Recent advances in rational design, synthesis and application of metal–organic frameworks as visible-light-driven photocatalysts

Abstract

The development of green renewable energy sources such as solar energy has become a focal point of research in addressing energy shortages and environmental pollution. Metal–organic frameworks (MOFs) as a novel photocatalytic material have garnered widespread attention due to their highly ordered porous structure and large surface area. This paper comprehensively reviews the advancements in MOF photocatalysts under visible light over the past five years; it provides concepts and directions for readers entering the field of MOF photocatalysis, covering construction, optimization and application. It specifically focuses on applications, including organic transformations, pollutant degradation, CO₂ reduction, N₂ fixation, and H₂ production. Finally, the future development prospects are also discussed to meet the requirements for using MOF photocatalysis as a low-cost, stable technology suitable for practical applications.

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Xu-Sheng Li

Xu-Sheng Li was born in Shannxi Province, China, in 1999. She received her B.S. degree in 2021 from Xi'an Technological University, China. She is now pursuing her M.D. degree under the supervision of Prof. Ping Liu (Northwest University, China). Her research interests focus on the design and preparation of photocatalytic MOFs.

Yu-Jie He

Yu-Jie He was born in Gansu Province, China, in 2000. She received her B.S. degree in 2022 from Zhengzhou University, China. She is now pursuing her M.D. degree under the supervision of Prof. Ping Liu (Northwest University, China). Her research interests focus on photocatalysis functional MOFs, and MOFs for photocatalytic organic reactions.

Jiao Chen

Jiao Chen obtained her B.S. degree (2016) in Applied Chemistry from Southwest University, and she received her Ph.D. (2021) in Organic Chemistry from Northwest University under the supervision of Prof. Jianli Li and Prof. Shengyong Zhang. She currently performs postdoctoral research in the College of Life Sciences at Northwest University. Her research interests focus on developing near-infrared fluorescent sensors for biomedical sensing, diagnosis, and treatment.

Quan-Quan Li

Dr Quan-Quan Li was born in Jiangsu, China, in 1990. She received her B.E. degree in 2013 from the Yancheng Institute of Technology and then her Ph.D. degree from Northwest University, China. In 2019, she joined the group of Prof. Jin-Xi Song (Northwest University, China) as a postdoctoral researcher, and now she is a lecturer at Yulin University. Her research interest focuses on the design and synthesis of photocatalytic MOFs and their functional applications.

Ping Liu

Prof. Ping Liu was born in Shandong Province, China. She obtained her Ph.D. degree in 2005 from Northwest University, China (Supervisor: Yao-Yu Wang). She joined the group of Prof. Zhensen Wu at Xidian University as a post-doctoral researcher (2006–2010), and in 2011, she was a visiting scholar at the University of Strasbourg, France. Now she is a full professor at Northwest University. Her research interest focuses on the rational design and synthesis of coordination polymers and their functional applications.

Jian-Li Li

Jianli Li studied chemistry at Northwest University and completed his Ph.D. (2007) in Organic Chemistry under the supervision of Prof. Zhen Shi. Then he did postdoctoral research at Northwest University in Prof. Guifang Zhao's group. He is currently working as a Full Professor at the College of Chemistry & Materials Science, Northwest University. His research expertise covers a broad spectrum of different fields of synthetic chemistry, including the design and development of photofunctional materials for biological sensing and theranostics, exploration of new synthetic methods for biomedical molecules, design and application of new catalytic materials, and photocatalytic energy and resource chemistry.

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1. Introduction

According to estimates by the International Energy Agency (IEA), the demand for fossil fuels has accounted for 81% of the world's total energy demand over the past few decades. Fossil fuels are non-renewable energy sources, and their combustion releases large amounts of CO₂, CO, and SO₂.¹ To alleviate the energy shortages and environmental pollution caused by high demand, developing green energy with a sustainable development concept has become a research hotspot. Solar energy, as a green and renewable energy source, offers new solutions to existing energy and environmental problems. However, the key challenge is how to efficiently and rationally utilize solar energy to convert it into energy required by humans under mild conditions.^2–4

Inspired by natural photosynthesis, in 1972, Fujishima et al. used single-crystal n-type TiO₂ (rutile) semiconductor electrodes to achieve the photolysis of water under UV irradiation, making the first breakthrough in the conversion of light energy to chemical energy.^5,6 TiO₂ can only absorb ultraviolet light, which constitutes about 7% of the total solar radiation, thus limiting the photocatalytic efficiency, due to its narrow absorption spectrum. In recent years, traditional visible light photocatalysts such as g-C₃N₄, CdS and Fe₂O₃ have been extensively developed and applied. To overcome the limitations of traditional materials and combine the advantages of existing materials, it is crucial to develop visible-light photocatalysts that have a broad light-response range.^7–19

In 2012, Li et al. first reported the photocatalytic reduction of CO₂ under visible light using NH₂-MIL-125(Ti).^23,24 Since then, MOF-based visible-light-driven photocatalysts have flourished.²⁵ MOFs are a class of porous complexes, typically formed by metal nodes (such as ions or clusters) linked with organic ligands (Fig. 1), and characterized by highly ordered porous structures and large surface areas, offering abundant active and adsorption sites. Their inherent narrow band gaps and high stability have sparked significant research interest.^20–22

Fig. 1 The MOF structure is created through the bonding of metal ions as nodes and organic molecules as linkers.

However, due to the poor light absorption capacity and the low efficiency with electron–hole separation of traditional MOFs, their practical application is limited. Therefore, some effective methods to improve performance have been proposed, including the modification of metal sites, functionalization of ligands, addition of various species (such as dyes and metal complexes) and construction of heterojunctions.^26–29 The number of publications on MOF photocatalysis has rapidly increased over the past five years, reaching over a thousand by 2023 (Fig. 2). Notable advances have been made in several fields, including organic transformations, H₂ production, pollutant degradation, CO₂ reduction and N₂ fixation.^30–34

Fig. 2 Publications trends of metal–organic framework (MOF) based photocatalysts. Publication results were obtained from Web of Science with the topic of “metal organic framework” and “photocatalysis” by the end of December 2023.

Although numerous reviews on MOF photocatalysis have been summarized, most of them focused on a single or a narrow application scope. To address this challenge this review provides a comprehensive summary and systematic outlook on the advances of MOF photocatalysts under visible light over the past five years. It offers ideas and directions for readers entering the field of MOF photocatalysis, from “construction–modification–application”. The article is structured in two parts: (i) performance optimization of MOF photocatalysts including the modification of metal sites, functionalization of organic linkers, addition of photosensitizers, construction of heterojunctions, encapsulation of functional guest species, morphology regulation and defect engineering; and (ii) focusing on the MOF photocatalysts in organic transformations, CO₂ reduction, N₂ fixation, pollutant degradation and hydrogen production. This not only gives guidance for the logical creation and modification of MOF photocatalysis but also provides fresh perspectives on their application.

2. Modulation strategies for the performance of MOF photocatalysts

In the heterogeneous photocatalytic system, the catalyst and reactants exist in different phases, facilitating easy recovery and reuse, showcasing advantages in industrial applications. MOFs possess atomic precision, customizable structures and unique physicochemical properties, rendering them highly promising in the field of photocatalysis.³⁵ MOFs are constituted by metal ions/metal clusters in conjunction with organic ligands, linked via moderately strong coordination bonds that connect intramolecular pore structures. Polydentate organic ligands often consist of oxygen or nitrogen.³⁶ The inorganic metal clusters are generally composed of transition metal ions, including common divalent ions such as Ni²⁺, and Zn²⁺, trivalent ions such as Fe³⁺, Sc³⁺, Co³⁺, Cr³⁺, and V³⁺, and group trivalent ions such as Al³⁺, In³⁺ and certain rare earth metal ions. The first MOF was synthesized and named by Yaghi et al. in 1995. Since then, MOFs have evolved into a broader family, currently encompassing notable subclasses such as MIL, UiO, ZIF, and PCN.^37,38 In recent years, MOFs such as UiO-66, MIL-100, MIL-88A, MIL-53 and MIL-101 have been investigated as photocatalysts, exhibiting semiconductor-like properties with the capability to absorb light upon irradiation.^39,40 In 2007, Horcajada et al. first synthesized MIL-100 by Fe–O octahedral clusters and benzene-1,3,5-tricarboxylic acid.⁴¹ More recently, in 2024, Li et al. successfully synthesized a novel CS/MIL-(Fe) photocatalyst (CS = charcoal sponge), which possessed a large surface area, excellent mechanical properties, and significantly enhanced photocatalytic performance.⁴² MIL-101 is also one of the representative MOFs responsive to visible light, a characteristic ascribed to the spin-allowed d-d transition of Cr³⁺.

However, issues such as the rapid recombination of photoinduced electron–hole pairs in traditional MOF heterogeneous catalysts can lead to low photocatalytic efficiency. To further enhance the efficiency and expand the application scope, understanding the specific processes of photocatalysis is beneficial and may aid in addressing the aforementioned issues.

The photocatalytic mechanism included a sequence of consecutive steps, including at least: (i) the absorption of light, (ii) the generation of photogenerated electron–hole pairs occurs at the site where the energy of the photon is absorbed, (iii) the photogenerated electron–hole pairs’ migration or recombination, (iv) on the surface, the photocatalyst adsorption of reactants, (v) occurrence of spatially separated redox reactions by the photogenerated electron–hole pairs on the active sites of the photocatalyst, and (vi) product desorption.^43–51 It is worth noting that molecular oxygen O₂ could be activated by the holes and electrons of photocatalysts, resulting in the production of various reactive oxygen species (ROS), e.g. superoxide anion radical (˙O₂⁻), hydrogen peroxide (H₂O₂), singlet oxygen (¹O₂), and hydroxyl radical (˙OH). These generated ROS can further act as important intermediates for advanced oxidation.⁵² In principle, by controlling the band gap to increase light absorption and reduce the recombination rate of photogenerated carriers, the photocatalytic activity of catalysts can be enhanced. The required functionalities can be introduced through direct synthesis or post-synthetic modification (PSM) strategies.^23,53,54

Recently, various effective methods have been developed to enhance the photocatalytic activity of MOFs under visible light exposure. This section will outline the commonly used and effective methods to improve photocatalytic performance in recent years, including modification of metal sites, functionalization of ligands, addition of photosensitizers, control of the heterostructure construction, encapsulation with functional guest species, morphology regulation and defect engineering.

2.1. Modification of metal sites

The choice of metal nodes is crucial in the development of MOFs customized for visible-light-driven applications. In MOFs, metal nodes function in performance like single semiconductor quantum dots. The charge separation state is generated during irradiation for photocatalysis. Consequently, choosing different metal nodes can modify the activity of MOF-based photocatalysts. These metal ions play a role in adjusting the electronic structure of MOFs, influencing parameters such as, for instance, bandgap, conductivity, and band position, and even promoting the separation of charges between excited-state ligands and metal ions, in addition to extending the lifetime of charge carriers.⁵⁵ Additionally, modifying the local structure of MOF base materials is effective in optimizing the charge transfer process. The incorporation of metal ions into this structural optimization strategy also results in metal-to-metal cluster charge transfer (MCCT), enhancing spatial electron–hole separation and simultaneously suppressing charge recombination. This also creates additional active sites.^56,57

Zhang et al. synthesized four homostructural porphyrin MOFs, namely UNLPF-10a, UNLPF-10b, UNLPF-11 and UNLPF-12, composed of free base, In^III-, Sn^IVCl₂- and Sn^IV-porphyrins; the structure of the porphyrin ligand is shown in Fig. 3.⁵⁸ Among these, UNLPF-12 demonstrated outstanding photostability and highly efficient photocatalytic activity in several significant organic transformations. The metalation of high-valent metal cations such as In^III and Sn^IV significantly transforms the electronic configuration of porphyrin macrocycles (Fig. 4A). This process resulted in the creation of a highly oxidized photoexcited state, which can undergo effective reductive quenching processes. These processes effectively enabled a range of organic reactions, encompassing aerobic hydroxylation of arylboronic acids, amine coupling reactions and Mannich reactions (Fig. 4B).

