Immobilization of metal active centers in reticular framework materials for photocatalytic energy conversion

Abstract

To enhance the economy and society's sustainability, prioritizing innovative, green energy conversion methods is essential. The conversion of solar energy into chemical energy through photocatalysis represents a viable solution in the pursuit of sustainable energy resources despite low solar-to-energy conversion efficiency. In recent years, the immobilization of metal active centers in nanospace has been considered an important strategy to improve photocatalytic performance. Metal–organic frameworks (MOFs), covalent organic frameworks (COFs) and hydrogen-bonded organic frameworks (HOFs), as exciting reticular framework materials, have attracted much attention due to their advantages of large specific surface area, high porosity and tunable functionalization sites, which are considered promising carriers for immobilizing metal sites. This paper introduces the structural features and advantages of each framework in photocatalysis, highlighting their potential applications in water splitting (H₂ or O₂ evolution) and CO₂ reduction. The review underscores the outstanding performance of metal-active sites within these frameworks. Finally, the current limitations and challenges of the reticular framework materials in photocatalysis are pointed out, and an outlook for future development is provided, which aims to inspire continued advancements in green energy solutions for a sustainable future.

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Honglei Zhang

Dr Honglei Zhang is an Assistant Professor/Investigator in Renewable Energy & Energy Storage at The Nottingham Ningbo China Beacons of Excellence Research and Innovation Institute (CBI). He completed his PhD in Chemical & Environmental Engineering in 2016, under the mentorship of Professor George Chen and Professor Tao Wu. His research primarily focuses on the synthesis, in situ characterization, and practical application of innovative photo/electrocatalysts for CO2 reduction and hydrogen production, which are critical processes in the field of renewable energy and environmental sustainability. Prior to joining CBI, he was a postdoctoral fellow and subsequently research fellow with the Hong Kong Polytechnic University.

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1. Introduction

Over the past few decades, the conflict between the substantial energy consumption for rapid economic growth and environmental degradation has become increasingly prominent.^1–5 To address the issue and reduce the consumption of non-sustainable energy resources, there has been a growing interest in developing alternative production methods and systems that utilize renewable and sustainable energy sources. Among them, photocatalysis, recognized for its prowess in solar-to-chemical energy conversion, is extensively explored as a solution for energy conservation and environmental pollution mitigation.^6–9 Its advantages, including renewability, cost-effectiveness, and minimal side effects, position it as a promising avenue for addressing these pressing challenges.

Photocatalysts, pivotal in photocatalytic technology, face a daunting challenge in achieving comprehensive control and optimization of performance, spanning efficiency, activity, and selectivity.¹⁰ Photocatalytic efficiency, an important indicator to evaluate their performance, is contingent on the optical properties of photocatalysts. The cornerstone for exceptional photocatalytic performance lies in enhancing quantum efficiency through bandgap structure tuning. Catalytic activity, closely linked to the number of catalytic sites, substrate adsorption, and product desorption, is influenced by morphologies, specific surface areas, and intrinsic or post-modified active centers. Selectivity hinges on the chemical properties, with redox behavior at catalytic sites being crucial. Modulating product type and concentration according to demand is integral to realizing high-value chemicals. Various photocatalytic systems, including traditional inorganic materials like metal oxides and sulfides,^11,12 have been explored for efficient energy conversion. However, inorganic materials face limitations such as wide band gaps, weak light absorption, and poor chemical stability.^13–15 These challenges, coupled with difficulty in rational modulation at the molecular level and dependence on noble metals, restrict the broad application of inorganic materials. Molecular catalysts allow pre-designed organic ligands but often compromise stability for increased activity and selectivity.^16–18 Additionally, controlling the assembly form of molecular catalysts in different solutions poses a significant challenge in studying the conformational relationships of photocatalysts. Hence, there is an urgent need for the discovery and study of a multifunctional photocatalytic material that seamlessly combines various properties.

Reticular chemistry extends molecular principles to construct precise structures through chemical bonds, facilitating the prediction and design of diverse periodically expanding structures. This approach enables the creation of various reticulated materials,^19,20 including discrete solids (metal–organic polyhedra, covalent organic polyhedra) and framework materials (Fig. 1) like metal–organic frameworks (MOFs), covalent organic frameworks (COFs), and hydrogen-bonded organic frameworks (HOFs).^21–23 Specifically, MOFs are a class of crystalline porous skeletal materials with periodic network structures, which are formed by self-assembly of organic ligands and metal ions via coordination bonds. Generally, metal ions act as the connecting points and organic ligands support to form spatial 3D extensions with three-dimensional pore structures. COFs are a new type of crystalline organic porous polymers based on covalent bonding connections, which are usually composed of lightweight elements (C, O, N, B, etc.), while HOFs, as another ordered framework materials, are constructed only by self-assembly of organic building units by non-covalent intermolecular forces (hydrogen bonding, π–π stacking, etc.). Reticular framework materials have emerged as prominent photocatalysts due to their tunable pore sizes and exceptional specific surface areas, enhancing reactivity by enabling substrate diffusion within restricted nano spaces during catalysis. The pre-designable structure further enhances their performance for diverse photocatalytic applications.^24–26 While most reviews focus on the overall design or organic unit importance, there is a limited exploration of the role of metal sites in these frameworks. Modulating original metal sites in MOFs or incorporating metal sites in COFs and HOFs addresses the vulnerability of homogeneous metal molecular catalysts,^27,28 optimizing kinetic and thermodynamic processes for enhanced photocatalytic performance. The robust topology and tailorable pore structure of reticular framework photocatalysts allow effective immobilization of metal active sites, influencing the feasibility and performance of the photocatalytic system.

Fig. 1 Schematic representation of reticular framework materials.

This review focuses on the progress in photocatalytic energy conversion using reticular framework materials (MOFs, COFs, HOFs) as ideal carriers for metal active sites. We will examine the structural features and unique advantages of each framework, emphasizing applications in water splitting (H₂ or O₂ evolution) and CO₂ reduction. The pivotal role of metal sites in influencing the photocatalytic performance will be highlighted. Finally, challenges and opportunities associated with reticular framework materials containing metal sites in the realm of photocatalytic energy conversion will be discussed.

2. Classification of reticular framework materials and respective advantages

2.1 Metal–organic frameworks (MOFs)

MOFs, as a novel periodic reticular framework material, have emerged as ideal candidates for multiphase photocatalysis (Fig. 2a).^29–31 The substantial specific surface area and high porosity of MOFs facilitate the accessibility of active sites, fostering the generation of charge–hole pairs.^32,33 Their structural customizability allows efficient band gap engineering and the incorporation of additional catalytic centers. The confined catalytically active sites within MOFs serve as unique microreactors, enhancing substrate adsorption and activation through host–guest interactions.^34,35 Furthermore, MOFs organize reactants strategically, minimizing entropy loss and reducing transition state energy, thereby facilitating photocatalytic reactions.^36,37 The periodic order and site isolation of catalytic columns in MOFs eliminate deactivation pathways, extending the functional lifetime of catalytic centers. This characteristic enables a thorough exploration of reaction mechanisms. Currently, efforts focus on improving MOFs' photocatalytic prowess by fine-tuning the type, number, and coordination environment of metal sites, attracting significant scientific attention.

Fig. 2 Structures of MOFs (a), COFs (b) and HOFs (c) constructed by organic monomers.

Typically, there exist two main forms of introducing metal catalyzed active sites in MOFs.³⁸ Firstly, metal-conjugated nodes with unsaturated coordination environments are utilized as the catalytic sites. The metal nodes in MOFs are usually accompanied by coordinated water molecules or other solvent molecules, which can be easily removed and do not disrupt the framework structure of the MOFs, allowing the accessible unsaturated metal centers that can be used as catalytic sites for reactions with substrate molecules. Secondly, the metal ions in the inorganic clusters can be replaced by different metal ions without compromising the structure. Thus, some MOFs can contain several kinds of metal ions, which can provide opportunities for versatility and tailoring the properties of the materials to specific applications. The possible synergistic effect due to the presence of multiple metals facilitates the realization of tandem or cascade reactions in the field of multiphase catalysis, making them suitable for a wider range of applications.

2.2 Covalent organic frameworks (COFs)

COFs, a subset of reticular organic framework materials, arise from the integration of organic building units linked by covalent bonds (Fig. 2b),^39–42 mirroring biopolymer formation. Non-covalent interactions, predominantly hydrogen bonding, and hydrophobic interactions, complement covalent bonds formed through polycondensation reactions. The distinctive fusion of covalent and noncovalent interactions in COFs sets them apart from other polymers and porous materials, offering a unique structure, properties, and functionality.^43,44 This distinctiveness positions COFs as an appealing platform across various fields. Notably, COFs feature pre-designed framework structures and inherent pore sizes, expanding their design versatility. Specifically, COFs excel in integrating desired architectural units or active sites into periodically ordered structures, creating permanent pores with atomic precision in sizes, shapes, and environments.^45,46

Since COFs were first reported by Yaghi and co-workers in 2005,⁴⁷ COFs have aroused excitement among researchers over the past nearly two decades. Extensive efforts have focused on COFs, emphasizing structural design, precise synthesis, and specific modifications across various disciplines. With inherent permanent porosity, rigid framework structures, and long-range ordering, COFs emerge as ideal candidates for multiphase catalysis in photocatalytic systems. Their design flexibility enhances properties crucial for photocatalysis, ensuring optimal light absorption, superior electron–hole separation, and efficient transfer. The substantial specific surface area and porous structure facilitate the exposure of catalytic sites and the mass transfer.^48,49 Notably, COFs exhibit remarkable chemical and thermodynamic stability under harsh photocatalytic conditions due to their robust covalent bonding.⁵⁰ These intrinsic properties position COFs with significant potential to rival or even surpass conventional inorganic semiconductors in photocatalytic applications, driving comprehensive exploration within the scientific community.

