cycle run. This work implied the importance of the type of linkage in the formation of COFs given that the COF with C=C bonds achieved strong light absorption, high catalytic sites, and improved charge separation and migration rate in comparison to the COF with C=N bonds, which are beneficial for the photo-degradation of tetracycline.²¹⁶ It is expected that the donor–acceptor COF system generates intermolecular charge transfer, which can improve the photocatalytic efficiency.

Considering this, Tong et al. synthesized two donor–acceptor COFs, COF-TD1 and COF-TD2, via the solvothermal reaction of thiadiazole and quinine-based organic moieties with triformylphloroglucinol and investigated their photocatalytic properties for the removal of paracetamol. In photocatalytic degradation under visible light, COF-TD1 displayed better efficiency by degrading >98% of paracetamol in 60 min, while COF-TD2 only managed 59.3% degradation of paracetamol over 120 min. Due to the electron-donating nature of the thiadiazole ring, effective intermolecular transfer occurs, generating a strong push–pull effect within a COF system, which leads to the efficient electron–hole separation and high photocatalytic efficiency observed for COF-TD1 compared to COF-TD2. Remarkably, COF-TD1 also showed efficient photocatalytic degradation of other water pollutants such as diclofenac, bisphenol A, naproxen, and tetracycline hydrochloride and it can be utilized for degradation of paracetamol in complicated water obtain from river, lake water, and sewage. Importantly, COF-TD1 can be used in a powder form or immobilized onto a glass slide for the efficient removal of paracetamol to achieve large-scale application in a scaled-up reactor under sunlight irradiation.²¹⁷

A. Bhaumik and his group developed a new metal-free COF for the efficient removal of organic pollutants from contaminated water. They prepared a donor–acceptor COF, C6-TRZ-TPA, using the triphenylamine moiety as the electron donor and triazine moiety as the electron acceptor building blocks. This COF possessed a highly crystalline structure due to the effective π-stacking of the triazine units and a large surface area of 1058 m² g⁻¹ with a pore size of 0.73 cc g⁻¹. The optical absorption profile of the COF had a broader absorption region from 400 to 800 nm, depicting its wide visible light absorption capacity. Having a donor–acceptor motif, C6-TRZ-TPA possessed an extensive π-conjugation system and planar triazine moieties with an abundance of nitrogen heteroatoms, which generate effective charge separation for photogenerated electrons with a smaller band gap of 2.2 eV. Leveraging its remarkable photophysical properties, C6-TRZ-TPA demonstrated high efficacy in the photocatalytic degradation of organic pollutants. The investigation focused on the photodegradation of model pollutants, namely Rose Bengal (RB) and methylene blue (MB), due to their considerable toxicity, health hazards, and bioaccumulative tendencies. The C6-TRZ-TPA COF catalyst displayed outstanding catalytic efficiency, achieving a degradation rate of over 97% for both Rose Bengal (RB) and methylene blue (MB) within 80 and 120 min under light irradiation, respectively. Notably, the C6-TRZ-TPA COF also showed the adsorption of 99% radioactive iodine from iodine/cyclohexane solution in a period of 24 h. Due to its large surface area, it demonstrated a high iodine uptake capacity of 4832 mg g⁻¹. This work explored the structure–property relation for the synthesis of crystalline and porous COFs for photocatalytic application.²¹⁸

Zhao reported a simple and efficient method to synthesize ultrathin 2D CONs for the photocatalytic degradation of organic contaminants. This new approach for the formation of COFs was carried out by the ultrasound treatment of monomers in a solvent system for longer hours to achieve gram-scale synthesis. Two different CONs, P-CON and T-CON, were prepared by applying triphenylamine as the core moiety and varying the triformylbenzene and triazine moieties as benzenetricarbaldehyde, respectively. Although the BET surface area of P-CON and T-CON was reduced compared to their crystalline COF counterparts, these ultrathin 2D CONs have great potential for photocatalytic activity because of the large number of accessible active sites within their porous surface, enabling the greater participation of reactants in catalytic reactions. This was further proven by the investigation of the photocatalytic activity of P-CONs and T-CONs for the degradation of methylene blue in aqueous solution. P-CON and T-CON achieved 96.2% and 99.8% degradation efficiency toward MB in 90 min, which were much higher than that of their P-COF and T-COF counterparts, respectively.²¹⁹