Fig. 3 Synthesis of porphyrinic MOFs from ref. 58. Copyright 2015, American Chemical Society.

Fig. 4 (A and B) Controlling the charge density of UNLPF-10a, -10b, -11, and -12 via the metalation of porphyrins; charges are shown with respect to cage occupancy in the overall framework. Schematic representation of four isomorphous porphyrin-based MOFs catalyzing a series of organic transformations from ref. 58. Copyright 2015, American Chemical Society. (C and D) Under visible light, DMPO was used to detect the EPR signals of ˙OH and ˙O₂⁻ generated by the trapping agent and the possible degradation mechanism from ref. 60. Copyright 2022 Elsevier Ltd.

Iron, an abundant element on Earth, possesses some degree of visible light absorption capability.⁵⁹ Yang et al. synthesized Fe-BDC₁ (ligand = terephthalic acid (H₂BDC)) through a straightforward stirring method;⁶⁰ compared with Fe-BDC₂ synthesized by the solvothermal method, the adsorption capacity for RhB (rhodamine B) increased by approximately 5.85 times. In Fig. 4C, the characteristic peak signal of Fe-BDC₁ can be clearly observed, while the peak intensity of Fe-BDC₂ is negligible. The improved catalytic activity of Fe-BDC₁ is mainly attributed to the expansion of its specific surface area (SSA) and the promotion of efficient electron separation and transfer processes, thereby accelerating the ≡Fe(II)/≡Fe(III) cycle and increasing the yield of ROSs. The possible degradation mechanism was proposed as follows (Fig. 4D): firstly, pollutants were adsorbed on the surface or within the pores of the MOF. Due to Lewis acid–base interactions, H₂O₂ rapidly coordinated with Fe CUSs. Meanwhile, under visible light irradiation, electrons and holes were effectively separated, with the separated electrons migrating along the Fe–O bond, leading to the reduction of ≡Fe(III). Subsequently, internal electron transfer between ≡Fe(III) and the coordinated H₂O₂ initiated the Fenton reaction, generating ROSs. Thereafter, the adsorbed organic pollutants were primarily degraded rapidly by ˙OH.

For instance, the incorporation of high-valent cations can also enhance photocatalytic performance. For example, Liu et al. introduced Ti⁴⁺ into Co(II) clusters, to serve as heterometallic nodes, which significantly enhanced the stability and high activities for the photocatalytic water oxidation of Co-MOF photocatalysts.⁶¹ A stepwise synthetic strategy was employed to first synthesize a trinuclear [Co₂Ti(μ₃-O)(COO)₆] metal cluster as shown in Fig. 5A followed by assembling of the cluster with visible light-responsive ligands (ligand = 3,3′,5,5′-azobenzenetetracarboxylic acid (ABTC)) into MOFs through a solvothermal process. This resulted in a novel single-site bimetallic Co-MOF (namely PFC-20-Co₂Ti) (PFC = porous materials from FJIRSM, CAS); the constructed PFC-20-Co₂Ti is isomorphic to PCN-250, serving as an efficient and reusable photocatalyst for the water oxidation of O₂. The incorporation of single-site Ti(IV) sites at the nodes modulated the charge distribution, and enhanced the valence state of the bimetallic nodes, endowing MOFs with strong coordination ability and improved catalytic kinetics. This step-by-step synthesis approach was also applicable to other transition metals (such as Mn and Ni) for preparing bimetallic MOF catalysts with water stability and activity. Yang et al. installed high-valent W⁶⁺ ions onto the Ti-oxo clusters of MIL-125(Ti) (ligand = H₂BDC).⁶² Without disturbing the crystal structure formation of the MOF substrate, the holes attracted by high-valence W⁶⁺ ions combine with the Ti-oxo clusters, serving as electron donors in the catalytic system. The resulting structure facilitated the metal-to-cluster charge transfer (MCCT), achieving a spatial separation of electrons and holes, further enhancing light absorption, suppressing charge recombination, promoting interface reactions and lengthening the lifetime of the photogenerated charge carriers. Improved H₂ production performance was seen. Jiang et al. installed Fe³⁺ onto Zr-oxo clusters, to synthesize a representative Fe-UIO-66 (ligand = H₂BDC) by a microwave-assisted approach.⁶³ The introduction of Fe³⁺ activated the “inert” MOF for photocatalytic C–H activation. Pan et al. selected ZIF-8 as an ideal model for metal ion doping which was cheap and was synthesized easily.⁶⁴ Co²⁺, Ni²⁺, or Cu²⁺ as photoreactive metal ions were added into the ZIF-8 metal nodes, resulting in a suite of M_xZn_y-ZIF (ZIF = ZIF-8, M = Co, Ni, Cu, ligand = 2-methylimidazole (2-MeIM)) (Fig. 5B). With Co(II) doping of the metal ion sites, the type of photogenerated ROS transitioned from the initial singlet ¹O₂ to multiple ROS like ¹O₂, ˙O₂⁻, which can play an important role in the field of photocatalysis.

Fig. 5 (A) The crystal structure of PFC-20 from ref. 61. Copyright 2021, Chinese Chemical Society. (B) Schematic illustration of the crystal structures of ZIF-8 and M/Zn-ZIFs (M = Co, Ni, Cu) from ref. 64. Copyright 2022, Elsevier Ltd.

Compared with monometallic catalysts, the introduction of a second metal alters the interfacial electronic structure and charge distribution. Jiang et al. combined Cu and Ni DMSPs (dual-metal-site pairs) in their respective single-site forms into an MOF to obtain MOF-808-CuNi (ligand = 1,3,5-benzenetricarboxylic acid (H₃BTC)) in a flexible microenvironment. The resulting MOF-808-CuNi exhibited a high CH₄ yield (158.7 μmol g⁻¹ h⁻¹).⁶⁵

Zhong and Wang et al. made a series of isostructural Co_xNi_y-MOFs based on 4,5-dicarboxylic acid (H₃IDC) and 4,4′-bpy, which were synthesized for photocatalytic CO₂ reduction.⁶⁶ X-ray diffraction (XRD) patterns demonstrated that mixed MOFs maintained similar structures to that of monometallic MOF (Fig. 6A). In Fig. 6B, comparing the energy-band of Co-MOF, Co_xNi_y-MOFs and Ni-MOF, the conduction band potentials of Co_xNi_y-MOFs are more negative than the redox potential of photocatalytic CO₂ reduction to CO (−0.52 V vs. NHE), which meets the thermodynamic requirement of CO₂–CO conversion. Compared with monometallic catalysts, the addition of a second metal altered the surface electronic structure and charge distribution, thereby affecting the photocatalytic performance.

Fig. 6 (A) XRD patterns and (B) the band structures of Co_xNi_y-MOFs from ref. 66. Copyright 2021, Elsevier B.V. (C) Schematic illustration of the preparation of 66-IS-M and its photocatalytic mechanism from ref. 67. Copyright 2023, American Chemical Society. (D and E) The emergence of a novel lowest unoccupied crystal orbital (LUCO) in a heterometallic Ce–Fe MOF leads to a reduced transition energy, in comparison with the homometallic Ce and Fe MOFs. Degradation of synthetic materials compared with TiO₂ from ref. 68. Copyright 2020, John Wiley & Sons, Ltd. (F) UV–vis and picture inset. (G) Schematic diagram of valence change from ref. 69. Copyright 2021, Elsevier B.V.

Yang et al. prepared catalysts by anchoring metal ions (Co, Ni, Cu) in NH₂-UiO-66 through the covalent coupling of isatin-Schiff basemetal complexes with microporous structures (Fig. 6C).⁶⁷ During the reaction process, the anchored metal ions serve as active sites and exhibit high activity. Then, Pichiah Saravanan et al. synthesized a structurally stable 3d–4f heterometallic compound constructed with transition metal (Fe) and one lanthanide metal cluster (Ce) linked with a photon-harvesting organic ligand coordination polymer through a solvothermal approach.⁶⁸ In comparison with homometallic Ce-MOF or Fe-MOF, the Ce–Fe MOF (ligand = 2-amino-1,4-benze-nedicarboxylic acid) exhibited exceptional light absorption capability. As shown in Fig. 6D, this enhanced activity is credited to the close intermingling of two distinct metal clusters in a heterometallic setting. Resulting in the creation of low-energy conduction orbitals, this process effectively decreased the transition energy. Therefore, the Ce–Fe MOF achieved a remarkable degradation rate of 94.6% for Acetaminophen (PCM), and the degradation of PCM is shown in Fig. 6E.

The coordination of heteroatoms and the mixing of valence states can also enhance transitions such as visible-light-driven MMCT. This effectively optimized the band structure and charge transfer dynamics of the metal clusters. Furthermore, the catalytic pathways may also be favorably modulated. Wang et al. synthesized an O,N co-ligated mixed valence state Fe(II/III) MOF named BIT-108 (ligand = 3′,5′-di(1H-1,2,4-triazol-1-yl)-[1,1′-biphenyl]-3,5-dicarboxylic acid (H₂DTzBPhDC)) by partially oxidizing the ions of the Fe(II)₂N₂O₁₀ clusters.⁶⁹ Using the Fe(II/III) MOF as a catalyst and platinum as a co-catalyst, a 25-fold increase in the visible light-induced H₂ production rate was achieved compared with the original Fe-MOF. The parent iron(II) framework BIT-107 (Fe) undergoes ageing in water to form high-spin iron(III), color from Fe(III) orange to green, resulting in mixed-valence iron nodes. BIT-108 (Fe) has an absorption spectrum spanning 400–700 nm and its excited state is capable of reducing protons in solution. Mechanistic studies indicated that the catalytic action is caused by the transfer of electrons from the sacrificial agent to the Fe(III) center at the nodes (Fig. 6F–G).

Optimization can also be achieved by adjusting the valence states of metal sites. Pang et al. used cellulose acetate (CA), a natural polymer, which is rich in electron-donating oxygen-containing groups.⁷⁰ These groups such as hydroxyl and acetyl groups exhibit strong coordination capabilities. Integrating CA into the visible-light-responsive PCN-222 (ligand = (4-carboxyphenyl)-porphyrin (H₂TTPP)) resulted in the formation of interface-optimized Zr-MOF composite materials via a simple one-pot reaction. In the synthesized composite material, the oxygen-containing groups in cellulose formed strong coordination interactions with the Zr⁴⁺ ions of Zr-MOFs. The oxygen atoms on the coordinating groups have a strong electron-donating capability, enabling them to transfer their electrons to the nearby Zr⁴⁺ ions at the interface of the composite material, resulting in a lower valence state (Zr³⁺) and a reduced bandgap, which is advantageous for photoelectron transfer.

2.2. Functionalization of organic linkers

While the regulation of metal sites plays a significant role in the catalytic process, precisely modulating the structure of the ligands can greatly influence the photocatalytic activity.⁷¹ When visible light irradiates MOFs, the organic linkers act as light-absorbing antennas. The variation of organic linkers within MOFs affects the efficiency of light absorption and charge transfer. Therefore, the functionalization of organic linkers by optimizing the LMCT (ligand-to-metal charge transfer) mechanism profoundly influences the stability of MOF frameworks, light collection capabilities and catalytic site performance.⁷² Introducing visible-light-responsive organic ligands such as pyridyl, porphyrin, and organic chromophores into the MOF can alter its ability to absorb visible light.^73,74 Additionally, the optical properties of the MOF can be modified by introducing substituents with different electron-donating and electron-withdrawing abilities. The mixed linker strategy can effectively narrow the band gap and expand the light absorption range. Furthermore, selectively removing ligands from mixed-ligand MOFs through thermal decomposition to introduce hierarchical porosity is an effective method.^75,76

Wang et al. introduced a novel MOF photocatalyst utilizing bipyridine (ligand = 9,10-bis(4′-pyridylethynyl)-anthracene (BPEA)) as a pillared ligand used in visible-light harvesting.⁷⁷ This Zn-MOF, named NNU-36, was synthesized using a controlled pillar-layer method (Fig. 7A–C). Under visible light exposure, NNU-36 demonstrated remarkable efficiency in reducing and degrading Cr(VI) in an aqueous solution. The success of NNU-36 underscores the feasibility of incorporating ideal visible light-responsive moieties into MOFs, offering promising prospects for photocatalytic applications.