2.3 Hydrogen-bonded organic frameworks (HOFs)

HOFs, a novel reticular framework material, stand apart from MOFs and COFs as they are stabilized by non-covalent hydrogen bonding (Fig. 2c).^51–53 Their distinguishing features include high crystallinity, sustainability, and utility, attributed to the flexibility and spontaneity of hydrogen bonding. Various topologies and monomer designs contribute to the fabrication of highly crystalline HOFs, enhancing structural analysis accuracy through techniques like powder or single-crystal X-ray diffraction. This exceptional crystallinity facilitates a thorough exploration of conformational relationships in photocatalysis.⁵⁴

The interest in assembling crystalline HOFs using hydrogen bonding was ignited in the 2010s, and several HOFs with permanent pores have subsequently been constructed using hydrogen bonding motifs,^55–57 such as pyridine, diaminotriazine, benzimidazolone, pyrazole, and carboxyl groups. The topology of HOFs is connected by weak hydrogen bonds, which leads to structural instability compared to MOFs and COFs. Indeed, several routes through π–π interactions, electrostatic interactions, and van der Waals force-assisted hydrogen-bonded assemblies have recently been used to fabricate stable HOFs.^58,59 However, weak hydrogen bonding connection also has its unique advantages; for example, HOFs can usually be assembled under milder conditions, which facilitates the confinement of metal units by in situ encapsulation. Moreover, the flexibility and reversibility of hydrogen bonding led to solution processability and simple regeneration of HOFs, which helps to realize their sustainable utilization under practical conditions.

3. Photocatalytic applications

3.1 Photocatalytic hydrogen evolution

Under the context of growing global energy demand, hydrogen (H₂) is an ideal alternative energy source to fossil fuels to achieve sustainable development, thanks to its high energy density (about three times that of gasoline), non-toxicity, renewability, and clean emission product. H₂ is considered an ideal energy carrier for the future, despite the risk of fire in the air and the remaining limitations of safety challenges for its consumption, storage, and transport. Currently, photocatalytic water splitting for H₂ production is one of the most promising strategies.⁶⁰ Semiconductor materials capable of efficiently realizing this process usually have distinct properties, such as a wide range of light absorption, high efficiency of electron–hole pair separation, long lifetime of the photoexcited electrons, minimal potential barriers to photoexcited charge migration, etc. Porous reticular framework materials have received increasing attention due to their ability to provide conventional solid-state platforms with porous structures, consecutive repeating units with few defects, countless active sites, and different metal centers as options. Therefore, such materials have the potential to be effective photocatalysts for H₂ evolution to overcome the limitations of the conventional semiconductors in this field.

3.1.1 MOFs for photocatalytic H₂ evolution. Pristine MOFs with adjustable central metal nodes can achieve the purpose of optimizing photon absorption and catalytic activity.⁶¹ Among them, a variety of pristine MOFs, such as MIL-101(Cr), UiO-66(Zr), and MIL-125(Ti), have already shown remarkable results in the field of photocatalytic H₂ evolution.^62–64 To investigate the conformational relationship in pristine MOFs, Jiang and co-workers prepared indium (In)-based porphyrin MOFs, named USTC-8(in), benefiting from an unprecedented In(OH)₃ precursor controlling the release of metal ions.⁶⁵ The structure is shown in Fig. 3a. In the MOF structure, In³⁺ not only formed In–O bonds but also resulted in a location above the porphyrin plane (out-of-plane, denoted as OOP) instead of entering the cavity in a coplanar manner due to the large size of In³⁺. Intriguingly, USTC-8(in) exhibited excellent photocatalytic H₂ evolution activity under visible light irradiation, which is far superior to that of the homostructured porphyrin MOFs with an in-plane central metal. This can be attributed to the fact that In³⁺ was easily detached from the porphyrin rings under photoexcitation, which avoided the rapid transfer of reverse electrons, and thus greatly improved the electron–hole separation efficiency and photocatalytic performance. In addition, USTC-8(in) also showed excellent stability in aqueous solutions at pH 2–11 due to the formation of strong coordination bonds. Similarly, the metal centers of pristine MOFs affect the energy band structure which can become a key factor in determining the photocatalytic activity. The performance and H₂ evolution rate of Cu-MOFs are several times higher than those of the analogous Cd-MOFs under the same conditions. This is because the conduction band (CB) value of Cu-MOFs is more negative than that of Cd-MOFs, which confers a stronger reduction ability to Cu-MOFs.⁶⁶ Furthermore, the abundant coordination sites of MOFs can provide sufficient space for stabilizing exogenous metals.

Fig. 3 (a) Views of the 3D network of USTC-8(In) and partial structures in USTC-8(In) and USTC-8(M) (M = Cu, Co, Ni). Adapted with permission from ref. 65. Copyright 2018, American Chemical Society. (b) The TEM image and AFM topography of Ni-TBAPy-NB nanobelts. Adapted with permission from ref. 68. Copyright 2022, American Chemical Society. (c) Schematic of the synergistic photocatalytic H₂ evolution process using Pt NP-incorporated MOF frameworks. (d) Diffuse reflectance spectra of 1, Pt@1, 2, and Pt. A photograph of the suspensions of these samples is shown in the inset. (e) Time-dependent H₂ evolution curves of as-prepared samples. Adapted with permission from ref. 70. Copyright 2012, American Chemical Society. (f) TA spectra of UiO-66-NH₂. (g) Time-resolved PL decay profiles for UiO-66-NH₂, Pt@UiO-66-NH₂, and Pt/UiO-66-NH₂, respectively. Adapted with permission from ref. 72. Copyright 2016, Wiley-VCH. (h) Schematic illustration of the structure of Pt/MOF-O. (i) HRTEM images of Pt/MOF-O. (j) EXAFS spectra and Pt L₃ edge EXAFS spectra of Pt/MOF-O, Pt/MOF-C and Ni MOF NSs. Adapted with permission from ref. 73. Copyright 2021, American Chemical Society.

Doping metals as mediators in MOFs also can promote charge migration and improve photocatalytic performance, which is inspired by the enhanced performance of metal-doped semiconductors. MOFs with Ti replacing part of Zr, NH₂-Uio-66 (Zr/Ti), were prepared by the post-synthetic exchange (PSE) method by Li et al.⁶⁷ The results showed that the photocatalytic H₂ evolution rate of NH₂-Uio-66(Zr/Ti) increased 1.5 times compared with NH₂-Uio-66(Zr), reaching 3.5 mmol mol⁻¹. DFT studies revealed that the probability of photogenerated electron transfer to the Ti⁴⁺ center was significantly higher than that to the Zr⁴⁺, resulting in the formation of the (Ti³⁺/Zr⁴⁺)₆O₄(OH)₄ excited state. Ti³⁺ played the role of an electron donor in which electrons are provided to Zr⁴⁺ to form Ti⁴⁺–O–Zr³⁺. Such an electron transfer mechanism helps to improve the transfer of excited interfacial charge to Zr–O clusters for enhanced visible-light photocatalysis. Zhang et al. reported a novel 2D p-type MOF nanoribbon (Ni-TBAPy-NB) photocatalyst with an average nanobelt thickness of approximately 60 nm (Fig. 3b). The MOFs consisted of 1,3,6,8-tetrakis(p-benzoic acid)pyrene (H₄TBAPy) ligand as the light-harvesting center and [Ni₃O₁₆] clusters as the catalytic center.⁶⁸ The in situ EPR experiments yielded a g value of 2.005 belonging to paramagnetic Ni(I) species, revealing the reduction of Ni²⁺ to Ni⁺ in the photocatalytic process. The reduced Ni(I) active center generated by the light-induced ligand-to-metal charge transfer (LMCT) process can serve as a reduction site for the adsorption and activation of water under visible-light irradiation, which subsequently enables the photocleavage of water to H₂ in the absence of any co-catalysts. Aziz et al. demonstrated that the doping metal center also can affect the band gap structure.⁶⁹ A narrowing of the bandgap was achieved by replacing Al with Fe in the part of the metal center, which resulted in an improved charge separation capability.