Similarly, Cai and co-workers designed a Z-scheme-based 2D/2D CON heterojunctions system for the photocatalytic degradation of antibiotics. Firstly, the olefin-linked pyrene-based PTO-COF and triazine unit containing TpMa-COF were synthesized and further developed into covalently linked 2D/2D CONs by connecting with remaining linker groups via a mechanochemical grinding method. 2D/2D CONs exhibited improved absorption and photophysical properties compared to their parent COFs. The construction of a Z-scheme heterojunction system in CONs enhanced their charge separation and conduction efficiency, which led to increased photocatalytic performances. 2D/2D CONs were evaluated for the degradation of the sulfamethazine antibiotic under visible light, where CONs displayed 98.4% degradation of sulfamethazine within 50 min, which was almost double that of PTO-CON and TpMa-CON. Thus, the above-mentioned work establishes an approach for the facile and scalable synthesis of COF nanosheets as photocatalysts for pollutant degradation and paves the way for practical applications.²²⁰

5.5.2 Hybrid COFs. Hybrid COFs synthesized by combining COFs with other materials have also been investigated for photocatalytic dye degradation. In this regard, Weng et al. developed a bipyridine-containing sp²-carbon-linked COF and further incorporated Pd nanoparticles in it to enhance its activity for the degradation of tetracycline hydrochloride (TC). The coordination of palladium nanoparticles improved its light-harvesting ability and tunable band gap due to the efficient charge transfer between the metal and COFs. Upon the photocatalytic degradation of tetracycline hydrochloride, Pd-COFs removed 93.45% of TC in 80 min, which was 63-fold higher than that of the pristine COFs. The efficiency is significant in comparison to that by Co-COF and Au-COF, which degraded only 23.46% and 46.21% of TC within the same time, respectively. Notably, Pd-COFs have high stability, enabling 98.5% removal efficiency after a continuous 8-cycle run.²²¹

Qiu prepared a silver-doped COF, Ag@TAPP-TEPT, for the selective degradation of sulfur mustard [bis(2-chloroethyl)]sulfide, named HD. TAPP-TFPT COF was constructed via the Schiff base reaction of 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (TAPP) and 2,4,6-tris(4-formyl phenyl)-1,3,5-triazine (TFPT) and modified by the deposition of silver nanoparticles on COF via a simple post-synthetic treatment. TAPP-TFPT COF possessed high crystallinity and the SEM and HR-TEM data revealed the uniform dispersion of the Ag NPs on the COF surface. Ag@TAPP-TFPT successfully degraded HD by oxidizing CEES into 2-chloroethyl sulfone under simulated sunlight in an O₂ environment. This COF formed a non-toxic sulphoxide product of CEESO and avoided the formation of toxic CEESO₂ products by over-oxidation. Due to the enhanced separation of charge carriers and the effective energy transfer/electron migration process from Ag@TAPP–TFPT to oxygen, visible light irradiation enabled the selective degradation of CEES into CEESO without the formation of harmful oxidants, leading to the efficient production of ¹O₂ and O₂˙⁻. The photocatalytic studies suggested that Ag coordination improved the charge carrier transportation to oxygen, forming the ¹O₂ and O₂˙⁻ intermediates, which selectively transform CEES into CEESO without the formation of any toxic product. This work demonstrates that the metal coordination in COFs is beneficial for the selective and efficient degradation of organic contaminants.²²²

Khaing and co-workers developed a heterojunction 2D–2D COF composite as a photocatalyst for the degradation of organic pollutants. TaPa-1-COF was fabricated using molybdenum sulfide (MoS₂) via a facile hydrothermal method. The X-ray analysis depicted peaks corresponding to MOS₂ and COF, indicating the successful formation of the MoS₂/COF composite. The MoS₂/COF composite demonstrated superior efficiency compared to its parent MoS₂ and COF, displaying the degradation of RhB and TC of about 98% and 85.9% under simulated sunlight irradiation in a period of 30 and 60 min, respectively. The enhancement in the photocatalytic activity of the COF composites is attributed to the formation of a 2D–2D heterostructure, which increases its absorption in the visible region and surface area and facilitates the separation and transfer of excitons upon solar illumination. This work indicated that the integration of inorganic substances in COFs in the form of a single composite can be an effective strategy for developing highly efficient photocatalysts.²²³