Fig. 7 (A–C) Structure of NNU-36; the layer structure is composed of BPDC ligands and zinc dimers; the BPEA ligand binding to the Zn-N bonds in the layer structure to form NNU-36 from ref. 77. Copyright 2017, American Chemical Society. (D) Porphyrin MOF photocatalysis water decomposition to produce hydrogen from ref. 78. Copyright 2018, American Chemical Society. (E) Crystal structures and coordination spheres of PCN-601 from ref. 79. Copyright 2020, American Chemical Society. (F) Schematic illustration of LTG-NiRu from ref. 80. Copyright 2023, American Chemical Society.

As a crucial component in natural light energy conversion, porphyrins exhibit distinctive photophysical properties owing to their unique π-conjugated structure. Furthermore, MOFs constructed with porphyrin rings offer versatile tunability in optical and catalytic properties, driving extensive research interest in these materials.

In 2018, Jiang et al. presented a groundbreaking study on MOF photocatalysis leveraging the distinct behaviour of metal porphyrins.⁷⁸ Employing In(OH)₃ as a precursor, they synthesized USTC-8(In) (ligand = tetra(4-carboxyphenyl)porphyrin (H₂TCPP)), a highly stable MOF based on out-of-plane (OOP) porphyrins, through the controlled release of metal ions. Under visible light irradiation, In(III) ions on the OOP porphyrin were reduced and easily detached, preventing back electron transfer and enhancing photocatalytic performance. In the structure, indium ions both form indium-oxygen carbonyl chains and metalate the porphyrin ring in situ, positioned above the porphyrin plane, providing a unique structure. USTC-8(In) demonstrated superior photocatalytic activity in H₂ production compared with the structurally similar USTC-8 and USTC-8(0.2In), highlighting its high stability and unique porphyrin structure (Fig. 7D). Then, In 2020, Cao et al. synthesized a Ni-MOF named PCN-601 (ligand = 5,10,15,20-tetra(1H-pyrazol-4-yl)porphyrin (H₄TPP)), incorporating pyrazole and porphyrin units, which combined light-harvesting capabilities, active catalytic sites and a substantial surface area.⁷⁹ PCN-601 has been proved to be an outstanding and highly durable photocatalyst, attaining room-temperature, visible light-powered full CO₂ reduction through water vapor (Fig. 7E). Kinetic studies indicated that the robust coordination provided by pyrazole groups and Ni-oxo clusters endowed PCN-601 with an optimal band alignment, which in turn promotes rapid electron transfer from the ligands to the nodes. PCN-601 exhibited CO₂-to-CH₄ generation rates surpassing those of similar carboxylic acid-based porphyrin MOFs and classical Pt/CdS photocatalysts by over 3 and 20 times, respectively.

Chen et al. ingeniously combined the intrinsic benefits of mesoporous MOFs with the [Ru(Phen)₃]²⁺ photosensitizers in a heterometallic Ni(II)/Ru(II) mesoporous MOF (LTG-NiRu, ligand = H₃L^Ru).⁸⁰ This integration involved linking pre-modified metal ligands with nickel metal centers. LTG-NiRu boasted a sturdy framework and larger one-dimensional channels, allowing the [Ru(Phen)₃]²⁺ to anchor onto the inner walls of the mesoporous MOF channels. This configuration effectively addressed the challenges of product/catalyst separation and catalyst recovery in multiphase systems. Notably, LTG-NiRu exhibited outstanding performance for aerobic oxygenation (Fig. 7F).

The modification of organic linkers (such as introducing new functional groups) can adjust the band gap in MOFs.^81–83 Typically, electron-donating substituents induce a redshift in the light absorption of MOFs, thereby enhancing their photocatalytic activity. For instance, Husam N. Alshareef et al. utilized a straightforward stirring hydrothermal method to fabricate 1D titanium phosphonate MOF photocatalysts.⁸⁴ The homogeneous incorporation of organic phosphine linkers reduced the bandgap. This is attributed to the remarkable electron-donating ability of the –OH functional group, which effectively shifts the top of the valence band, and then tunes light absorption to fall within the visible spectrum. Additionally, the unique one-dimensional nanowire topology enhanced the transport and separation of photoinduced charge carriers. Consequently, under visible and full-spectrum simulator irradiation, phosphonic acid titanium nanowires exhibited significantly enhanced photocatalytic hydrogen evolution activity. At the same time, Shang et al. achieved a groundbreaking advancement by functionalizing hierarchical porous ZIF-8 nanocrystals through a simple thermal treatment in air at temperatures as low as 200 °C.⁸⁵ This transformation converted the ZIF-8 nanocrystals from conventional ultraviolet-driven photocatalysts to novel broad-spectrum-driven photocatalysts. The thermal treatment not only led to a significant increase in surface area but also broadened the light absorption range from ultraviolet to infrared. The formation of isocyanate groups (–N=C=O) in the thermally treated ZIF-8 (ZIF-8-T) was identified as the crucial factor enabling ZIF-8-T to exhibit outstanding photocatalytic performance in the degradation of formaldehyde.

Jiang et al. reported a titanium-based MOF, MIL-125, for its ability to perform dark photocatalytic hydrogen production.⁸⁶ By introducing various functional groups onto the ligand, distinct MOFs were synthesized (MIL-125-X, X = NH₂, NO₂, Br), each exhibiting markedly different activities (Fig. 8A). The electron-donating or electron-withdrawing nature of these functional groups influenced the electron storage capacity and lifetime of Ti³⁺ intermediates under light exposure, resulting in varied dark photocatalytic activities among the MOFs with different functional groups. Among these, MIL-125-NH₂ displayed superior activity compared with MOFs with other functional groups. Next, Wang et al. conducted a significant study where a range of MIL-101-X(Fe) MOFs (X = –H, –NH₂, –OH, –OCH₃) with different substituents was synthesized successfully.⁸⁷ These MOFs were then subjected to adsorption-degradation experiments involving various antibiotics. The innovative approach involved altering the substituents within the side chains of MIL-101(Fe), resulting in changes in particle morphology (Fig. 8B) and active surface properties, thereby significantly enhancing their efficacy in removing stubborn pollutants. This research provided mechanistic insights into modulating Fenton-like reactions by manipulating substituents on the side chains of MOFs.

Fig. 8 (A) Showing the dark photocatalysis over MIL-125 and MIL-125-X (X = NH₂, NO₂, Br) from ref. 86. Copyright 2022, Royal Society of Chemistry. (B) Schematic illustration of the synthesis of Fe-based MOFs from ref. 87. Copyright 2023, Elsevier B.V. (C and D) Hammett plot and possible mechanism of photocatalytic selective reduction of Cr(VI) to Cr(III) over MIL-68(In)-X from ref. 88. Copyright 2018, Elsevier B.V. (E) The synthesis of CdS/UiO–Ni–X–Y (X = S, O; Y = H, Cl, CF₃) for photocatalytic H₂ production from ref. 71. Copyright 2024, American Chemical Society. (F) Schematic illustration of the synthesis process of SCu-CZS from ref. 92. Copyright 2020, Elsevier B.V. (G) Schematic illustration of linker engineering in the synthesis of ultra-stable Ni₈-MOFs from ref. 93. Copyright 2023, American Chemical Society.

Wu et al. selected a classic indium-based MOF MIL-68(In)-X for study due to its affordability, adjustability and stability.⁸⁸ They synthesized MIL-68(In)-X (X = H, NH₂, NO₂, Br) in one step using a solvothermal method and organic linkers with different substituents corresponding to H₂BDC (–H), H₂BDC-NH₂ (–NH₂), H₂BDC-NO₂ (–NO₂) and H₂BDC-Br (–Br), to modulate electronic structure changes. The photoactivity of MIL-68(In)-X was influenced by the different substituents strongly and a Hammett-type linear free-energy relationship (LFER) was observed between the reaction rates and the electronic properties of the substituents. In other words, modifying MIL-68(In) with electron-donating substituents (like –NH₂) can enhance the photoreductive performance of Cr(VI), while substituents that withdraw electrons (like –NO₂, –Br) diminish its photoactivity (Fig. 8C and D). Jiang et al. prepared a series of sandwich-structured composites UiO-66-NH₂@Pt@UiO-66-X (X = –H, –Br, –NA (naphthalene, in short), –OCH₃, –Cl, –NO₂).⁸⁹ By varying the X groups in the UiO-66-X shell, they balanced the proton reduction capability of Pt and the charge separation of the UiO-66-NH₂ core. UiO-66-NH₂@Pt@UiO-66-H exhibited optimal photocatalytic H₂ production activity. UiO-66 which were amino-functionalized can elevate charge separation or expand the light absorption of MOFs into a visible range.^81–83,90

In addition to amino groups, thiol (–SH) groups also act as electron-donating groups, capable of effectively coordinating with metal centers (particularly noble metals), thereby creating additional unsaturated metal sites for catalysis. When thiol (–SH) groups are incorporated into the UiO-66, metal ions can be incorporated into the MOF's cavities through coordination bonds. Using thiol as a connector in UiO-66/Cd could effectively prevent CdS quantum dot aggregation, improving photocatalytic efficiency. Shi et al. initially created a UiO-66 MOF (UiO-66-(SH)₂) using thiol as a ligand, then added CdS and ultimately formed the UiO-66-(S-CdS)₂ photocatalyst.⁹¹ Well-dispersed CdS quantum dots are trapped in UiO-66-(SH)₂ pores and the sulfur bonds linking UiO-66 and CdS act as an efficient charge transfer bridge, boosting interfacial carrier transfer and hydrogen production. In 2024, Jiang et al. decorated CdS nanoparticles (NPs, 27 ± 0.3 nm) on the surface of UiO-67,⁷¹ which is assembled from Zr-oxo clusters and 2,2′-bipyridine-5,5′-dicarboxylic acid, thus forming a CdS/UiO composite material. Subsequently, the chelated N sites on the MOF linker can install single Ni(II) sites, which are further six-coordinated by different molecules to obtain CdS/UiO–Ni–X–Y (X = S, O; Y = H, Cl, CF₃), where X and Y act on the primary and secondary coordination spheres. The results indicated that the optimized CdS/UiO–Ni–S–CF₃ composite material achieved a high H₂ production rate (Fig. 8E). The increased in activity is due to the functional groups in the primary coordination atom and secondary coordination sphere, which critically regulate the charge separation kinetics and energy barriers of the rate-determining step (RDS); in particular, the primary coordination sphere has a more pronounced effect compared with the secondary coordination sphere.

Subsequently, Shi et al. loaded CdS/ZnS quantum dots onto Cu(II)-modified thiol-functionalized UiO-66 (UIOS-Cu) MOFs, as shown in Fig. 8F.⁹² CdS and ZnS quantum dots are evenly distributed within the MOF channels and on the surface, with Cu(II) ions attached to thiol groups. The HOMO of UIOS-Cu is elevated above the VB of CdS due to Cu(II) modification, promoting the efficient transfer of photoinduced holes from the VB of CdS to the HOMO of UIOS-Cu.

By adjusting the electron-accepting capacity in the D–A–D building blocks, the charge separation efficiency can be further enhanced. Li et al. reported a series of isostructural fcu-topology Ni₈-MOFs using linearly bridged dipyrazole as the organic linker (named JNU-212, JNU-213, JNU-214 and JNU-215, JNU = Jinan University).⁹³ As shown in Fig. 8G, by changing the units from benzene to benzothiadiazole, to benzoselenadiazole and to naphthoselenadiazole, their corresponding MOFs exhibited progressively improved photoelectrochemical performance. The benzoselenadiazole unit in JNU-214 exhibited the most compatible band gap and the highest ROS generation efficiency, making it a marked photocatalyst in the oxidative coupling reaction. Tuning the electron-accepting capability of pyrazole-bridging units, studies have demonstrated that through ligand engineering, the charge separation and transfer effectiveness of Ni₈-MOFs can be significantly improved. Similarly, Li et al. reported a porphyrin-based donor–acceptor (D–A) MOF consisting of a meso-tetra(4-carboxyphenyl)porphyrin (TCPP) donor and a thieno [3,2b:20,30-d]thiophene-S,S-dioxide (BTDO) acceptor via coordination with Zn²⁺ ions and denoted as TCPP-Zn-BTDO.⁹⁴ This combination enhanced the electron push–pull effect, and the formation of the donor–acceptor (D–A) structure effectively broadened the spectral absorption range, which facilitated the separation of photogenerated carriers, thus enhancing the photocatalytic performance.