However, despite pristine MOFs exhibiting H₂ evolution performance, poor conductivity and difficulties in electron transfer kinetics limit further improvement. Therefore, metal nanoparticles (NPs) are often employed as a co-catalyst in the photocatalytic H₂ evolution from MOFs. Lin and co-workers successfully encapsulated Pt nanoparticles with diameters of 2–3 nm and 5–6 nm in the cavities of MOFs (Fig. 3c), to investigate the relationship between the size of Pt nanoparticles deposited in MOFs and the photocatalytic performance.⁷⁰ As shown in Fig. 3e, the TON of the Pt@MOF photocatalytic H₂ precipitation was about 5-fold higher than that of the control samples, which can be up to 7000, attributed to the rapid transfer of electrons from the photo-reduced Ir center to the trapped Pt NPs. Furthermore, the expanded absorption range of visible light (Fig. 3d) also played a crucial role in enhancing the performance. Similarly, Su et al. successfully prepared a series of spatially isolated metal single-atom catalysts (SACs) by binding metal single atoms (M-SAs) in the pores of porphyrin-type MOFs.⁷¹ The prepared MOFs exhibited excellent sustained efficiency in photocatalytic H₂ evolution with a TON as high as 21 713 over 32 h and a start/sustained turnover frequency (TOF) greater than 1200/600 h⁻¹ based on M-SAs under visible light irradiation. Mechanistic studies showed that the tight binding of catalytically active Pt-SAs with a Pd-porphyrin photosensitizer led to enhanced chemical binding and stabilization effects, which can accelerate the electron–hole separation and charge transfer in the pore space, thus largely promoting the photocatalytic water splitting process.

In addition, the photocatalytic efficiency is closely related to the position of Pt relative to the MOF. Jiang and co-workers incorporated Pt nanoparticles inside UiO-66-NH₂ (Pt@UiO-66-NH₂) and supported Pt nanoparticles on UiO-66-NH₂ (Pt/UiO-66-NH₂) with a diameter of 3 nm by two different methods, respectively.⁷² Pt@UiO-66-NH₂ showed better photocatalytic activity than Pt/UiO-66-NH₂, which is attributed to the fact that it greatly shortens the distance from the electrons to the catalytic center and promotes the separation of electrons and holes. The TA spectra of UiO-66-NH₂ (Fig. 3f) and corresponding time-resolved PL (Fig. 3g) revealed that the lifetimes of UiO-66-NH₂, Pt/UiO-66-NH₂ and Pt@UiO-66-NH₂ were 10.28 ± 0.06, 7.26 ± 0.04 and 2.86 ± 0.02 ns. The shortened lifetimes proved that the introduction of Pt NPs inhibited the recombination of photogenerated carriers in the MOFs due to the opened new channel of electron transfer. Recently, Huang et al. demonstrated that the oriented growth of ultrasmall noble metal nanoparticles (e.g., Pt, Pd, Ag, and Au) on MOFs could be achieved by precisely regulating the reduction kinetics of metal ions (Fig. 3i).⁷³ In particular, when methanol and ethanol were used as reducing agents, the rapid reduction process of metal ions led to the random distribution on the surface of MOFs. In contrast, the oriented growth of metal particles on the edges of MOFs was achieved when ethylene glycol and diethylene glycol were chosen, due to their inhibition of the reduction kinetics, which can be seen in Fig. 3h. Ni–O, Pt–Pt, and Pt–O bonds can be clearly observed in the extended X-ray absorption fine structure (EXAFS) spectrum (Fig. 3j), which further provided strong evidence for the existence of Pt. Moreover, detailed experiments indicated that electrons may be transferred from Pt atoms to O atoms, leading to low electron density of Pt and high electron density of O in Pt/MOF-O. As a result, elevated activity of photocatalytic H₂ evolution was exhibited under both acidic and alkaline conditions.

3.1.2 COFs for photocatalytic H₂ evolution. Extensive research has been conducted on H₂ evolution photocatalysts utilizing COFs, which focus on the strategic design and modulation of organic frameworks to enhance their performance. Notably, the introduction of metal sites in COFs is an effective strategy to promote charge separation and migration. One of the most common means is the use of units containing metals in the construction of COFs. At present, organic ligands commonly used to introduce single metal sites include porphyrin, phthalocyanine, bipyridine, salen, etc.

Porphyrins and their derivatives are conjugated π-electron macrocycles with unique physical and chemical properties. In principle, the incorporation of different metal ions into the porphyrin can rationally adjust the photophysical and electronic properties, thus affecting the photocatalytic activity of the corresponding COFs. Considering this, Wang et al. designed and synthesized four COFs with similar structures, which are named MPor-DETH-COF (M = H₂, Co, Ni, Zn).⁷⁴ Interestingly, these COFs exhibited tunable photocatalytic H₂ evolution ability, in the order of CoPor-DETH-COF < H₂Por-DETH-COF < NiPor-DETH-COF < ZnPor-DETH-COF. The difference in photocatalytic activity was attributed to the effect of different metal ions on the carrier dynamics in the COFs. In particular, the migration of photogenerated electrons relies on metal-to-metal channels rather than specific metal centers under photoexcitation of the porphyrin, and the photogenerated holes are transferred mainly via the pathway of macrocycle-on-macrocycle. As seen in Fig. 4a, for the metal-free H₂Por-DETH-COF, the migration of both electrons and holes takes place via the macrocycle-on-macrocycle pathway, which undoubtedly increases charge recombination and leads to poorer photocatalytic activity. However, for CoPor-DETH-COF and NiPor-DETH-COF, the promoted photoactivity can be attributed to the ligand-to-metal charge transfer (LMCT) process, which is partially inhibited with the increase of d electrons (Ni²⁺ for 3d⁸), resulting in improved hole transfer ability and enhanced charge separation efficiency. Finally, the LMCT process was prohibited in Zn²⁺ ions with 3d¹⁰ configuration, resulting in a long-time charge separation state. Therefore, ZnPorDETH-COF showed the best photocatalytic H₂ evolution activity under the same conditions.

Fig. 4 (a) The schematic illustrations of the hole–electron transport processes in MPor-DETH-COFs. Adapted with permission from ref. 74. Copyright 2021, Springer Nature. (b) Illustration of the synthetic procedures of RuCOFs. Adapted with permission from ref. 76. Copyright 2022, Wiley-VCH. (c) Triplet spin density surfaces for Rubpy-ZnPor COF. (d) Proposed photochemical process for H₂ evolution. Adapted with permission from ref. 77. Copyright 2023, Wiley-VCH. (e) Gibbs Free energy diagrams for hydrogen evolution on the salen metal site in Zn-salen-COF and M/Zn-salen-COF (M = Fe, Co, Ni). Adapted with permission from ref. 78. Copyright 2023, Wiley-VCH. (f) Mott–Schottky Pt/TP-COF (left) and Pt-PVP-TP-COF MIS (right) photocatalyst. Adapted with permission from ref. 79. Copyright 2019, Wiley-VCH.

In response to the frequent leakage of transition metal ions during the photocatalytic reactions, bipyridine ligand metals are a good choice. Guo et al. used a solvothermal method to anchor Ni(II) ions in bipyridine-containing COFs.⁷⁵ In contrast to the traditional method of anchoring Ni(II) ions in COFs under mild conditions, this method can result in torsion of the 2,2′-Bpy part of the single bond, leading to a planar conformation coordinated to Ni(II). The COF–Ni²⁺ complexes allowed the unique transfer mode of metal-to-ligand charge as well as a broadened absorption range of visible light. Based on this, the H₂ evolution rate of COF-Ni²⁺ was nearly 2.5 times higher than that of pristine COFs, and the H₂ evolution can also be observed upon 700 nm light irradiation. On the other hand, Gu and co-workers extended bipyridines to 3D COFs.⁷⁶ Three different COFs, RuCOF-ETTA, RuCOF-TPB, and RuCOF-ETT-BA (Fig. 4b), were synthesized based on the Ru(II)tris(2,2′-bipyridine) unit. Interestingly, each RuCOF contained three isomorphic COFs that were interlocked with the Ru center as the registration point. The homogeneous distribution of Ru units resulted in excellent chemical stability, strong light harvesting ability, and outstanding H₂ evolution (20 308 μmol g⁻¹ h⁻¹) of the RuCOFs. Moreover, Lan et al. prepared a series of bis-photosensitised COFs with strong visible light absorption by organically combining two conventional photosensitizers, pyridine ruthenium/iron (Ru(bpy)/Fe(bpy)) and porphyrin/metal porphyrin complexes (2Por/ZnPor).⁷⁷ DFT calculations (Fig. 4c and d) demonstrated that both Rubpy and ZnPor can be excited and subsequently undergo a two-way electron transfer process, and this dual-excitation behavior lays the foundation for achieving fast photoelectron transfer and efficient photocatalytic water splitting to H₂.