Inspired by this, a novel hybrid photocatalyst, Cu₂O-ACOF-1@Pd, was developed by embedding an azine-based COF in Cu₂O cubes and further anchoring Pd nanoparticles on it. A-COF was synthesized via the condensation reaction of 1,3,5-triformylbenzene and hydrazine. In the hybrid material, the COF provides a stable, crystalline, and porous surface, Cu₂O has semiconductor properties, enhancing light absorption, and Pd nanoparticles can enhance the charge separation of photoexcited electron–hole pairs, reducing the charge recombination process. Thus, their combined synergistic effect in the hybrid material can result in superior photocatalytic performances. The light-induced degradation of chlorinated biphenyls was examined using a hybrid COF, with a focus on three distinct monochlorinated PCB (polychlorinated biphenyl) congeners, where chlorine was positioned at the ortho, meta, or para positions. Cu₂O-ACOF-1@Pd showed a better photocatalytic performance than A-COF, Cu₂O-ACOF-1, and ACOF1@Pd, resulting in the complete degradation of chlorinated biphenyl in a period of 30–40 min. The photocatalytic activity of all the tested photocatalysts followed the order of Cu₂O-ACOF-1@Pd > Cu₂O-ACOF-1 > ACOF1@Pd > ACOF-1. The difference in photocatalytic performance indicated that Cu₂O and Pd nanoparticles are important for the photocatalytic degradation of PCB and their combination with COF achieved the rapid decomposition of PCB.²²⁴

Xu and co-workers prepared a Fe₃O₄-decorated MOF/COF hybrid with magnetic properties for photocatalytic dye degradation from polluted water. To prepare the Fe₃O₄@MOF_UIO-66@TzDa-COF, Fe₃O₄ nanoparticles were decorated on UIO-66-MOF to form Fe₃O₄@MOF_UIO-66, which was further treated with the precursor of TzDa-COF under solvothermal conditions to grow the COF on the modified surface (Fig. 19). The MOF and COF possessed a highly crystalline structure with a large porous surface area, excellent stability, and photocatalytic properties. The structure of the MOF/COF hybrid revealed a uniform layer of COF dispersed on the surface of Fe₃O₄@MOF_UIO-66@TzDa-COF, which was stable under different pH conditions. The hybrid material had a specific BET surface area of 4144 m² g⁻¹ with a pore volume of 1.2186 cm³ g⁻¹, and thus high adsorption of dyes can be achieved for a fast degradation process. This study explored the photocatalytic degradation of various dyes using Fe₃O₄@MOF_UIO-66@TzDa-COF, revealing its effectiveness in rapidly degrading anionic dyes, particularly malachite green (MG) and Congo red (CR). The hybrid completely degraded MG and CR dyes within 10–120 min irrespective of their concentration and the removal rate was found to be approx. 97–99% (Fig. 19). Additionally, it also enabled the degradation of methylene blue and methyl orange dyes with a removal rate of 73% and 97%, respectively. This work can inspire the construction of hybrid photocatalysts with increased magnetic and functionality separability by combining the properties of COF, MOF, and nanoparticles.²²⁵

Fig. 19 Schematic representation of Fe₃O₄@MOF_UIO-66@TzDa-COFs and photocatalytic dye degradation of methylene blue and Congo red. Reproduced with permission from ref. 225. Copyright 2020, the American Chemical Society.