Incorporating an electron-enriched skeleton into a single-ligand MOF is an effective strategy to optimize charge separation efficiency and improve photocatalytic performance.^95–97

Naphthalenediimides (NDIs) are a class of chemically active, π-electron deficient compounds, widely used in the photocatalysis field.^98–101 Jin et al. utilized a straightforward one-pot synthesis strategy to develop Zr-NDI-H₂DPBP (ligand = 5,15-di(p-benzoato)porphyrin (H₂DPBP)) as a heterogeneous photocatalyst for the aerobic oxidation coupling of anilines at room temperature (Fig. 9A).⁹⁴ Its imine production rate greatly surpasses other non-noble metal MOF photocatalysts and it can be effectively used for many aniline derivatives. Experimental and DFT computational results show that in the mixed-ligand Zr-NDI-H₂DPBP, the PET occurs between the donor H₂DPBP and acceptor NDI ligands and, along with the sensitization of H₂DPBP, significantly enhances the conversion of oxygen in the ground-state to active ˙O₂⁻ and ¹O₂, ultimately enhancing the photocatalytic activity of Zr-NDI-H₂DPBP.

Fig. 9 (A) The targeted PET model of mixed-ligand MOF Zr-NDI-H₂DPBP from ref. 94. Copyright 2023, American Chemical Society. (B) In a Zr(IV)-MOF in situ porphyrin substitution from ref. 102. Copyright 2021, Wiley-VCH GmbH. (C) Proposed mechanisms for the photocatalytic oxidative benzylamine coupling reaction over CCNU-16 from ref. 105. Copyright 2022, Royal Society of Chemistry. (D) Schematic diagram for the synthetic process of NH₂/R-MIL-125 (R = Cl, H, CH₃) from ref. 106. Copyright 2022, American Chemical Society.

Researchers have also used ligand substitution strategies to boost the photo-catalytic efficacy of MOFs while stabilizing their structure. For instance, Li et al. proposed a strategy to enhance the stability and functionality of Zr(IV)-MOFs through in situ porphyrin substitution.¹⁰² A linear porphyrin ligand, 4,4′-(porphyrin-5,15-diyl)dibenzolate (DCPP²⁻), that is size and geometry matched, was chosen to replace the NDIDB²⁻ (4,4′-(1,3,6,8-tetraoxobenzo[lmn][3,8] phenanthroline-2,7(1H,3H,6H,8H)-diyl) dibenzoate) ligand. This strategy enhanced the stability and functional linkage of BUT-109 (Zr), resulting in the BUT-110 series with varying DCPP²⁻ content (Fig. 9B). The chemical stability of the BUT-110 series was greatly enhanced and the incorporation of metalloporphyrins endowed the BUT-110 MOF with exceptional photocatalytic activity for CO₂ reduction without photosensitizers.

Carbazole, triphenylamine and porphyrin are typical multifunctional materials known for their efficient light absorption and excellent electron-donating properties.^103,104 When loaded into MOF frameworks, carbazole ligands enhance charge transfer and photocatalytic activity. Wen et al. have synthesized a new MOF, Zn₅(TPyP)(BCTC)₂(H₂O)₃ (CCNU-16), from porphyrin- and carbazole-based organic linkers, which photocatalytically oxidized amines and sulfides in visible light (Fig. 9C).¹⁰⁵ The TOF (turnover frequency) for benzylamine and thioanisole reached 121.0 h⁻¹ and 190.7 h⁻¹, respectively, Experimental results indicate that the incorporation of the H₄BCTC ([9,9′-bicarbazole]-3,3′,6,6′-tetracarboxylic acid) ligand enhances CCNU-16 activity.

The construction of dual-ligand MOFs has mainly focused on the introduction of two linkers with excellent light absorption properties, and the rational design of metal sites and organic ligands is of great significance in promoting electron transfer in dual-ligand MOFs. Jiang et al. employed NH₂-MIL-125(Ti) as a model system to enhance the photoinduced electron transfer between organic ligands and metal clusters using the dual-quantum-induced push–pull effect.¹⁰⁶ Utilizing the Hammett constant (σ_m) as a guide, they synthesized a dual-ligand MOF NH₂/Cl-MIL-125, with –NH₂ (σ_m = −0.16) and –Cl (σ_m = 0.37) chosen as the electron-donating and electron-withdrawing group, respectively (Fig. 9D). Concurrently, –CH₃ (σ_m = −0.07, electron-donating) and –H (σ_m = 0, neither electron-donating nor electron-withdrawing) were selected as reference groups to validate the electron push–pull mechanism. Under full-spectrum irradiation, NH₂/Cl-MIL-125 exhibited higher photocatalytic hydrogen evolution activity compared with single-ligand NH₂-MIL-125, NH₂/CH₃-MIL-125 and NH₂/H-MIL-125. This improvement is likely due to the electron push–pull interaction between –NH₂ and –Cl, which effectively facilitated electron transfer.

2.3. The addition of photosensitizers

The introduction of guest molecules is a powerful means to improve the photocatalytic performance of MOFs.¹⁰⁷ The main interaction types usually include π–π stacking, coordination bonds, hydrogen bonds, halogen bonds, and electrostatic interactions with charged compounds. These interactions can significantly affect the host–guest interactions (Fig. 10).¹⁰⁸

Fig. 10 Three factors: the site of metal sites (I); modifications to the organic linker moiety (II); the introduction of dopants either during synthesis or post-synthesis (III); and the main interaction types between host–guest.

Using photosensitizers, such as organometallic complexes, represents another effective approach for crafting MOFs with visible light absorption capabilities. Zhang et al. pioneered the utilization of Ru(bpy)₃Cl₂ as a template and rigid tricarboxylate ligands to synthesize a battery of Ru(bpy)₃@MOFs, wherein the [Ru(bpy)₃]²⁺ fragments were encapsulated within sodalite-type cages.¹⁰⁹ The open channels and polyhedral cages within the MOFs effectively distributed the encapsulated photocatalysts, enhancing the mobility of reactants and products, and thereby improving the catalytic activity. Ru(bpy)₃@MOFs exhibited excellent catalytic performance under visible light in the aerobic oxidation of benzyl halides and the cyclization of tertiary anilines and maleimides (Fig. 11A).

Fig. 11 (A) The aerobic oxidation of benzyl halides and the cycloaddition reaction of tertiary amines with maleimides under visible light catalysed by Ru(bpy)₃@MOFs from ref. 109. Copyright 2019, American Chemical Society. (B) Scheme diagram of the preparation of 2D Fe-MNS and dye-sensitized system from ref. 110. Copyright 2021, Elsevier Inc. (C) Schematic diagram of the formation of MIL-125-NH₂ by RhB doping from ref. 111. Copyright 2023, Wiley-VCH GmbH. (D) Construction schematic diagram of Eosin Y-based MOF, displaying an illustration of photocatalytic inert C–H bond activation and transformation from ref. 112. Copyright 2022, American Chemical Society.

Dye-sensitized systems offer a means to enhance both light absorption and charge separation efficiency, while also allowing for the modulation of the catalyst band structure. Li et al. achieved this by incorporating the visible light-responsive dye sensitizer [Ru(bpy)]₃²⁺ into two-dimensional Fe-MOF (ligand = H₂BDC) nanosheets (Fig. 11B), resulting in a negative shift of the LUMO potential of two-dimensional Fe-MNS from 0.11 V to 0.15 V (relative to the reversible hydrogen electrode, RHE).¹¹⁰ Consequently, the [Ru(bpy)]₃²⁺/Fe-MNS catalytic system demonstrates excellent photocatalytic CO₂ reduction activity.

A major limitation in the application of transition metal complexes as photosensitizers lies in their prohibitive expense. To overcome this restriction, several research groups have investigated potential alternatives by employing inexpensive dyes or dye-like groups for sensitizing MOFs. For instance, Jong Hyeok Park et al. utilized the rigid dye modifier rhodamine B (RhB) to controllably adjust the pore size of Ti-based MOF MIL-125-NH₂ (Fig. 11C).¹¹¹ This approach allows for the enlargement of the pore size while preserving the integrity of the MOF crystals, thereby enhancing their photocatalytic activity. Notably, this method exhibits outstanding performance in purifying toluene.

Zhao et al. developed a heterogeneous MOF with excellent performance by incorporating Eosin Y into a porous MOF.¹¹² This modification efficiently facilitates the activation of dormant C–H bonds in reactants through hydrogen atom transfer mechanisms. Notably, the introduction of Eosin Y significantly enhances the chemical stability of the catalyst. Moreover, by introducing a secondary auxiliary ligand, the carbonyl groups of xanthene on the Eosin Y dyes were entirely preserved and periodically aligned within the confined channels of this rigid framework. This arrangement facilitates the formation of excited-state radicals, thereby promoting the activation of inert C–H bonds and enhancing the reaction efficiency (Fig. 11D).

2.4. Heterostructure construction

One effective method to enhance photocatalytic efficiency is to design rational heterojunction photocatalysts, to effectively separate the photogenerated electron–hole pairs in semiconductor photocatalysts.^113,114 A heterojunction typically refers to the interface formed by two different semiconductors that satisfy lattice and thermal matching. Currently, there are various types of heterojunction, including Type I, II, III, p–n junctions, Z-scheme heterojunctions, and S-scheme heterojunctions, as in Fig. 12; Type I, II, and III are conventional heterojunctions (A corresponds to semiconductor A, B corresponds to semiconductor B, which semiconductor can be an n-type semiconductor, or a p-type semiconductor).^115–117 Combining MOFs with other semiconductors to exploit the synergistic effect of heteromaterials can effectively improve the charge dynamics, and reduce electron–hole recombination.

Fig. 12 (A–F) Type I heterojunction, Type II heterojunction, Type III heterojunction, p–n junction, Type Z heterojunction, and Type S heterojunction, respectively.

Type I (straddle) heterojunctions have the following characteristics: (1) the conduction band (CB) and valence band (VB) potentials of A are higher and lower than B respectively; and (2) electrons migrate to the B, and holes migrate to the A. Thus electrons and holes are both on B¹¹⁸ (Fig. 12A).

Type II (staggered) heterojunctions have the following characteristics: (1) the CB and the VB levels of semiconductor A are higher than the corresponding levels of the semiconductor B; and (2) there are preferential VB holes at A and CB electrons on B. This results in a spatial separation of electron–hole pairs (Fig. 12B).

Type III (broken gap) heterojunctions have the following characteristics: (1) the staggered gap becomes extreme, resulting in non-overlapping bandgaps; and (2) the electron–hole migration and separation cannot occur. This is not suitable for enhancing the separation of e⁻–h⁺ pairs (Fig. 12C).

Among the aforementioned conventional heterojunctions, in Type II heterojunctions, photoinduced electrons and holes are trapped in two spatially separated potential wells, with the resulting holes and electrons moving in opposite directions, effectively promoting the separation of electron–hole pairs.^119–122

For example, in 2022, Wang et al. constructed three MOFs with different metal ions but the same ligands (2-aminoterephthalic acid), named In-MOF, Zr-MOF, and Fe-MOF, which were in situ grown on a covalent organic framework (COF) containing triazine (TP-TA), forming a Type II heterojunction for light-driven CO₂ reduction reactions.¹²³ Technically, a more negative CB potential would allow a relatively more ready photocatalytic reduction. The order of photocatalytic activity, In-MOF@TP-TA > Zr-MOF@TP-TA > Fe-MOF@TP-TA, aligns with the conduction band (CB) potentials of the MOF components (from negative to positive) (Fig. 13A), thereby corroborating the direction of charge flow within the type II heterostructures. Compared with In-MOF, TP-TA has a more negative CB and a more positive VB, leading to an internal electric field between the VB of TP-TA and the CB of In-MOF, facilitating the separation and migration of electrons and holes.