Furthermore, salen as an efficient molecular catalyst for H₂ evolution integrated with the light collector in a photocatalyst is an effective strategy for solar light conversion. As an example, Deng and co-workers reported a series of rigid M/Zn-salen-COFs (M = Zn, Fe, Co, and Ni), which were synthesized from salen-M molecular catalysts with pyrene as the light trapping unit.⁷⁸ Among them, Co/Zn-salen-COF exhibited the most efficient rate of H₂ evolution, reaching 1378 μmol g⁻¹ h⁻¹. The presence of salen-Co increased the separation of photogenerated charge in the COFs and reduced the transfer resistance of interfacial charge, so salen-Co in Co/Zn-salen-COF exhibited a lower Gibbs free energy (Fig. 4e). Furthermore, a binuclear Cu-salphen COF was designed by Mak et al. to achieve an H₂ evolution rate of 36.99 mmol g⁻¹ h⁻¹ under visible light irradiation without the decoration of Pt.⁷⁹ The independent Cu sites with high loading can not only effectively increase the probability of photogenerated charge into the reaction center on the surface of the photocatalyst, but also prevent the recombination of photogenerated carriers. In addition, the authors exfoliated COFs into ultrathin nanosheets to overcome the problems of low active site utilization and slow ion diffusion prevalent in large-volume COFs. That also improved the photocatalytic efficiency thanks to the accelerated charge transfer kinetics greatly.

In the actual process of photocatalytic H₂ evolution, metal Pt is often involved in COF photocatalytic H₂ evolution as a proton reduction co-catalyst due to its excellent properties. Pt acts as an electron collector throughout the photocatalytic system, with the COFs forming Schottky junctions at the contact interface. In other words, the COFs act as photosensitizers to provide photogenerated electrons for proton reduction on the co-catalyst. Therefore, it is crucial to promote the electron transfer from COFs to the co-catalyst to enhance the photocatalytic H₂ evolution. Zhang et al. anchored single-atom Pt to the pore walls of β-ketamine-bonded COFs (TpPa-1-COF).⁸⁰ The advanced AC HAADF-STEM and XAFS techniques demonstrated that the atomically dispersed Pt existed in the form of a six-coordinated C₃N–Pt–Cl₂. The good dispersion of single-atom Pt as photocatalytic active sites lays the foundation for the achievement of excellent photocatalytic H₂ evolution activity, which was increased 3.9 and 48 times compared to TpPa-1 COFs with Pt nanoparticles and parent TpPa-1, respectively. In addition, the presence of single-atom Pt facilitated effective photogenerated charge separation and lowered the energy barrier for the formation of H*.

Surpassing the conventional Schottky-type photocatalysts, Long's group cleverly designed a novel metal–insulator–semiconductor (MIS) photosystem (Pt-PVP-TP-COF) based on the electrostatic self-assembly of polyvinylpyrrolidone (PVP) insulator-covered Pt NPs with hydrophilic imine-linked TP-COFs.⁸¹ As shown in Fig. 4f, the photoelectrons can tunnel through the thin insulating layer (typically∼1.0 nm) to the collector when the MIS heterostructure is irradiated, where the H₂ evolution reaction occurs. The electrostatic field formed perpendicular to the COFs between the TP-COF and the insulating layer provided the impetus for charge tunneling for the extraction of hot π-electrons from the photoexcited COF semiconductor. In normal Schottky-type photocatalysts, both excitons can be transferred to Pt, leading to non-radiative recombination and significant reduction of hot carriers and hole oxidation. In contrast, the MIS photosystem hindered the transfer of photogenerated holes to Pt, which can improve the separation efficiency. The combined enhancement of the photoexcitation, charge separation, and oxidation rates of holes accumulated in the VB of the TP-COF resulted in a maximum H₂ evolution rate of 8.42 mmol g⁻¹ h⁻¹ and the highest apparent quantum efficiency of 0.4% at 475 nm.

3.1.3 HOFs for photocatalytic H₂ evolution. The study of HOFs as catalysts for photocatalytic H₂ evolution is still in its infancy due to the instability of HOFs. Lu et al. have taken into account the role of hot electrons generated during the photocatalytic process to improve the photochemical stability and activity of HOFs.⁸² The efficient elicitation of hot electrons by introducing Pt nanoparticles as electron sinks into high-crystalline nano-HOFs was demonstrated as a new strategy for the synthesis of photostable HOFs, which overturns the traditional viewpoint of “fragile HOFs”. The obtained Pt@nano-HOF not only had an efficient H₂ evolution rate of 31.2 mmol g⁻¹ h⁻¹, but also maintained a stable structure and substantial activity after 5 cycles. In addition, it is further demonstrated that the introduction of planar [Co(qpy)(OH₂)₂]²⁺ molecules can further accelerate the transfer of the hot electrons by forming a π–π stack with the frameworks. The additional instabilities in HOFs are attributed to the weak hydrogen bonding, thus exploring HOF materials that are competent as H₂ evolution photocatalysts is a long-term and challenging task.

3.2 Photocatalytic oxygen evolution

Photocatalytic O₂ evolution is another half of the water-splitting reaction. Combining the two half-reactions of photocatalytic hydrogen and oxygen production, the target of light-to-energy can be achieved. However, the photocatalytic O₂ evolution is often accompanied by slow kinetics because it involves multi-electron transfer processes and high overpotential requirements, especially the high energy barrier required for O–O bond formation. Therefore, the progress of reticular frameworks in the field of photocatalytic O₂ evolution has been relatively slow compared to the photocatalytic H₂ evolution, and only a few cases have been reported.

3.2.1 MOFs for photocatalytic O₂ evolution. In an advanced strategy, Qin et al. developed a new way to obtain bismuth-based MOFs (Bi-mna),⁸³ which resulted in an O₂ evolution rate of 216 μL h⁻¹ under visible light irradiation. Theoretical calculations revealed an enhanced photocatalysis by the ligand-to-ligand charge transfer (LLCT) process due to the closed shell character of Bi³⁺ (d¹⁰s²). Furthermore, Liu and co-workers employed a stepwise synthesis method to obtain new bimetallic Co-MOFs (named PFC-20-Co₂Ti, Fig. 5a) by utilizing highly active Ti⁴⁺ cations.⁸⁴ In the classical [Ru(bpy)₃]₂-S₂O₈²⁻photocatalytic system, PFC-20-Co₂Ti exhibited high activity with an optimal turnover frequency of 8.06 × 10⁻³ s⁻¹ and an apparent quantum yield (AQY) of 8.56% under 500 nm irradiation, which is shown in Fig. 5b. The doping of single-dot Ti(IV) cations increased the charge density of Co-MOFs at the metal nodes and optimized the catalytic kinetics, resulting in a satisfactory photocatalytic activity of O₂ evolution.

Fig. 5 (a) The different metal clusters in the PFC-20 series and the crystal structure of PFC-20. (b) Comparison between AQYs of O₂ evolution and PL intensity of [Ru(bpy)₃]²⁺-S₂O₈²⁻ solution under monochromatic-light irradiation. Adapted with permission from ref. 84. Copyright 2022, Chinese Chemical Society. (c) The fs-TA spectra of CoTPP, [CoII(bpy)₃](OAc)₂, and CoTPP-CoBpy₃. (d) ICT from CoTPP to [CoII(bpy)₃](OAc)₂. (e) The decay trace of ESA signal at 500 nm in CoTPP-CoBpy₃ and [CoII(bpy)₃](OAc)₂. (f) Electron transfer pathway including ISC and ICT. (g) Proposed water oxidation mechanism in CoTPP active sites. Adapted with permission from ref. 84. Copyright 2024, American Association for the Advancement of Science. (h) The possible process of the HER on the Tp segment and the OER via a dual-site process on the Bpy segment in TpBpy-NS, and the comparison of calculated Gibbs free energy change for C2d paths at pH = 7. Adapted with permission from ref. 87. Copyright 2023, Springer Nature.

3.2.2 COFs for photocatalytic O₂ evolution. Moreover, covalent organic frameworks are also emerging in the field of O₂ evolution. Yang et al. synthesized COFs with an imine linkage using 2,2-bipyridine-5,5′-dicarboxaldehyde (Bp) and 1,3,5-tris(4-aminophenyl)benzene (TAPB) as initial materials.⁸⁵ As a result, the as-prepared Bp-COF showed impressive visible-light-driven water–oxidation activity in conjunction with Co²⁺, reaching an O₂ precipitation rate of 152 μmol g⁻¹ h⁻¹, which was processed in the presence of Ru as the co-catalyst and AgNO₃ acting as the sacrificial agent. Compared with the pristine Bp-COF, the obtained BpCo-COF exhibited a more suitable HOMO level for water oxidation. Similarly, Wang et al. obtained an ionic covalent organic framework named (CoTPP-CoBpy₃) COF by combining cobalt bipyridine and cobalt porphyrin.⁸⁶ The porous sheet structure (21.45 nm) endowed the as-prepared (CoTPP-CoBpy₃) COF with excellent dispersibility in water, which lays the foundation for oxygen extraction from water. Secondly, as shown in Fig. 5c–e, the ultrafast trilinear state charge transfer (1.8 ps) and prolonged charge separation (1.2 ns) further facilitated the hole's efficient accumulation of the CoTPP part, promoting the process of an otherwise slow photocatalytic water oxidation. It should be mentioned that the decreased ESA peak of CoTPP-COBpy₃ at 470 nm (S1 → S2 of CoTPP) indicates the enhanced inter-system crossover (ISC) from the S1 state to the T1 state. The synergistic modulation of Co complex building blocks is shown in Fig. 5f, which established a smooth and flexible electron transfer pathway from the S1 (CoTPP) to T1 (Co-bipyridine) excited states. Notably, the end-on bridge between the superoxide radical and CoTPP was also confirmed by in-depth in situ characterization and theoretical calculations, and the preferred photocatalytic O₂ evolution pathway of CoTPP-CoBpy₃ COF was obtained (Fig. 5g). The aforementioned unique advantages resulted in a surprising O₂ evolution rate of 7323 μmol h⁻¹ for (CoTPP-CoBpy₃) COF. Furthermore, Lan and coworkers achieved overall water splitting under visible light irradiation by in situ addition of sub-nanometer platinum as the co-catalyst in the pores of COF nanosheets.⁸⁷ Thanks to the more favorable reactive path because of its favorable energetics (Fig. 5h), the optimal photocatalytic activities of Pt@TpBpy-NS reached 9.9 and 4.8 μmol in 5 h for H₂ and O₂, respectively.