Furthermore, Xue and his group fabricated carbon nanotubes (CNTs) on the surface of a COF to form a COF/CNT membrane to enhance their photocatalytic application for dye degradation. TpBD COF was grown on the carbon nanotubes via in situ treatment. The CNT/COF membrane emerged as a hybrid photocatalyst, which has a large surface area and photocatalytic properties. The contact angle of the COF/CNT surface decreased to 54° with the formation of the COF layer, which was lower than the initial 126° for the CNT membrane. The hydrophilic nature of the CNT membrane was significantly enhanced, improving the adsorption of water during photocatalysis. Consequently, the COF/CNT hybrid achieved an excellent degradation efficiency against mordant black 17 (MB17), exhibiting a total degradation capacity of 708.2 mg g⁻¹. Notably, the mechanical properties of CNTs significantly increased, and the efficiency of COF/CNT decreased only 10.6% in degradation capacity after seven cycles. This work employed the benefits of COF and CNT integration and described a new strategy to prepare hybrid materials for dye degradation application.²²⁶

6. Summary

In summary, this review focused on the recent advancements in COFs as photocatalysts for various photocatalytic applications. COFs have been widely investigated as promising photocatalyst candidates due to their highly crystalline structure, porosity, light absorption ability, and excellent stability. The structure of COFs can be predesigned by tuning the topology of their organic building blocks, linkage chemistry, and functionalization. The topology of COFs can be modulated by varying their precursor organic building blocks. The topology of COFs is the best way to predetermine their morphology and pore channel networks. The pore size and shape of COFs can be modulated by using different topological designs. The possibilities of using different types of organic monomers for the development of COFs have been investigated extensively. However, there are many combinations of organic precursors in the form of knots and linkers that have to be explored to gain topological advantages for photo-based applications.

Subsequently, crystallinity is associated with strong absorption, large surface area, and efficient charge separation, improving the photocatalytic performance. Long-range ordered COFs possess superior structural and photoelectrical properties compared to their amorphous polymer. The challenge of obtaining crystalline COFs can be overcome by applying the solvothermal method for their synthesis. Solvothermal synthesis yields COFs with controlled dehydration and slow diffusion of monomers, leading to a highly crystalline structure. However, the solvothermal method has many drawbacks such as harsh reaction conditions, long reaction time, and low yield, which need to be overcome. Furthermore, several other synthetic methods such as microwave synthesis and room temperature synthesis have been developed but they produce COFs with low crystallinity and porosity. Thus, there is still scope for developing facile and cost-effective synthetic methods for highly crystalline COFs with large scalability.

In addition, the stability of COFs is a major concern for photo-based applications. Stable COFs are not only capable of avoiding photo-corrosion and degradation under different reaction conditions but are also necessary for long-term stability and large-scale application. COFs with β-ketoenamine, imine, and azine linkages have been explored for photocatalytic activity considering their facile synthesis and highly stable nature. It has also been revealed that the covalent linkage between organic units in COFs dictates their thermal and chemical stability under different conditions. In this regard, olefin-linked COFs display feasible delocalization of π-electrons through the –C=C– bond, forming a strong conjugated COF network with high crystallinity.

Moreover, COFs must have broad absorption in the visible region to develop effective light harvesting ability, enhancing their photocatalytic efficiency. The utilization of highly π-conjugated aromatic systems as building blocks generates high photon absorption capacity in the visible region. Additionally, the fabrication of COFs with photoactive materials can also increase their light-harvesting in the broad range of UV (ultraviolet) to NIR (near infrared) regions. Simultaneously, the availability of a large number of catalytic sites maximizes the photocatalytic performances, and thus the high porosity of COFs becomes an important factor. The high porous surface of COFs enhances their adsorption capacity and provides numerous active sites for catalytic reactions, increasing their photocatalytic activity. Microporous COFs have small pore sizes and large surface areas and can perform efficient photocatalysis for light-driven water splitting, CO₂ reduction/conversion, etc. On the contrary, large aromatic system-based organic monomers form mesoporous COFs, demonstrating high molecular adsorption for trapping large molecules, enabling efficient organic transformations or dye degradation. It has been shown that the bigger or longer the building units, the larger the pore size and shape, subsequently enhancing the surface area.