Fig. 13 (A) MOFs’ band-structure and mechanism of photocatalytic CO₂ reduction of In-MOF@TP-TA hybrid material, and XPS spectra studies (inset) from ref. 123. Copyright 2021, American Chemical Society. (B) (a) The UV–vis DRS spectra, (b) the band gaps, (c) the EIS curves, (d) photocurrent intensity, and (e and f) Mott–Schottky plots. (C) Possible visible light photocatalytic mechanism of NH₂-MIL-53(Al)/CdS p–n heterojunctions from ref. 124. Copyright 2021, Elsevier B.V.

Type-II heterojunctions can spatially separate e⁻–h⁺ pairs, and the enhancement in e⁻–h⁺ separation achieved is insufficient to overcome the ultrafast e⁻–h⁺ recombination in the semiconductor. On the other hand, p–n heterojunctions can accelerate e⁻–h⁺ migration across the heterojunction, improving the efficiency of charge separation by creating an additional electric field.

p–n junctions (staggered) have the following characteristics: (1) electrons migrate from the CB of the p-type photocatalyst to the n-type photocatalyst, and move from the VB of the p-type photocatalyst to the n-type; and (2) between the interfaces the two semiconductors generated an internal electric field, because there are opposite preferential charge carriers at the closely contacted interface (Fig. 12D).

Zhou et al. used a simple two-step hydrothermal method to synthesize NH₂-MIL-53(Al)/CdS p–n heterojunctions (ligand = 2-aminoterephthalic acid (NH₂-BDC)). The photocatalytic reduction of Cr(VI) and the removal of the trichlorophenol achieved efficiencies of 99.23% and 98.85% under visible light.¹²⁴ In Fig. 13B, ultraviolet–visible diffuse reflectance spectroscopy (DRS) indicated that the introduction of CdS effectively expands the absorption range and improves the optical response. Compounding CdS reduces the band gap to 2.50 eV, proving that heterojunction formation narrows the band gap and enhances visible light absorption. Electrochemical impedance spectroscopy (EIS) and photocurrent–time (IT) curves show that NH₂-MIL-53 (Al) combined with CdS promotes the separation and transfer of photogenerated charge carriers. Mott–Schottky (MS) tests showed that NH₂-MIL-53 (Al) is a p-type semiconductor, and the CdS is an n-type semiconductor. Before contact, the Fermi energy level (E_f) of p-type NH₂-MIL-53 (Al) is near the top of the VB, and that of n-type CdS is near the bottom of the CB. After contact, NH₂-MIL-53 (Al) moves negatively, CdS moves positively, and the Fermi level drives to equilibrium. The formation of p–n heterojunctions creates a built-in field, favoring spatial charge separation (Fig. 13C).

In 1979, Bard introduced the traditional Z-scheme heterojunction photocatalyst structure, named for its Z-shaped electron transfer process.¹²⁵ The traditional Z-scheme photocatalytic system consists of two different semiconductors, along with an acceptor/donor pair, but is only applicable in the liquid phase. In 2006, Tada and colleagues arranged the concept of an all-solid-state Z-scheme photocatalyst, consisting of two different semiconductors and a solid electronic mediator between them.¹²⁶ Not only does it optimize the spatial separation of electrons and holes and the redox potential, but it can also be applied in solution, gas and solid media, thus expanding its range of applications, although the electronic mediators are costly and scarce.¹²⁷ To address this issue, in 2013, Yu et al. proposed a direct Z-scheme heterojunction photocatalyst that enhances photocatalytic activity in the presence of an electronic mediator, significantly advancing photocatalytic performance.¹²⁸

In direct Z-scheme heterojunctions (staggered), e⁻ on the CB of B recombine with VB holes on A, and electrons and holes that are longer lived are mostly located on A in the CB and B in the VB, respectively (Fig. 12E).¹¹³

In 2023, Feng et al. utilized ultrasonic impregnation to uniformly load metal sulfide ZnCo₂S₄ (ZCS) nanoparticles with extra metal atoms onto the surface of MOF-199 (Cu, ligand = H₃BTC), forming a direct Z-scheme heterojunction for stable and efficient photocatalytic H₂ evolution.¹²⁹ Under simulated sunlight, ZCS/M (ZnCo₂S₄/MOF-199) exhibited high photocatalytic activity with a hydrogen evolution rate of 11.6 mmol g⁻¹ h⁻¹, surpassing those of MOF-199 and ZCS alone by factors of 48.4 and 83.3, respectively and exhibiting an apparent quantum yield (AQY) of 4.92% at 420 nm. Moreover, the uniform anchoring of ZCS onto the surface of MOF-199 effectively mitigated ZCS aggregation issues, thereby exposing a greater number of active sites. Based on ESR analysis findings, ZCS/M is capable of generating ˙OH under light exposure, suggesting a Z-scheme electron transfer pathway. Due to the lower Fermi level (E_f) of MOF-199 compared with that of ZCS, upon contact between MOF-199 and ZCS, electrons within ZCS will autonomously migrate to MOF-199 until an E_f equilibrium is established between them. Simultaneously, due to electron accumulation on the surface of MOF-199 and electron depletion on the surface of ZCS, a plasmonic field is formed from ZCS to MOF-199, inducing band bending in both MOF-199 and ZCS (Fig. 14A). Owing to band bending and the internal electric field (IEF), photogenerated electrons on the conduction band (CB) of MOF-199 can cascade along the bent band to the valence band (VB) of ZCS, where they recombine with holes, preserving the strong reducing electrons on the CB of ZCS. This study presented a novel strategy for designing and fabricating direct Z-scheme heterojunctions with transition metal sulfides on the MOF platform to enhance photocatalytic hydrogen generation in water splitting. In 2022, Shi et al. modulated the heterojunction interface to promote electron–hole separation; an ultrathin C₃N₅ nanosheet (N-CN) was prepared by facilely exfoliating the bulk C₃N₅ and was further applied to the construction of the NH₂-UiO-66/N-CN heterojunction.¹³⁰ Before contact, there was no charge transfer between the NH₂-UiO-66 and N-CN (Fig. 14B(a and b)). Fig. 14B(c and d) showed that after the interface contact between NH₂-UiO-66 and N-CN, electrons transfer from N-CN to NH₂-UiO-66. An electron depletion layer and an electron accumulation layer were formed in the N-CN and NH₂-UiO-66, respectively. This forms a space charge region and induces corresponding band bending. Driven by the internal electric field, a Z-scheme charge transfer pathway (Fig. 14B(e)) caused electrons from the CB of NH₂-UiO-66 to migrate to the VB of N-CN and recombine with holes there. Photogenerated electrons in the CB of N-CN and holes in the VB of NH₂-UiO-66 were retained for the photocatalytic reaction. The NH₂-UiO-66/N-CN-2 remained stable over 10 cycles over 30 hours and exhibited a significantly enhanced H₂ evolution rate, which is one of the best photocatalysts in UiO-66 family.

Fig. 14 (A) Schematic diagram of the direct Z-scheme formation process and electron migration path from ref. 129. Copyright 2023, Elsevier. (B) (a and b) Atomistic models of the NH₂-UiO-66/N-CN interface and energy-level diagrams, respectively, before contact; (c) calculated charge density differences of the NH₂-UiO-66/N-CN interface (yellow color: positive charges; light blue color: negative charges; C: gray; N: blue; O: red; H: white; green: Zr); (d and e) heterojunction energy-levels and proposed mechanism from ref. 130. Copyright 2022, American Chemical Society. (C) (a–c) The degradation of TC, (d) kinetic study, (e and f) ESR spectra of Cu-BTC/ZnWO₄ heterostructure, (g) work function, (h) sonophotocatalytic TC degradation in presence of various radical scavengers, (i) degradation of TC using sonophotocatalysis on Cu-BTC/ZnWO₄ S-scheme photocatalysts from ref. 134. Copyright 2023, Elsevier.

In 2019, a new concept of S-type heterojunction was proposed, first introduced by Professor Jiaguo Yu and his research group. By correcting misunderstandings related to Type II, liquid-phase (traditional) Z-scheme and all-solid-state Z-scheme electron transfer mechanisms, they introduced the innovative concept of the S-type heterojunction.^131–133

S-scheme heterojunctions have the following characteristics: (1) if A has a smaller work function and a higher Fermi level, the electron density at the A/B interface is larger on B to create an electron depletion layer and an accumulation layer; and (2) the CB of the reducing part and the VB of the oxidizing part retain strong photogenerated electrons and holes, avoiding the recombination of insignificant photogenerated carriers and introducing a strong redox potential (Fig. 12F).

Bernaurdshaw Neppolian et al. utilized an ultrasonic-assisted hydrothermal method to combine Cu-BTC (ligand = H₃BTC) with zinc tungstate (ZnWO₄).¹³⁴ Under visible light irradiation and ultrasound treatment, the Cu-BTC/ZnWO₄ heterojunction completely removed tetracycline (TC) within 60 minutes. When subjected to ultrasound and light irradiation at the same time, the degradation efficiency of Cu-BTC increased to 65%, ZnWO₄ to 74% and Cu-BTC/ZnWO₄ to 98%, as shown in Fig. 14C(a–d). The degradation efficiency of TC by Cu-BTC and ZnWO₄ catalysts exhibited the order of sonocatalysis < photocatalysis < sonophotocatalysis. Electron spin resonance (ESR) showed that Cu-BTC/ZnWO₄ S-scheme photocatalysts produce ˙O₂⁻ and ˙OH under light irradiation. The addition of trapping agents during the sonophotocatalytic degradation of TC indicated that h⁺ also plays a dominant role Fig. 14C(e–h). The coupling of Cu-BTC and ZnWO₄ can produce active species. The prepared Cu-BTC/ZnWO₄ heterostructure provided additional nuclei for the formation and growth of cavitation bubbles in the liquid, thereby enhancing the acoustic cavitation. Additionally, these cavitation bubbles generated a significant amount of heat energy during their collapse, increasing the pyrolysis rate of H₂O molecules. In sonophotocatalysis, ultrasound eliminates the intermediates produced during degradation through mass transfer, continuously refreshing the surface area and active sites of the Cu-BTC/ZnWO₄. Fig. 14C(g) showed that the work functions (Φ) of n-type Cu-BTC and n-type ZnWO₄ were 5.5 and 5.13 eV, respectively. Due to the higher E_F of Cu-BTC compared with ZnWO₄, electrons migrated from the Cu-BTC to ZnWO₄ until an E_F equilibrium was reached, resulting in Cu-BTC band edge and upward bending, ZnWO₄ downward bending, establishing an internal electric field at the interface. The interaction of the internal electric field and Coulomb force promotes the rapid migration of excited electrons in the ZnWO₄ CB to the interface, where they recombine with holes in the Cu-BTC VB. The electrons accumulated in the Cu-BTC and the holes accumulated in the ZnWO₄ and were retained and actively participated in the reaction. The charge transfer mechanism is shown in Fig. 14C(i). Zr-based MOF, developed by Xu et al., involved combining UiO-66-NH₂ with cadmium sulfide nanoparticles (CdS-NPs) using a chemical bath deposition method to form a core–shell structure.¹³⁵ The presence of type-S heterojunctions not only significantly enhanced the selective oxidation of benzyl alcohol under illumination but also effectively generated H₂.

The interface engineering between MOF and COF has proved to be effective at optimizing charge transfer.¹³⁶ Bi et al. employed a solvothermal method to construct an olefin-linked COF (TTCOF)/NH₂-UIO-66(Zr), S-type heterojunction of COF/MOF, which is used for photocatalytic CO₂ reduction to CO in gas–solid reaction systems.¹³⁷ Under light irradiation with only H₂O (g) as a mild reducing agent, the CO yield is 6.56 μmol g⁻¹ h⁻¹, which is 4.4 times and 5 times higher than that of the original TTCOF and NH₂-UIO-66(Zr), respectively.