In addition, the suitable energy band structure is the groundwork for achieving efficient overall water splitting, while the highly exposed active site is an important part of improving photocatalytic activity. Based on this, Yang et al. revealed the catalytic activities of several COFs for H₂ and O₂ evolution reactions by first-principles calculations, which provided useful guidance for the design of COFs that can be used for visible-light-driven overall water splitting.⁸⁸ Twelve COFs were designed by assembling nitrogen-containing bonds (including imine, nitrogen, and azo groups) and four phenyl building blocks (including benzene, triphenylamine, 1,3,5-triphenyl benzene, and 2,4,6-triphenyl-1,3,5-triazine). The results showed that 9 of these COFs satisfy both the potential requirements for photocatalytic H₂ and O₂ production, and only three TA-based COFs can undergo the H₂ evolution reaction in theory. In addition, the fact that I-TST COFs can produce H₂ and O₂ under visible light irradiation was further demonstrated experimentally. The approach of theory-guided experimental provided a new way to design efficient COF-based photocatalysts for overall water splitting under visible light.

3.3 Photocatalytic carbon dioxide conversion

Carbon dioxide (CO₂) is considered one of the most dominant greenhouse gases, with approximately 60% of the global warming effect attributed to CO₂ emissions. At the same time, it is also an abundant, cheap, non-toxic, and bio-renewable carbon resource. Therefore, the conversion of CO₂ into other useful chemicals is an effective way to curb problems of global warming and energy shortage. Thermodynamically, abundant energy is required due to the large enthalpy (523 kJ mol⁻¹) of the double bond in O=C=O. Recently, the economic solar-driven decomposition of the bond between C and O has been gaining attention. Reticular framework materials have great advantages as photocatalysts due to their high capacity of catalytic sites, semiconducting properties, and tunneling morphology. In addition, the uniform permeable structure and high specific surface area of the reticular framework materials provide an ideal platform for the introduction of metal sites into the frameworks, which lays a solid foundation for achieving more efficient photocatalytic CO₂ reduction.

3.3.1 MOFs for photocatalytic CO₂ conversion. The metallic part of MOFs consists mainly of divalent or trivalent ions of transition metals or lanthanides.^89–91 These metal nodes as active sites induce the photocatalytic reaction by sinking excited state electrons and maintaining a stable low valence state to ensure the separation of excited state charges, thus ensuring sufficient processing time for the photocatalytic reaction.⁹² The results show that MOFs consisting of metal ions in variable valence states have a significant reducing ability which can effectively drive the photocatalytic CO₂ reduction reaction. For example, Lan and co-workers constructed a series of stable heterogeneous metal MOF structures (NNU-31-M, M = Co, Ni, Zn).⁹³ The efficient separation of electrons and holes is attributed to the visible-light-excited heterogeneous metal cluster units and the photosensitive ligands, respectively. The as-prepared MOFs achieved the conversion of CO₂ and H₂O into HCOOH and O₂, without the presence of additional sacrificial agents and photosensitizers. In this structure, the high valence Fe³⁺ acted as an oxidation site while the low valence M²⁺ played the role of a reduction site. Among the MOFs, the highest HCOOH yield of 26.3 μmol g⁻¹ h⁻¹ was observed for NNU-31-Zn, which was accompanied by nearly 100% selectivity. This work provided a suitable framework for the design of crystalline photocatalysts to achieve the reduction of CO₂ with H₂O. In addition, they also obtained a series of polyoxometalate-based metal–organic frameworks (M-POMOFs),⁹⁴ which achieved photo- and electrocatalytic carbon dioxide reductions, respectively. As shown in the mechanism diagram (Fig. 6a), the specific multielectron products were generated on iron-POMOF through switching driving forces to control electron flow direction between single metal site and cluster catalysis.

Fig. 6 (a) Proposed reaction mechanism in PCR and ECR reactions for Fe-POMOF, respectively. Adapted with permission from ref. 94. Copyright 2022, American Association for the Advancement of Science. (b) Copper K-edge XANES spectra of pristine and (c) Fourier transformed K³-weighted χ(k) function of EXAFS for different samples. Adapted with permission from ref. 98. Copyright 2023, Wiley-VCH. (d) Schematic illustration of the NIR-light-induced electron transfer pathway fromπ-conjugated systems to Zr clusters in TNP-MOF for CO₂ reduction. Adapted with permission from ref. 99. Copyright 2022, American Chemical Society. (e) Different coupling modes of CO₂ and HCOOH interact with the Co site at different spin states. Adapted with permission from ref. 103. Copyright 2020, American Chemical Society. (f) AFM image and the height profiles along the marked white line of COF-367 NSs. Adapted with permission from ref. 106. Copyright 2019, American Chemical Society. (g) TA kinetics of two samples at various emission wavelengths. Adapted with permission from ref. 109. Copyright 2018, American Chemical Society. (h) Corresponding normalized TA kinetic curves probed at 450 nm. Adapted with permission from ref. 119. Copyright 2023, Elsevier. (i) Normalized XANES spectra and FT-EXAFS spectra at the Co K-edge of Co-2,3-DHTA-COF, Co₃O₄, CoO, CoPc and Co foil samples. (j) Normalized XANES spectra and FT-EXAFS spectra for discriminating the radial distance and k-space resolution of Co-TP-COF. Adapted with permission from ref. 114. Copyright 2023, Springer Nature.

Similarly, Ti-based MOFs have attracted great interest in the photocatalytic field due to the photoinduced transition from Ti⁴⁺ to Ti³⁺.⁹⁵ Li et al. prepared NH₂-MIL-125(Ti) MOFs with the satisfactory ability to reduce CO₂ to HCOOH under visible light.⁹⁶ NH₂ in the prepared MOFs mainly acted as an adsorbent, whereas the Ti⁴⁺ center accepted the photogenerated electrons from the NH₂-functionalized ligands, then reduced to Ti³⁺. Subsequently, Ti³⁺ reacted with the excess electrons with CO₂ for the generation of HCOOH. Cu-based MOFs also exhibit similar properties. Typically, Cu has higher activity in the oxidized state than metallic copper, thus pre-oxidation of Cu-MOFs before photocatalysis is the most common method. However, this method suffers from a drawback in that maintaining the oxidized state of Cu sites is difficult during CO₂ reduction, which leads to a decrease in photocatalytic capacity.⁹⁷ To address this, Sheng et al. achieved photocatalytic CO₂ reduction to CO and CH₄ by Cu-MOFs in the presence of O₂ levels (20% v/v) in air, using water as the hole scavenger.⁹⁸ The results of structural analyses (Fig. 6b and c) showed that the hydroxylated Cu generated by Cu sites during pre-oxidation under aerobic conditions could maintain a stable structure during CO₂ reduction, whereas it undergoes structural changes and loses the reducing ability under anaerobic conditions. This resulted in a 5-fold higher CO₂ reduction rate in the presence of O₂ than under anaerobic conditions, which opened the possibility of CO₂ reduction in an air atmosphere.

Another wide strategy is the integration of large π-conjugated organic units into MOFs, such as porphyrin. As an example, mesoporous MOFs with six macrocyclic π-conjugated units that progressively increased in number were synthesized.⁹⁹ The MOFs containing the porphyrin unit (TNP-MOF) were able to convert CO₂ to HCOOH at an extremely high rate of 6630 μmol g⁻¹ h⁻¹ under near-infrared light irradiation. The richness of the linker's electron density can broaden the photo-absorption range of the MOFs. Moreover, as shown in Fig. 6d, the transfer pathway of photogenerated electrons from the conjugated linkers to the metal clusters led to rapid charge separation. These combined factors resulted in an unprecedentedly high photocatalytic performance.