Furthermore, band gap engineering, charge separation efficiency, and high charge migration rate are key factors to improve the photocatalytic efficiency of COFs. COFs with narrow bandgaps are preferred as photocatalysts due to their increased light absorption, resulting in the photo-excitation of more electrons for catalytic activity. The bandgap of COFs can be engineered by employing highly aromatic π-conjugated organic units, functional groups, and extended conjugation in the COF. The incorporation of metal atoms or metal complexes also reduces the HOMO–LUMO gap and activation energy of COFs for photocatalytic reactions. Accordingly, the appropriate band structure of COFs also controls the charge separation and migration mechanism of photoexcited electron–hole pairs, which leads to higher efficiency. In this context, several donor–acceptor-based COF systems have been developed, where electron-rich and electron-withdrawing moieties are aligned perfectly in the crystalline COF structures. The charge transfer from donor to acceptor moieties in the COF network contributes to effective charge separation. Many donor–acceptor systems were briefly discussed, where electron-rich moieties such as pyrene, triphenylamine, triazine, and porphyrin are applied as donors, while thiazole, benzothiadiazole, and halogen-containing organic moieties serve as the acceptor. Also, the presence of extended conjugation in the COF network through organic monomers is another strategy to enhance the charge separation and migration of photogenerated electrons. Additionally, the formation of a heterojunction system by incorporating COFs with other materials that consist of the required optoelectronic properties can also improve the charge separation and mass transportation of charge carriers by inhibiting the charge recombination process. In this regard, many hybrid COFs have been developed by the in situ incorporation of metal oxides, inorganic compounds, carbon-based materials, metal–organic frameworks, etc.

The judicious choice of organic building blocks for the structural design of COFs can play a decisive role in developing COF-based photocatalysts with high crystallinity, porosity, light harvesting ability, optimal band gap tuning, efficient charge separation, and migration for effective photocatalytic activity. Although the crystallinity and stability of COFs depend on their synthetic process and linkage chemistry, large π-array organic units are more effective in forming long columnar structures due to the π–π interaction between them and enhance the COF network crystallinity. Besides, organic moieties with planar π-conjugation systems produce photoactive properties, lower the band gap, and facilitate charge transportation in COFs, thus developing robust COFs for photocatalytic activity. In addition, donor–acceptor COFs have been studied for several photo-based applications such as CO₂ conversion, HER and OER. In the case of CO₂ photoreduction, organic monomers consisting of active sites for metal coordination such as bipyridine units have great importance given that single metal atoms/metal complexes can readily be incorporated in COF systems to enhance their efficiency. Current research revealed that COFs consisting of triformylphloroglucinol, triazine, diethoxy-terephthalohydrazide, and sulphone moieties as building blocks have shown great promise in terms of efficiency, photostability, and long-term durability for photocatalysis.

Finally, COFs as photocatalysts have been widely investigated within the last few years and shown great improvement but their large-scale application is still far away. The methods for the synthesis of COFs need to be optimized at least to the gram-scale level for large-scale application. The in-depth analysis of fundamentals of COF-based photocatalytic systems can lead to strategically design COFs with high crystallinity, porous surface, appropriate band gap, and effective charge separation to improve their photocatalytic performance. Regarding H₂ evolution and CO₂ reduction, the dependence on noble metal catalysts or co-catalysts must be reduced to develop cost-effective COF photocatalysts. Further, the development of overall water-splitting photocatalytic systems or artificial photosynthetic systems based on COFs needs to be encouraged for efficient H₂ or O₂ evolution with CO₂ reduction. Regarding light-driven organic transformations and degradation, COFs need to be functionalized to be highly porous and more selective toward organic transformations or the degradation of organic contamination. Overall, the development of COFs as photocatalysts with ideal properties has great potential for solar-driven applications, and the progress in this field will lead to a green and sustainable economy.

Data availability

The data generated and/or analyses during in this review are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

S. M. M. sincerely thanks SERB-DST, New Delhi, India (Project CRG/2020/001769), BRNS, Mumbai, India (Project 58/14/17/2020-BRNS/37215), and Ministry of Textiles {Project 2/1/2021-NTTM(Pt.)}, India for financial assessment. K. P. thanks Science and Engineering Board (SERB), India for National Postdoctoral Fellowship (PDF/2022/000110). R. D. thanks UGC, New Delhi for the fellowship.

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