In summary, selecting materials with matching energy levels to construct staggered heterostructures like Type-II, Z, and S-type heterojunctions can effectively enhance the photocatalytic efficiency. Additionally, a deeper understanding of the charge-separation dynamics in composite materials can further develop more efficient photocatalysts.¹³⁸

2.5. Encapsulation with functional guest species

2.5.1. Metal NPs. The wide application of noble metal single-atom nanoparticles such as Pd, Pt, Ru, Au, Ag, etc., with higher Fermi levels, as co-catalysts, broadens the spectral response range of MOF-based photocatalysts, facilitates photogenerated charge carrier separation and improves the photocatalytic activity of MOFs.^139,140

For example, Jiang et al. used UiO-66-NH₂ incorporating uniform Pt NPs as an ideal platform to study the effects of interfacial microenvironment modulation (by removing polyvinylpyrrolidone (PVP) from Pt) on electron transfer and photocatalysis.¹⁴¹ The PVP appeared to influence the electron transfer between the Pt and MOF. Compared with Pt_PVP@UiO-66-NH₂, the removal of interfacial PVP boosts photocatalytic H₂ production. This provided important insights for adjusting the microenvironment of the catalytic center in photocatalysis. Then Jiang et al. indicated that the position of metal NPs in the MOF affects photocatalysis.¹⁴² By incorporated inside or supported Pt NPs on a UiO-66-NH₂, Pt@UiO-66-NH₂ and Pt/UiO-66-NH₂ were obtained. Both composite materials showed increased H₂ production activity, but there were significant differences.

Zhao et al. synthesized the Pt/Cu^II-MOF (Cu^II-MOF: {[Cu(HL)(BDC)]·0.75H₂O}_n HL = 4′-(4-hydroxyphenyl)-4,2′:6′,4′′-terpyridine) photocatalyst using the photodeposition method.¹⁴³ The synergistic effect of dual catalytic active centers promoted the photocatalytic H₂ evolution from water. The beneficial electronic interaction between unsaturated coordination Cu^II ions and uniformly dispersed Pt NPs enhanced the H₂ production ability of Pt (4.38 wt%)/Cu^II-MOF nanoribbons by 2.51 mmol g⁻¹ h⁻¹. This is 4.7 times and 1.9 times higher than that of individual Pt NPs and crystalline Cu^II-MOF respectively. Herme G. Baldoví et al. also used the photodeposition method to deposit Ru on MIL-125(Ti)-NH₂ for the dual photothermal reaction mechanism of carbon dioxide methanation.¹⁴⁴ In addition to this, a charge transfer between metal sites and supports is crucial for catalysis. Jiang et al. selected three MOFs that do not respond to visible light, ZIF-8, UiO-66, and MIL-125, to stabilize small-sized Pt nanoparticles; the order of activity for the photocatalytic oxidation of benzylamine is Pt/ZIF-8 > Pt/UiO-66 > Pt/MIL-125.¹⁴⁵ XPS and CO-DRIFT (CO adsorption) spectroscopy indicate that different MOFs contribute differently to the electron donation to Pt. Under 450 nm light, the electrons excited in the Pt bands of Pt/UiO-66 and Pt/MIL-125 are injected back into the MOFs, whereas this does not occur in Pt/ZIF-8 due to its higher energy gap. The higher electron density in Pt/ZIF-8, induced by the aforementioned processes, enhances its ability to generate ˙O₂⁻, resulting in the highest electron density and improved oxidative coupling efficiency.

In 2021, Dai et al. first utilized a two-step method to synthesize the ternary heterostructure nanocomposite Pd@UiO-66-NH₂@ZnIn₂S₄, where ZnIn₂S₄ and UiO-66-NH₂ feature a closely interfaced and suitable band gap.¹⁴⁶ The introduction of Pd nanoparticles into the photocatalyst, the narrowing of the band gap and the high band gap alignment in Pd@UiO-66-NH₂@ZnIn₂S₄ significantly enhanced the visible light utilization, broadened the spectral response and promoted photogenerated charge carrier separation. In the realm of photocatalytic hydrogen production, 0.3% Pd@UiO-66-NH₂@ZnIn₂S₄ composite material achieved activities of 5.26 and 22.35 mmol g⁻¹ h⁻¹ under visible light and simulated sunlight, respectively. The band gap alignment between the ZnIn₂S₄ and UiO-66-NH₂, along with the incorporation of Pd NPs, broadened the spectral response range, promoted the separation of photogenerated carriers, and enhanced the photocatalytic performance (Fig. 15A and B).

Fig. 15 (A and B) hydrogen production rates of various photocatalysts, UN = UiO-66-NH₂, ZIS = Pure ZnIn₂S₄, UZ = UiO-66-NH₂/ZnIn₂S₄, PUZ = Pd@UiO-66-NH₂@ZnIn₂S₄. Schematic illustration of the band structures of UN, ZIS, UZ and PUZ-3 from ref. 146 Copyright 2021, Royal Society of Chemistry.

Furthermore, Jiang et al. designed Pt@MIL-125/Au and Pt/MIL-125/Au, based on the integration of the plasmonic effect and the Schottky junction; Pt@MIL-125/Au exhibited excellent photocatalytic H₂ production activity under light irradiation.¹⁴⁷ Jiang et al. synthesized UCNPs-Pt@MOF/Au (UCNPs = upconversion nanoparticles, MOF = UiO-66-NH₂) composites using plasmonic and upconversion effects,¹⁴⁸ where MOF absorbs UV light and Au responds to visible light, and UCNPs convert NIR light to UV-vis that MOF and Au can absorb. Thus, this material has broadband light absorption characteristics from ultraviolet to near-infrared regions, applied for photocatalytic H₂ production. In 2021, Wang et al. immobilized AuNPs within UiO-66 particles through impregnation and reduction, forming a uniquely designed gas membrane solution (GMS) reaction interface composed of porous Au@MOF particles.¹⁴⁹ The high dispersion and stability of AuNPs facilitated N₂ molecule and hydrated proton transfer, enhancing the plasma photocatalytic reaction at the gas–liquid interface. Results showed that under visible light, the ammonia release rate was 18.9 mmol g_Au⁻¹ h⁻¹, with an AQE of 1.54% at 520 nm (Fig. 16A).

Fig. 16 (A) Schematic illustration for direct P²NRR on AuNPs encapsulated in a UiO-66 matrix from ref. 149. Copyright 2021, Royal Society of Chemistry. (B) Efficient microwave-assisted method in contrast to Ni_NP/MOF and one-pot synthesized NiS_NP/MOF from ref. 156. Copyright 2021, American Chemical Society.

Then, Jiang et al. investigated the single-atom alloy (SAA) catalyst for photocatalysis.¹⁵⁰ As a result, the Pt component on the Pd surface can be controlled from the shell-form to SAA dispersion precisely, based on the rational control of Pt loading. The photocatalytic H₂ production activity of Pd₁₀@Pt_x/UiO-66-NH₂ can be greatly increased by precisely fabricating the Pd₁₀@Pt₁ SAA co-catalyst, based on the rational control of Pt loading.

Transition metal (TM) catalysts, apart from noble metals, have garnered widespread attention in catalysis. Loading active metal NPs onto heterogeneous supports is a prevalent method that maximizes the proportion of active sites and tunes the performance of the catalysts.^151–154 For instance, Wang et al. showed that Cu-doped UiO-66 creates modified mesoporous defects, increasing the surface area by 58% compared with UiO-66, which facilitates the adsorption of molecular oxygen.¹⁵⁵ Additionally, Cu doping enhances the photocurrent and reduces the interfacial charge transfer resistance, thereby improving the charge separation efficiency. Jiang et al. employed a microwave-assisted method to introduce various earth-abundant metals such as Ni²⁺, Co²⁺, and Cu²⁺ in single-atom form onto Zr₆-oxo clusters (Fig. 16B).¹⁵⁶ The study specifically focuses on Ni₁-X/MOF systems (MOF, UiO-66-NH₂ unless otherwise mentioned; Ni₁–X, Ni₁–S, Ni₁–O, Ni₁–S_ox; ox, oxidation). Among these single-atom catalysts (SACs), Ni₁-S/MOF features a unique sulfur-coordinated Ni(I) center with a reduced oxidation state, which offers a significantly lower proton activation barrier compared with its counterparts. The results demonstrate that under visible light, Ni₁-S/MOF exhibits the highest photocatalytic H₂ production activity, over 270 times that of the original MOF, far surpassing other Ni₁-X/MOF catalysts.

Furthermore, metal NPs can also be transformed into structurally precise metal nanoclusters (NCs) for photocatalysis. In 2024, Jiang et al. encapsulated four MAg₂₄ (M = Ag, Pd, Pt, Au) NCs into UiO-66-NH₂, through electrostatic interactions. Among them, AuAg₂₄@UiO-66-NH₂ exhibited the highest H₂ production rate of up to 3.6 mmol g⁻¹ h⁻¹.¹⁵⁷ The MAg₂₄ clusters encapsulated into the MOF produced an electron transfer pathway similar to the Z-scheme heterojunction, which promoted charge separation, and optimized Ag electronic state.

2.5.2. Metal–oxide NPs. TiO₂ was a commonly used classical photocatalytic material with the band gap of 3.2 eV.^158,159 The use of ZnO in the field of photocatalysis has expanded increasingly.^160–162 The ZnO band gap energy (3.3 eV) is very close to that of TiO₂, theoretically endowing it with similar photocatalytic properties to TiO₂. Fe₃O₄ has garnered considerable research interest in the field of photocatalysis, particularly because of its magnetic properties, enabling facile separation of the catalyst from solutions.¹⁶³

Recently, researchers have made efforts to develop metal oxide@MOF nanocomposites as novel photocatalysts.¹⁶⁴ A comprehensive summary detailed the effects and applications of semiconductor-like metal oxides such as TiO₂, ZnO and Fe₃O₄ in photocatalysis by Zeng et al.¹⁶⁵ Additionally, certain metal oxides such as BiOX (X = I, Br and Cl) have been extensively researched as ideal photocatalysts, where the unique [Bi₂O₂]²⁺ layers interlaced with halide ions facilitate electron transfer in various directions while reducing the transfer distance.¹⁶⁶ In 2023, Yao et al. developed core–shell Bi-MOF@BiOX (X = I, Br and Cl) heterostructured hybrid materials using a one-pot solvothermal method.¹⁶⁷ Utilizing the competitive coordination effects of CAU-17 (ligand = trimesic acid) and BiOX on Bi³⁺, CAU-17 was transformed into another structurally and compositionally similar Bi-MOF. Bi³⁺ acted as a “bridge”, facilitating intimate interfacial contact between BiOX and Bi-MOF, greatly accelerating the charge and mass transfer.

CeO₂ is a common and frequently used rare-earth metal oxide and has a positive photocatalytic response due to its excellent unique structure and electro-optical properties. Xu et al. synthesized a novel CeO₂/Ce-MOF (ligand = 1,3,6,8-tetra(4-carboxylphenyl)pyrene (H₄TBAPy)) homologous S-type photocatalyst, by a one-step hydrothermal method.¹⁶⁸ CeO₂ nanoparticles grow on the surface of cubic Ce-MOF, forming a CeO₂/Ce-MOF S-heterojunction structure. Significantly, the CO₂ concentration generated by CeO₂/Ce-MOF (404.2 ppm) was ≈6.7 and 3.4 times superior to those generated by CeO₂ and Ce-MOF, respectively. The AQE at 420 nm is 7.51%, which is 6.7 times and 3.4 times that of CeO₂ and Ce-MOF, respectively. The improvement in efficiency is primarily attributed to two factors: (i) the crucial role of the effective separation capability of photogenerated carriers and prolonged exciton reaction lifetime; and (ii) the formation of an internal electric field (IEF) being sufficient to overcome the relatively large exciton binding energy. This also increases the dissociation efficiency of the electron–hole pairs by 50.4% (Fig. 17A(a and b)).