In response to the synergistic effect of individual metal sites on the CO₂ reduction reaction, dual-metal-site pair catalysts have been considered as an effective strategy for inducing highly selective products in multi-step catalytic processes.¹⁰⁰ In particular, the synergistic effect of these dual-metal-site pairs on the bonding of C1 intermediates provides a viable way to enhance the induction of specific reaction pathways in the catalysis of CO₂-to-CH₄. Based on this, recently, Zhong et al. merged Cu and Ni sites into a MOF (i.e., MOF-808) in the form of a single-site, which exhibited dynamic adaptive behavior to accommodate mutated C1 intermediates, leading to the photocatalytic CO₂-to-CH₄ process and achieving a yield of 158.7 μmol g⁻¹ h⁻¹.¹⁰¹ The theoretical calculations showed that the spatial configuration of the Cu/Ni MOF is dynamic during CO₂ photoreduction, just as the bonding with substrates and intermediates induces suitable conformations in a natural enzyme system. This adaptive behavior played a crucial role in stabilizing the various C1 intermediates, thus inhibiting the desorption of undesired by-products, which is thought to be responsible for the highly selective photoreduction of CO₂ to CH₄.

3.3.2 COFs for photocatalytic CO₂ conversion. COFs not only have high crystallinity, specific surface area, and pore size in addition to an extended conjugated skeleton but also exhibit a π-delocalization system and highly ordered π–π stacking, resulting in the charge carriers being transported along the planar and stacking directions excellently. However, research on the photocatalytic reaction of CO₂ reduction by using COFs as photocatalysts is still in its infancy. Doping of metal sites has been well-received as an effective strategy to improve charge separation. In this part, we discuss the potential of COF anchored metal active centers for photocatalytic CO₂ reduction reactions. Porphyrin-based COFs with highly conjugated π-electron macrocycles have promising applications in photocatalysis due to their excellent ability of visible light absorption. Various metal ions can be easily implanted into the porphyrin center as the active sites for photocatalysis.¹⁰² Considering the close relationship between the spin state of Co and the photocatalytic performance, Jiang et al. tuned the spin state of the Co sites in porphyrin COFs by changing their oxidation states.¹⁰³ Intriguingly, Co in COF-367-CoII is in a spinless state (S = 0), its energy is 0.15 and 0.41 eV lower than that of the low-spin state (S = 1) and the high-spin state (S = 2), respectively. It resulted in COF-367-CoIII exhibiting enhanced selectivity for CO₂ reduction to HCOOH, in contrast to COF-367-CoII. The DFT results showed that the oxidation state (+2 or +3) of Co significantly affected the spin state, which in turn determined the electron distribution and orientation of the Co-3d orbital, thus significantly altering the mode of Co–CO₂ molecule interaction (Fig. 6e), which led to the preferential formation of HCOOH intermediates in COF-367-CoIII and an unfavorable one in COF-367-CoII. Furthermore, considering the practical applications, the utilization of COFs for photocatalytic CO₂ reduction without the addition of additional photosensitizers and sacrificial agents is urgent. Lan et al. prepared a series of crystalline COFs, TTCOF-M (M = 2H, Zn, Ni, Cu), by utilizing porphyrin (TAPP) and tetrathiafulvalene (TTF) as the structural units.¹⁰⁴ Effective photogenerated electrons were transferred from TTF to TAPP via covalent bonding, which can be attributed to the electron-deficient nature of TAPP and the superior electron-supplying ability of TTF. Subsequently, CO₂ reduction and H₂O oxidation reactions occurred on the electron-rich TAPP and the hole-rich TTF, respectively, followed by efficient photocatalytic activity. With the development of topology, porphyrin-based 3D COFs have also begun to emerge in the photocatalytic field. Fang et al. reported a 3D crystalline COF in 2023, which achieved an ultra-high CO yield of 15.1 mmol g⁻¹ h⁻¹.¹⁰⁵ This was attributed to the exposure of large interpenetrating channels (4.6 nm), high surface area (2204 m² g⁻¹), and abundant porphyrin centers (0.845 mmol g⁻¹). In addition, modulation of the axial π–π stacking of porphyrin-based COFs is a strategy to further improve the photocatalytic ability. In 2019, Jiang's group achieved the preparation of ultrathin porphyrin-based COF nanosheets with a thickness of around 1.2 nm (Fig. 6f) by hindering the axial π–π stacking during the growth process.¹⁰⁶ Based on this idea, their group obtained 2D MPor-DPP-COFs (M = H, Co, and Ni) by condensation of metal-porphyrin and 3,8-diamino-6-phenylphenanthridine (DPP).¹⁰⁷ The bulky phenyl substituent at the periphery of the TTP unit reduced the axial π–π stacking which enlarges the interlayer spacing of COFs to 6.0 A, resulting in enhanced photocatalytic CO₂ reduction activity and stability.

The advantages of Re(I) complexes as photocatalysts for CO₂ reduction are as follows: high product selectivity and outstanding CO₂ reduction efficiency.¹⁰⁸ Therefore, it is promising to support Bpy-Re in COFs. At first, Huang et al. revealed three key intermediates in charge separation, induction period, and CO₂ reduction decisive step in the photocatalytic process of COFs containing Re complexes for the first time.¹⁰⁹ The photogenerated electrons in this system can be transferred to the Re complexes via intramolecular charge transfer, which achieves better activity than homogeneous Re catalysts. Then, Re-TpBpy was used as an example to study the excitation kinetics and charge transfer process of such catalysts by Zheng and co-workers.¹¹⁰ Femtosecond transient visible (fs-TA) and time-resolved infrared (fs-TRIR) revealed the whole process. As shown in Fig. 6g, the excited electrons were in the position of the Bpy conduction band when the energy of photons was close to the Re-TpBpy band gap, then injected into the Re site within sub-picoseconds. However, these electrons recombined with photogenerated holes in 13 ps. In contrast, when the COFs are excited with high energy, the hot electrons were injected into the high-energy orbital of Re in 2 ps, and then bounced back to the Bpy fragment within 24 ps, while the hot holes slowly (340 ps) relax to the energy level of HOMO. At this point, the long lifetime of excitation electrons in the Re sites provided favorable conditions for two-electron mediated photocatalytic CO₂ reduction to CO. Furthermore, Han et al. achieved both CO₂ reduction and H₂O oxidation by sp² COF constructed by using the rhenium complex as the reduction site and the triazine ring as the oxidation site.¹¹¹ The excellent charge separation ability of COF-Bpy-Re allowed the photogenerated electrons and holes to concentrate on the two unit sites in a periodic arrangement, respectively. This resulted in a CO yield of 190.6 μmol g⁻¹ h⁻¹ and an O₂ evolution rate of 90.2 μmol g⁻¹ h⁻¹. Similarly, Bpy-Cu and Bpy-Ni were integrated into the COFs, which also achieved the same purpose of improving the efficiency of the photocatalytic CO₂ reduction reaction.¹¹² This is because of the enhanced adsorption of CO₂, the expanded light absorption, and the improved electron separation efficiency of COFs.

Moreover, 1,2-acenaphthenequinone has likewise been used to anchor monatomic metal sites. Hou et al. utilized it to introduce different single atomic metal sites (e.g., Fe, Co, Ni, Zn, Cu, Mn, and Ru) to the frameworks of triazine COFs with the metal–N–Cl structure.¹¹³ It is worth noting that the CO₂ conversion rate of Fe SAS/TrCOF as the representative catalyst was 26 times higher than that of the original Tr-COF, which was up to 980.3 μmol g⁻¹ h⁻¹. The superior photocatalytic performance could be due to the synergistic interaction of the atomically dispersed metal sites with the COF framework, which not only lowered the reaction energy barriers for the formation of the *COOH intermediates but also facilitated the adsorption of CO₂ while also facilitating the desorption of CO.

The coordination environment of the metal site is also an important factor affecting the performance of COF photocatalysts except for different organic units. Wang et al. took advantage of the controllable functional groups in the precursor module and the strong coordination of Co(II) to spontaneously form Co-COFs,¹¹⁴ which can be based on the pre-designed hydroxyl positions in the aldehyde module by a programmable method to provide a precise coordination microenvironment. The EXAFS results in Fig. 6i and j provide ample evidence for this. In this research, two Co-COFs with different ways of oxygen coordination were designed. The results showed that the as-prepared Co–COF with Co–O₄ coordination after adjusting the coordination environment, reached a CO yield of up to 18 000 μmol g⁻¹ h⁻¹ and a selectivity of up to 95.7% under visible light irradiation. In situ characterization and theoretical calculations jointly demonstrated that the pre-designed Co–O₄ sites significantly promoted the migration efficiency of charge carriers in the Co–COF matrix and inhibited the recombination of photogenerated electron–hole pairs during the photocatalytic process.

Similar to MOFs, the simultaneous integration of dual photosensitive units into the COF backbone not only further extends the range of light absorption, but also enables more efficient intermolecular multi-electron transfer towards the catalytic centers across the π-conjugated COFs,^115,116 which could potentially alleviate the rapid quenching and diffusion limitations of photoexcited electrons in physical hybrid systems.¹¹⁷ Wang et al. used [Rel(bpy)(CO)₃Cl] and protoporphyrin as building blocks to synthesize imine-conjugated H₂PReBpy-COF.¹¹⁸ The self-properties of porphyrin and the extended π-conjugated plane of the 2D COFs resulted in excellent light absorption, efficient charge separation, and fast interfacial charge transfer, making H₂PReBpy-COF a preeminent photocatalyst for CO₂ reduction. On the other hand, Cao et al. utilized [Ru(bpy)₃]²⁺ (bpy = 2,2′ bipyridine) and activated cobalt porphyrin (Co-Por) to prepare COF-RuBpy-Co.¹¹⁹ The efficient photoelectron transfer from [Ru(bpy)₃]²⁺ to Co-Por enabled a CO generation rate of 547 μmol g⁻¹ h⁻¹, which was a 1.4-fold improvement over both physical mixtures. It is worth noting that the fs-TA results provided evidence for a faster transfer of photogenerated charge (44.2 ps, Fig. 6h) on the large π-conjugated system.

Density Functional Theory (DFT) calculations play an important role in exploring and understanding potential catalytic mechanisms, as well as guiding experimental design.^120,121 A series of COFs with bimetallic active sites were designed to readily aggregate to form nanoparticles due to the higher surface energy of the isolated metal atoms. The possible intermediates, free energy, reaction pathways, and overpotentials of the CO₂RR were considered and calculated by DFT. Moreover, with the descriptor identification study, the Bader charge associated with the Pauling electronegativity of the embedded bimetallic atoms was found to be an important factor affecting the catalytic activity.

3.3.3 HOFs for photocatalytic CO₂ conversion. HOFs, as another type of crystalline reticular framework material, are self-assembled from organic or metal–organic building blocks by hydrogen bonding. However, their instability and low directionality of the hydrogen bonding remain a stumbling block for the establishment of permanent pores in the HOFs, thus hindering its development in the field of CO₂ reduction. Therefore, it is necessary to establish new synthetic strategies to prepare robustly functional HOFs for more efficient performance. To achieve this, Jiang et al. obtained post-functionalized 2,2′-bipyridine (bpy) derived biological hydrogen-bonded framework (HOF-25) by the reaction of a guanine-quadruplex with Re(CO)₅Cl with the assistance of π–π interactions, as shown in Fig. 7a.¹²² Interlayer π–π interactions favor the stabilization of this robust HOF and the maintenance of permanent pores. The as-prepared HOF-25-Re photocatalyst exhibited good non-homogeneous catalytic activity for CO₂ photoreduction under visible light irradiation when dispersed in CH₃CN, with a yield of CO as high as 1448 μmol g⁻¹ h⁻¹. Accordingly, Cao and coworkers developed a series of HOFs as biomimetic heterophasic catalysts by adjusting the proportion of metalloporphyrin centers in the whole HOFs.¹²³ The HOFs were prepared with 100%, 61%, and 30% of Cu in the porphyrin center, which was named PFC-58, PFC-58-61, and PFC-58-30, respectively (Fig. 7b). The experimental and computational results demonstrated that varying the Cu content in the porphyrin not only achieved the fine-tuning of the photosensitizer/catalyst ratio but also significantly altered the microenvironment around the active site and the charge separation efficiency. As a result, the obtained HOFs achieved challenging CO₂ reduction in the presence of H₂O vapor, which exhibited a high HCOOH yield (29.8 μmol g⁻¹ h⁻¹, without a sacrificial agent).

Fig. 7 (a) AA packing mode of HOF-25 and a proposed structure of HOF-25-Re for CO₂ reduction. Adapted with permission from ref. 122. Copyright 2021, Wiley-VCH. (b) Schematic diagram of the structures of PFC-58 series HOFs with different metalloporphyrin contents. Adapted with permission from ref. 123. Copyright 2022, Wiley-VCH. (c) Interlayer NCI isosurfaces for PFC-71 dimer and PFC-73-Cu dimer show a larger overlap area of PFC-73-Cu than PFC-71. (d) PFC-71 and PFC-73-Cu show similar in-plane NCI isosurfaces with a strong hydrogen bond interaction (only PFC-73-Cu is depicted as a representative). (e) Comparison of DOS and the corresponding OPDOS of PFC-71 and PFC73-Cu dimer. Adapted with permission from ref. 125. Copyright 2021, Wiley-VCH.

Drawing on the fact that in homogeneous porphyrin systems, the metalloporphyrin center is usually considered to be one of the most important determinants of the robustness and activity of the materials, changing the metal center leads to drastic changes in the electronic structure of the macrocycle, axial bonding/interactions, and peripheral substitution geometries.¹²⁴ Based on this, Liu et al. obtained metalloporphyrin HOFs with different non-covalent interactions, orbital overlap, and molecular geometries but the same topology by introducing different transition metals into the porphyrin center.¹²⁵ The results in Fig. 7c–e demonstrate that the metalated porphyrins dramatically altered the electrostatic potential, density of states, and thus the non-covalent interactions between the interlayer porphyrin fragments. Subsequently, effective catalysis of the photoreduction of CO₂ to CO was achieved. The photocatalytic performance of CO₂ was heavily dependent on the type of chelating metal at the porphyrin center, which provided an effective guide for the design of porphyrin HOFs in the field of photocatalytic CO₂ reduction.

3.4 Hydrogen peroxide production

Hydrogen peroxide (H₂O₂) is an environmentally friendly oxidizing agent with a wide range of applications in bleaching, disinfection, semiconductor manufacturing and chemical synthesis.¹²⁶ At the same time, H₂O₂ is a potential energy carrier that can be used directly in fuel cells to generate electricity. In addition, the energy density of aqueous H₂O₂ is comparable to that of compressed H₂, and H₂O₂ is more convenient than H₂ in terms of storage, transport, and portability, making it a promising alternative to H₂ as a clean energy source. For all the above reasons, the global H₂O₂ market is expected to grow to 5.7 million tonnes by 2027. Currently, more than 95% of industrial H₂O₂ is produced by anthraquinone oxidation, an energy-intensive, multi-step method to generate a large amount of waste.¹²⁷ Therefore, the direct production of H₂O₂ through a photocatalytic reaction using H₂O and O₂ is a potentially viable and green sustainable approach.^128,129

The partial replacement of metal–oxygen clusters by other metal ions or the incorporation of new metal ions can lead to new physicochemical properties of MOFs.¹³⁰ In particular, hydrophobicity, superior charge separation efficiency and longer lifetime of the photoexcited state can be effectively achieved in MOFs. As an example, the hydrophobic MOF MIL-125-Rn (n = 4 and 7) was firstly synthesized by Yamashita's group,¹³¹ but the drastically reduced specific surface area was not conducive to further improvement of the H₂O₂ production. On this basis, the authors alkylated the Ti cluster of MIL-125-NH₂ by using octadecyl phosphonic acid (OPA).¹³² Unlike before, the as-prepared hydrophobic OPA/MIL-125-NH₂ maintained the original large surface area due to the alkylation reaction only occurring on the outermost surface of the pristine MOFs with open coordination sites. The yield of H₂O₂ was again improved 3 times compared to MIL-125-Rn. The increased photocatalytic activity was attributed to the faster diffusion of reactants and products as well as the slower decomposition of H₂O₂ in OPA/MIL-125-NH₂. Wang et al. loaded Pd single atoms onto defect-rich UiO-66-NH₂ and subsequently processed gas permeable membranes (Pd/UiO-66-NH₂).¹³³ Photocatalytic generation of H₂O₂ at the gas–solid interface was achieved using humid O₂ (Fig. 8a). The defect sites favored light absorption and interaction with metal single atoms, while the positively charged Pd single atoms both enhanced H₂O₂ generation and inhibited H₂O₂ decomposition. The H₂O₂ generation rate on the Pd/UiO-66-NH₂ membrane was 10.4 mmol g⁻¹ h⁻¹ in the gas–membrane–gas mode, which was 4.9 times higher than that on the Pd/UiO-66-NH₂ particles. Furthermore, the bimetallic PMOF-RuFe(OH) was prepared by immobilizing mono-iron hydroxyl sites in MOFs by Schröder et al.¹³⁴ The photocatalytic processes were triggered by photogenerated electron transfer from the excited state PS* of [Ru(bpy)₂(bpydc)]* to generate the electron-rich PW9V3IV/V. Then it drove the photoreduction of O₂ to H₂O₂via proton-coupled (from H₂O) electron transfer (from the reduced PW9VIV/V3). Intriguingly, the bimetallic catalyst PMOF-RuFe(OH) can rapidly convert the in situ formed H₂O₂ to ·OH radicals facilitated by the {Fe–OH} fraction and subsequently achieved the conversion of CH₄ to CH₃OH with 100% selectivity in the presence of H₂O and O₂. The restricted mono-iron hydroxyl sites in MOFs were converted to CH₃OH by the formation of the [Fe–OH⋯CH₄] intermediate to CH₄, thus lowering the barrier to C–H bond activation. The PMOF-RuFe(OH) framework provided a stable platform for such a H₂O₂-coupled reaction, which could achieve higher value in energy conversion.