Fig. 17 (A) (a) Time course curve of CO₂ liberation from CH₃CHO decomposition for samples, (b) AQE of CeO₂/Ce-MOF from ref. 168. Copyright 2023, Wiley-VCH GmbH. (B) (a) Transient photocurrent response, (b) EIS Nyquist plots of MOF-808, CdS@ MOF-808-X, and CdS from ref. 176. Copyright 2021, American Chemical Society. (C) (a) H₂ production rates with different contents of x mg Ni₂P@UiO-66-NH₂, (b) x% 10 mg Ni₂P@UiO-66-NH₂/Zn_0.5Cd_0.5S (x% 10 N@U/ZCS) with different contents of 10 N@U cocatalyst from ref. 187. Copyright 2022, Elsevier B.V. (D) (a) Ball-and-stick representation of NiW-DPNDI, (b) 3D open network of NiW-DPNDI, (c) the design concept of NiW-DPNDI for photocatalysis styrene oxidation from ref. 194. Copyright 2023, Elsevier B.V.

2.5.3. Metal-sulfide NPs. Traditional semiconductor photocatalysts can only absorb UV light, with wavelengths less than 387 nm.^169–171 Metal sulfides, with suitable conduction band positions and narrow band gaps, such as CdS with a band gap of ∼2.4 eV and a fairly negative conduction band potential, exhibit an excellent response to visible light.^172–175

Consequently, in 2021, Hou et al. synthesized porous CdS@Zr-MOF photocatalysts based on representative catalysts such as CdS and Zr-MOF (ligand = H₃BTC), which were used for the synthesis of imines in air.¹⁷⁶ The transient photocurrent response and EIS Nyquist plots showed that CdS@MOF-808-X (1–4 for different ratios of CdS loading of MOF-808) had a higher photocurrent response, more efficiency of separation and a transfer of photogenerated carriers (Fig. 17B(a and b)). Among the CdS@MOF-808 composites, CdS@MOF-808-3 showed both a photocatalytic activity yield (95%) and selectivity (nearly 100%) in the oxidative coupling of amines.

Shi et al. synthesized a new photocatalyst, PdS@UiOS@CZS (UiOS: thiol-functionalized UIO-66, CZS: Cd_0.5Zn_0.5S).¹⁷⁷ PdS quantum dots were encapsulated within its cage and CZS solid solution NPs were anchored on its outer surface. The resulting PdS@UiOS@CZS exhibited a photocatalytic hydrogen evolution rate of 46.1 mmol h⁻¹ g⁻¹ irradiation from 420 to 780 nm. The encapsulation of PdS QDs enhanced the ability of the UIOS pores to attract holes. Huo et al. coated CdS NPs on Ni-MOF, and a 3D stratified CdS/Ni-MOF composite with a tightly connected interface was successfully fabricated.¹⁷⁸ In the prepared photocatalysts, the 20%-CdS/Ni-MOF demonstrated the highest CO production rate.

Moreover, MoS₂ features a tunable bandgap structure, ranging from an indirect bandgap of 1.2 eV in bulk MoS₂ to 1.8 eV in monolayer MoS₂, which can be activated under visible light conditions.^179–182 Woong Kim et al. investigated a photocatalyst driven by the MoS₂@MIL-88(Fe) (ligand = BDC) interface, which consists of molybdenum disulfide (MoS₂) anchored to an MOF and is prepared for activation under visible light illumination.¹⁸³ MoS₂@MIL-88(Fe) composite material exhibited a narrow optical bandgap of 2.75 eV. The MoS₂ and MIL-88(Fe) provide effective redox sites for the mineralization of MB and RhB dyes. Reusability studies indicate that after five cycles, its photocatalytic activity remains excellent.

2.5.4. Metal-phosphide NPs. Transition metal phosphides (TMPs), such as Ni₂P, Ni₁₂P₅, and Co₂P, have been reported as alternative noble metal co-catalysts for photocatalytic H₂ production owing to their unique structure and electronic properties.^184–186 In 2022, Ge et al. used an in situ solvothermal method to encapsulate Ni₂P within UiO-66-NH₂. The Zn_0.5Cd_0.5S was decorated with a core–shell Ni₂P@UiO-66-NH₂ cocatalyst, forming ternary Ni₂P@UiO-66-NH₂/Zn_0.5Cd_0.5S composite materials.¹⁸⁷ When Ni₂P was wrapped in pure UiO-66-NH₂, the catalytic H₂ evolution activity significantly increased (Fig. 17C(a)), with the hydrogen production rate peaking at 10 mg of Ni₂P addition. Fig. 17C(b) illustrated the effect of 10 Ni₂P@UiO-66-NH₂ content on the photocatalytic capability of ZCS sulfide. However, further increases in decorating capacity reduced the hydrogen production rate, and this likely due to excessive cocatalyst shielding of active sites on ZCS. Ni₂P played a crucial role in this ternary photocatalyst.

Sasanka Deka et al. have for the first time developed a novel type of hollow ZnCo-MOF (ligand = 2-methylimidazole) spheres at room temperature, which were loaded with monodisperse transition-metal phosphide (TMPs), including NiCoP, FeCoP, Ni₂P and CoP nanoparticles. These composite materials could act as efficient non-noble metal catalysts for hydrogen evolution reactions.¹⁸⁸ Compared with systems based on Pt cocatalysts, these are not only more economical but also exhibit comparable excellent photocatalytic hydrogen evolution activity to Pt@ZnCo-MOF (Pt@ZCM), achieving 8583.4 μmol h⁻¹ g⁻¹ with an AQY of 28.2%. This achievement is related to the efficient n–n heterojunction charge separation and transfer, as well as the NiCoP cocatalyst reducing the reaction activation energy, thereby facilitating rapid hydrogen evolution kinetics. Jin et al. synthesized the Ni-MOF-74/Ni₂P (ligand = H₂BDC) precursor using a low-temperature phosphorization method.¹⁸⁹ Using a solution mixing method, they produced a uniquely structured Ni-MOF-74/Ni₂P/MoS_x ternary heterostructure; the introduction of Ni₂P enhanced the composite catalyst utilization of visible light, leading to increased electron generation and thus improved photocatalytic hydrogen evolution activity.

2.5.5. Polyoxometalates (POMs). Polyoxometalates (POMs) are multinuclear, anionic metal oxide clusters composed typically of early transition metals in their highest oxidation states, such as W(VI), Mo(VI), or V(V).^190,191 In POMs, the high oxidation states of the metal centers mean their lowest unoccupied molecular orbitals are composed of linear combinations of d-orbitals from metals with a strong tendency to accept electrons, allowing for multiple reversible reductions within a relatively narrow potential range. Leveraging their high stability and tunability, POMs have been successfully used to enhance the photocatalytic activity of heterogeneous catalysts.¹⁹² Consequently, Carsten Streb et al. reviewed the embedding of POM guests into MOF hosts and the synergistic effects of the resulting composites in multiphase catalysis.¹⁹³ Following this, the field continued to develop. Han et al. synthesized a photoactive polyoxometalate-based MOF (NiW-DPNDI) using a solvothermal approach, comprising Ni(II) ions, [ZnW₁₂O₄₀]⁶⁻ anions and N,N′-bis(4-pyridylmethyl)naphthalene diimide (DPNDI) molecules.¹⁹⁴ The strong anion–π interaction between the [ZnW₁₂O₄₀]⁶⁻ anion and the electron-deficient naphthalenic ring centroid, along with the π–π interaction between DPNDI groups, facilitates electron–hole separation and creates a short charge transfer pathway. NiW-DPNDI exhibits high efficiency and diverse chemical selectivity in the oxidation of styrene under both thermal and photocatalytic conditions (Fig. 17D).

2.5.6. Metal-free semiconductors. Combining zero-dimensional (0D) quantum dots (QDs) with two-dimensional (2D) MOFs to create 0D/2D hybrid catalysts is an effective method to address the rapid recombination of photogenerated electron–hole pairs.¹⁹⁵ Carbon dots (CDs) are a type of special carbon nanomaterial less than 10 nm in size.^196,197 They are widely noted for their luminescence, low toxicity, stability, and easy production.^198–200 Since CDs can act both as electron acceptors and photosensitizers and their ultra-small size allows for strategic encapsulation within the pores of MOFs, enhancing their photoelectron transfer efficiency, it is feasible to develop CD-hybridized MOFs photocatalysts. The MOF structure facilitates charge/mass transfer and increases the surface area to volume ratio of CD active sites.²⁰¹

Li et al. successfully synthesized NH₂-UiO-66 particles modified and embedded with CDs, as shown in Fig. 18A.²⁰² The placement of the CDs significantly affects the photocatalytic activity of the NH₂-UiO-66 particles. In addition, the incorporated CDs form small heterojunctions with the Zr–O clusters in the MOF, resulting in the MOF photocatalysts demonstrating rapid charge separation and transfer. Besides acting as electron acceptors, under long-wavelength light, CDs also serve as photosensitizers in MOF photocatalysts. The main difference between the two materials lays in the differing positions of CDs within the hybrid particles. As shown in Fig. 18B(A) and (B), CD/NH₂-UiO-66 particles display a uniform size with a mean diameter of 180 nm, and under TEM (Fig. 18B(C)) the particle shows an octahedral shape with 3–4 nm tiny dot-like particles on the MOF surface. When CDs are encapsulated within MOF particles, the resulting CD@NH₂-UiO-66 particles exhibit a similar octahedral morphology, but with an average size of about 150 nm (Fig. 18B(D) and (E)), while some black dots are observed under the MOF shell (Fig. 18B(F)). CD@NH₂-UiO-66 exhibited higher CO₂ reduction activity.

Fig. 18 (A) Schematic illustration of CD-decorated NH₂-UiO-66 particles and CD-embedded NH₂-UiO-66 particles, respectively. (B) TEM, SEM and high-resolution TEM characterization. (A, B, and C) CD-decorated NH₂-UiO-66 particles, (D, E, and F) CD-embedded NH₂-UiO-66 particles from ref. 202. Copyright 2020, Royal Society of Chemistry. (C) Proposed mechanism of CO₂ reduction over g-CNQDs/PMOF hybrids under visible-light irradiation from ref. 203. Copyright 2019, American Chemical Society. (D) Charge transfer of g-C₃N₄/Ce-BDC heterojunction from ref. 214. Copyright 2021, Elsevier B.V. (E) The synthesis of BP/R-Ti-MOFs/Pt from ref. 224. Copyright 2020, Elsevier B.V. and Science Press.

Wu et al. synthesized a novel hybrid catalyst by integrating two-dimensional (2D) ultrathin porphyrin MOF (PMOF) with zero-dimensional (0D) graphitic carbon nitride quantum dots (g-CNQDs).²⁰³ In the synthesized g-CNQDs/PMOF hybrid, g-CNQDs coordinate with the Co active sites in the porphyrin MOF, directly synergizing with the Co in the PMOF as compared with the bare PMOF (Fig. 18C). The migration of photogenerated carriers and gas substrates from g-CNQDs to the Co active centers is faster and more efficient. The production rates of CO and CH₄ are 6.10 μmol g⁻¹ h⁻¹ and 6.86 μmol g⁻¹ h⁻¹, respectively.

Additionally, Wang et al. provided a detailed introduction to typical synthesis methods of CDs and MOFs composites, and then they prospected composites in photocatalysis and sensing applications.²⁰⁴

The unique properties of C₆₀ in photocatalytic reactions have attracted the attention of researchers.²⁰⁵ Zhu et al. developed a new type of MOF photocatalyst by encapsulating C₆₀ into the nanoscale zirconium-based MOF, NU-901 (ligand = 1,3,6,8-tetrakis(p-benzoate) pyrene).²⁰⁶ Host–guest interactions, the uneven charge distribution, and the electrostatic potential difference in the C₆₀@NU-901 created a strong built-in electric field, effectively separating and transporting charge carriers to the surface. The electric field in C₆₀@NU-901 is 10.7 times higher than in NU-901 alone and the photocatalytic hydrogen evolution of C₆₀@NU-901 reached 22.3 mmol g⁻¹ h⁻¹.