Fig. 8 (a) Schematic illustration of the SA/MOF membranes. Adapted with permission from ref. 133. Copyright 2021, Springer Nature. (b) Calculated oxygen adsorption energies on the Co atom and N atom of CoPc-BTM-COF. (c) Free energy diagrams for 2e⁻ (orange line) and 4e⁻ (cyan line) ORR processes on CoPc with optimized configurations for each step. Adapted with permission from ref. 135. Copyright 2022, American Chemical Society. (d) Schematic diagram of the photocatalytic H₂O₂ mechanism on TAPT-TFPA@Pd ICs. Adapted with permission from ref. 136. Copyright 2023, American Chemical Society. (e) Photocatalytic activity of Cu₃-BT-COF, Cu₃-pT-COF and TFP-BT-COF for H₂O₂ photosynthesis. (f) Schematic diagram of the proposed reaction mechanism for H₂O₂ photosynthesis. Adapted with permission from ref. 137. Copyright 2023, Wiley-VCH.

For COFs, achieving a highly active and selective 2e⁻ oxygen reduction reaction remains the key to efficient H₂O₂ production. Jinag et al. obtained two piperazine-conjugated COFs by nucleophilic substitution reactions with 1,2,4,5-benzenetetramine (BTM) or 3,3′-diaminobenzidine (DAB), namely, CoPc-BTM-COF and CoPcDAB-COF.¹³⁵ Both COFs exhibited excellent light absorption capacity in the visible range and enhanced photo-induced charge separation and migration efficiencies, resulting in an H₂O₂ yield of 2096 μmol g⁻¹ h⁻¹ and an apparent quantum yield of 7.2% at 630 nm. As shown in Fig. 8b, the oxygen adsorption energy on the Co atom of CoPc-BTM-COF was −0.40 eV with a 1.88 Å Co⋯O, which was much smaller than that of the N site (+2.01 eV, N⋯O distance of 2.66 Å), suggesting that the Co atom in CoPc-BTM-COF has the active site nature of the ORR. In addition, DFT demonstrated the good selectivity of CoPc-COFs for H₂O₂ production by the 2e⁻ ORR, as the activation barrier of CoPc for the 2e⁻ ORR was 0.97 eV compared to 1.27 eV for the 4e⁻ ORR, resulting in faster kinetics of the reaction of the 2e⁻ ORR to CoPc-COFs (Fig. 8c).

Palladium metal-isolated clusters act as an intermediate between nanomaterials and single atoms, which exhibit excellent activity and selectivity in the photocatalysis of H₂O₂ production due to their highly overlapping electronic orbitals. It is an effective way to optimize their poor stability by confining them in COFs. It can be clearly seen that, as shown in Fig. 8d, Guo et al. strongly confined Pd clusters in fluorinated COFs by a strong metal–carrier interaction (MSI),¹³⁶ significantly improving the stability (over 100 h) and activity (2143 μmol g⁻¹ h⁻¹) of H₂O₂ production, which can be attributed to the fact that the strong electronegativity of fluorine in COFs can not only enhance MSI, but also optimize the d-band center of Pd.

The combination of H₂O₂ photosynthesis and bio-value addition not only maximizes energy efficiency but also enables the production of value-added products. To this end, Lan et al. prepared a series of redox molecular COFs with Cu-containing units (Cu₃-BT-COF, Cu₃-pT-COF, and TFP-BT-COF) and investigated the coupling of H₂O₂ photosynthesis to the preparation of furoic acid (FA) from the photo-oxidation of furfuryl alcohol (FFA).¹³⁷ Cu₃-BT-COF, constructed by the covalent linkage of Cu₃ with the thiazolyl group (BT), was a newly designed COF with nanosheet morphology (∼1.25 nm), which exhibited the best performance for the photocatalytic coupling reaction. Among them, the FA generation efficiency was up to 575 mM g⁻¹ (∼100% conversion and >99% selectivity), while the H₂O₂ generation rate was up to 187 000 μM g⁻¹, which was much higher than that of Cu₃-pT-COF, TFP-BT-COF, and their monomers, as shown in Fig. 8e. And as seen in Fig. 8f, the covalent linkage between Cu₃ and BT in the Cu₃-BT-COF enabled the visible-light-driven electrons to be efficiently transferred from Cu₃ to the BT portion, resulting in the generation of photoexcited electrons (on BT) and holes (on Cu₃) that can be used for H₂O₂ production and FFA photo-oxidation reactions, respectively. Significantly, the system was highly versatile and can be extended to other alcohol photo-oxidation systems, such as H₂O₂ photosynthesis coupled with 2-thiophene methanol photooxidation. This opens the way for work on COFs for H₂O₂ photosynthesis with biomass value addition.

4. Outlooks and challenges

In conclusion, the integration of reticular framework materials- MOFs, COFs, and HOFs-into photocatalysis has sparked unprecedented interest, offering immense potential for sustainable and clean energy conversion. The unique characteristics (Table 1) of reticular frameworks promise enhanced photocatalyst efficiency, addressing existing challenges and propelling the field forward. These materials provide a versatile platform for designing catalysts with improved selectivity and activity. Looking ahead, the outlook for reticular frameworks containing metal sites in energy-related photocatalysis is promising. Ongoing research aims to optimize the design, structure, and composition for enhanced efficiency and selectivity. The customization of metal sites, pore structures, and surface properties holds potential for materials tailored to specific catalytic processes. Emphasizing sustainability, these materials can contribute to eco-friendly energy conversion processes and promote a greener future. The development of multifunctional reticular frameworks for integrated and efficient energy conversion systems is anticipated. Collaboration across disciplines is crucial for innovative breakthroughs and potential commercialization. Reticular framework materials containing metal sites stand poised to revolutionize energy-related applications through efficient and sustainable photocatalysis processes, marking a significant stride toward a cleaner and greener future. However, as illustrated in Fig. 9 and Table 1, the field is not without its current limitations and challenges that warrant attention and further research.

Table 1 The advantages and disadvantages of reticular framework materials for photocatalytic energy conversion

Reticular framework materials
Advantages	Disadvantages
Large specific surface area	Synthetic complexity
Adjustable hole size	Relatively expensive
Easy to functionalize	The structure of multi-component frames is difficult to confirm
Structural design in advance	Sacrificial agents are usually required in photocatalytic reactions

---

Fig. 9 Current limitations and challenges of reticular framework materials.

1. Stability concerns: many reticular frameworks, particularly certain MOFs, may face challenges related to stability under the rigorous conditions of photocatalysis. Exposure to high temperatures, corrosive environments, or extended reaction times can lead to framework degradation, affecting both catalytic activity and long-term performance. Developing strategies to enhance the stability of reticular frameworks without compromising their photocatalytic efficiency remains a critical challenge. The exploration of novel materials or modification techniques to mitigate stability issues is an area requiring dedicated investigation, such as, the structure of MOFs should be carefully designed to avoid easily destructive groups so that the frameworks are difficult to destroy under extreme conditions.

2. Mass transfer limitations: the inherently porous nature of reticular frameworks, while advantageous for providing a high surface area, can pose challenges related to mass transfer limitations. In some instances, small pore sizes or intricate frameworks may impede the efficient diffusion of reactants to active sites, influencing overall reaction rates. Addressing mass transfer limitations necessitates innovative designs or post-synthetic modifications that maintain the structural integrity of the framework while optimizing pore size and accessibility. Developing frameworks with larger pores or hierarchically structured materials, or adding metal sites or functional groups with greater adsorption capacity for transition states to enhance mass transfer accessibility, may be potential solutions.

3. Synthetic complexity: the synthesis of certain reticular frameworks, especially COFs and HOFs, can be intricate and time-consuming. Precise control over reaction conditions, extended reaction times, and the need for specialized equipment contribute to the challenges associated with large-scale production. Furthermore, precise control over the synthesis of these frameworks with specific metal sites is essential but can be challenging. Achieving uniform distribution of metal sites and controlling their coordination environment remains a key hurdle. Modulation of the microenvironment of the metal sites through the use of different ligands, followed by more efficient photogenerated charge segregation and migration is worthy of further investigation.

4. Limited active sites: some reticular frameworks may exhibit limitations in the density of active sites available for catalysis. This can impact overall photocatalytic efficiency and may require optimization for specific reactions, which is a benefit for achieving more efficient photocatalytic systems and high selectivity for desired products.

In conclusion, the potential of reticular framework materials in advancing photocatalysis is considerable. However, addressing the current limitations and challenges is imperative for the progression of reticular framework materials containing metal sites in energy-related photocatalysis. Continuous research and innovation are essential to surmount these obstacles and fully unleash the capabilities of these materials in sustainable energy technologies.

Author contributions

The conceptualization was carried out by S. W., W. G., J. L., and H. Z.; the formal analysis was conducted by C. L., B. W, S. L., N. H., and H. Z.; the original draft preparation was done by S. W., N. H. and H. Z.; the review and editing were carried out by W. G., S. L., J. L., and H. Z.; the supervision was done by W. G., J. L., and H. Z.; the funding was provided by H. Z.; all authors have read and agreed to the published version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the Youth Foundation of the National Natural Science Foundation of China (No. 52203300), Zhejiang Provincial Key Laboratory (No. 2020E10018), 2023 Ningbo Natural Science Funding Commonweal Programme Major Project (No. 2023S019), and the Zhejiang Provincial Natural Science Funding Major Project (No. LDT23E06011E06).

Notes and references

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