Carbon nitride can exist as allotropes with different properties, with graphitic carbon nitride considered the most stable.²⁰⁷ Due to its moderate band gap (about 2.7–2.9 eV), reasonable electronic band structure, non-toxicity, low cost, good stability and ease of preparation, it has attracted widespread attention.^208–211 In 2008, Wang and his colleagues first used g-C₃N₄ as a metal-free photocatalyst for hydrogen production.^212,213 V. Muelas-Ramos et al. used a g-C₃N₄/NH₂-MIL-125 (ligand = 2-amino benzene dicarboxylic acid) composite photocatalyst under LED irradiation to degrade diclofenac in water.²¹⁴ The presence of MOF reduces the e⁻–h⁺ recombination rate of the g-C₃N₄. The valence band maximum (VBM) was estimated from XPS, with the VBM of NH₂-MIL-125 and g-C₃N₄ being 2.87 and 1.75 eV, respectively. The CB of NH₂-MIL-125 and g-C₃N₄ were approximately calculated as 0.31 and −0.93 eV (E_CB = VBM − E_g). As shown in Fig. 18D, NH₂-MIL-125 acts as an oxidative photocatalyst, while g-C₃N₄ serves as a reductive photocatalyst. The electrons in the CB of the MOF recombine with the holes in the VB of g-C₃N₄, leaving behind species with strong redox capabilities which can improve diclofenac degradation.

Graphene is a two-dimensional carbon allotrope that was first isolated in 2004 through mechanically exfoliating a single layer of graphite atoms.²¹⁵ Graphene sheets are single layers of sp² hybridized carbon atoms arranged in a flat honeycomb lattice. This structure gives graphene its unique physical and chemical properties, including distinctive surface characteristics, excellent conductivity and high surface area, which have been used to construct various nanocomposite materials.^216–219

Lu et al. achieved a unique enhanced synergistic effect on MIL-53 (Fe) using ultra-small γ-Fe₂O₃ and GO via a hydrothermal method.²²⁰ Compared with traditional surface coating methods, incorporating γ-Fe₂O₃ (average particle size 2–3 nm) into the interiors of MOF particles increased the photocatalytic absorption and formed numerous micro-heterojunctions with the MOF units, and then effectively facilitated photogenerated carrier separation. By providing attachment points for MOF crystals, functionalized GO reduced the size of the MOF particles to 100–200 nm and improved their dispersibility. The large specific surface area and high electronic conductivity of graphene oxide also enhanced the active sites and quantum yield of the photocatalytic reaction. Consequently, γ-Fe₂O₃-MIL-53(Fe)-GO composites significantly enhanced the photocatalytic activity against norfloxacin.

Black phosphorus (BP), one of the allotropes of phosphorus and a 2D material, shows broad application prospects in the field of photocatalysis.²²¹ It features tunable direct bandgaps ranging from approximately 2.1 eV for monolayer phosphorene to about 0.3 eV for bulk BP. It possesses high free carrier mobility and an absorption range from visible to near-infrared light. Combined with MOFs, it is expected to have even broader application prospects.^222,223 Xu et al. reported a simple method for the synthesis of BP/R-Ti-MOF/Pt using an in situ synthesis approach (Fig. 18E).²²⁴ Ti⁴⁺ ions initially accumulate on the surface of BP to form BP/Ti complexes. Subsequently, in the presence of chloroplatinic acid (H₂PtCl₆), a solvothermal method is used to further grow BP/Ti complexes in a high concentration of NH₂-BDC, yielding BP/R-Ti-MOF. Strong interactions between Ti and P lead to the reduction of Ti⁴⁺ valence states on the surface of the BP, forming abundant reducible Ti⁴⁺ species within R-Ti-MOFs/BP. This reduction in Ti⁴⁺ results in BP/R-Ti-MOFs/Pt showing enhanced photocatalytic hydrogen production efficiency compared with Ti-MOFs/Pt and BP + Ti-MOFs/Pt (physically mixed), with a bandgap width of 2.12 eV, lower than Ti-MOFs/Pt 2.58 eV, showing efficient charge transfer and light absorption abilities.

2.6. Morphology regulation

The morphology of the catalyst is one of the key factors influencing photocatalytic activity. The morphological control of photocatalysts includes the regulation of crystal shape (1D, 2D, and 3D), size, and planarity. Research has shown that different shapes of MOF nanomaterials can be synthesized through simple coordination modulation strategies.^225–227 Gu et al. constructed nanorod self-assembled Bi-based MOF microspheres (Bi-MOF-M, ligand = H₂BDC) using an ethylene glycol-assisted solvothermal method.²²⁸ The SEM of Bi-MOF-M was shown in Fig. 19A(a–c). After the in situ growth of Bi₂MoO₆ on the surface of Bi-MOF-M, the surface structure had an alteration, with the nanorods nearly invisible. The surface layer of the as-prepared composite shows a flower-like structure composed of petal-like nanosheets, and as shown in Fig. 19A(d–f) the petal thickness is about 20 nm. This method easily created a chemically bonded heterojunction interface. This allows it to maintain high catalytic activity of the primary nanocomponents while also being easily recoverable due to its micrometer-level material. It has exhibited high performance under visible-light in the degradation of tetracycline.

Fig. 19 (A) The SEM images of Bi-MOF-M and Bi-MOF-M/BMO-0.3 from ref. 228. Copyright 2021, Elsevier Inc. (B) Schematic diagram for the formation of the MOF-5/CuO@ZnIn₂S₄ tandem heterojunction from ref. 229. Copyright 2022, Royal Society of Chemistry. (C) Synthesis procedure of the highly ordered macroporous MIL-125 from ref. 232. Copyright 2019, American Chemical Society. (D) Schematic diagram of the synthesis of MIL-125(Ti)@TiO₂/ZnSe HNDs from ref. 237. Copyright 2022, Elsevier Inc.

Zhou et al. synthesized a 3D floral spherical shaped MOF-5/CuO@ZnIn₂S₄ core–shell Z-scheme heterojunction using a two-step solvothermal and oxidation method (Fig. 19B).²²⁹ The flower-ball-shaped core–shell structure provided numerous active sites for photocatalysis. The 3D floral spherical shape and its photothermal effect promote charge separation and accelerate photocatalysis. When applied to H₂ production and the degradation of 2,4-dichlorophenol and phenol, the photocatalytic H₂ production rate (1938.3 μmol g⁻¹ h⁻¹) was 18 times that of the original MOF-5, with photocatalytic degradation efficiencies for 2,4-dichlorophenol and phenol reaching 98.7% and 97.3%, respectively.

Liu et al. utilized Ce-MOFs (ligand = 1,4-dicarboxybenzene) as a template to rationally design and construct a Ce-doped ZnIn₂S₄ tetrakaidecahedron nanocage (ZTNs-Cex) photocatalyst for photocatalytic hydrogen evolution through a one-step hydrothermal method.²³⁰ The Ce-doped ZnIn₂S₄ featured a non-spherical 3D hollow nanostructure, inheriting the tetrakaidecahedron shape from the Ce-MOF template. By modifying the morphology, the light-harvesting ability is significantly improved, and the separation of photogenerated charge carriers is facilitated. The optimal ZTNs-Ce20 showed a hydrogen evolution photocatalytic activity of 7.46 mmol h⁻¹ g⁻¹ and an AQE of 6.56% at 380 nm, which are three times that of the original ZnIn₂S₄ (2.61 mmol h⁻¹ g⁻¹).

As reported by the IUPAC definition, porous materials are classified into microporous (pore size <2 nm), mesoporous (pore size 2–50 nm) and macroporous materials (pore size >50 nm), each showing significant value in specific applications. Macroporous materials can serve as reaction sites for polymer polymerization.²³¹ Li et al. synthesized highly ordered hierarchical macroporous MOF materials (macroporous MIL-125) via a one-step solvent evaporation-induced self-assembly route (Fig. 19C).²³² The obtained macroporous MIL-125 exhibits an extremely high specific surface area, with a BET surface area of up to 1083 m² g⁻¹. Furthermore, the macroporous MIL-125 exhibits an orderly hierarchical porous structure and excellent light absorption capabilities. Macroporous MIL-125, used as a photocatalyst for CO₂ capture, exhibits high cycling stability, good tolerance to substrates and excellent yields in the CO₂ carbonyl coupling reactions under ultraviolet light.

Compared with ultrathin MOFs, hollow structures with internal cavities and thin shells not only expose more active sites but also provide confined spaces for catalytic reactions and product separation.^233–236 Tian et al. synthesized ZnSe-sensitized hierarchical MIL-125(Ti)@TiO₂ hollow nanodisks (HNDs) using a two-step solvothermal process followed by a selenization technique (Fig. 19D).²³⁷ In the ternary hybrid, MIL-125(Ti)@TiO₂ displays a hierarchical hollow sandwich-like heterostructured nanodisk, with TiO₂ nanosheets distributed on both sides of the MIL-125(Ti) nanoshell. The in situ formed ZnSe nanoparticles enhance the visible light absorption of the MIL-125(Ti)@TiO₂/ZnSe HND hybrid. Consequently, the optimized mixture features abundant catalytic sites, high charge separation efficiency and broad visible light absorption, and thus enhanced photocatalytic CO₂ reduction performance and recyclability compared with single-component catalysts and binary hybrid catalysts.

2.7. Defect engineering

Defects in MOFs refer to regions where the regular periodic arrangement is disrupted due to the absence or misplacement of atoms or ions within the framework.²³⁸ Defect engineering is regarded as an effective method to regulate the properties of MOFs, such as photogenerated carrier mobility, pore size, number of catalytic active sites, and band gap.^239,240 Maria Yaseen et al. effectively prepared a series of defect MOFs (D-NUiO66) Zr-UiO-66-NH₂ using a modular acidic method (HCl).²⁴¹ The optimized D-CeNUiO66 photocatalyst efficiently reduced CO₂-to-CO with a peak rate of 38.6 μmol g⁻¹ h⁻¹ and demonstrated significant reproducibility in CO production. The integration of defect sites in D-NUiO66 enhanced the photoresponse to visible light, resulting in a reduced band gap of 2.9 eV. Jiang et al. synthesized an MOF featuring controlled structural defects, named UiO-66-NH₂-X (X represents the molar equivalents of the modulator, acetic acid, with respect to the linker in synthesis).²⁴² Structural defects in MOF can turn on photocatalysis. The H₂ production showed a volcano-type trend with increasing structural defects, where Pt@UiO-66-NH₂-100 exhibited the highest activity. This highlights that creating moderate structural defects is crucial for promoting efficient electron–hole separation.

3. Applications

In recent years, there has been rapid progress in the development of visible light-responsive MOFs. Extensive research on these materials has led to the emergence of numerous visible light-responsive MOFs and their composite materials. Many researchers have widely applied MOFs and their composites as efficient and recyclable photocatalysts in the field of photocatalysis. This part includes pollutant degradation, carbon dioxide (CO₂) reduction, nitrogen (N₂) fixation, hydrogen (H₂) production, and organic transformations under light irradiation.

3.1. Pollutant degradation

These pollutants include dyes,^243–245 antibiotics,^246–248 and polycyclic aromatic hydrocarbons.^249,250 Various types of heavy metal and organic pollutant are often stable in water, posing significant hazards despite their low concentrations. Among heavy metals, hexavalent Cr(VI) (chromium) is a common pollutant in surface and groundwater from many industrial processes,^251,252 such as electroplating, tanning, metallurgy, metal finishing and petroleum refining. Studies have shown that Cr(VI) concentrations exceeding 0.1 ppm can have lethal effects on aquatic life. Similarly, over the past few years, more than 100 000 tons of antibiotics have been used globally, and extensively applied to treat bacterial infections in humans. Antibiotics are difficult for the human to metabolize and are directly excreted into the natural environment. Antibiotic resistance genes are a result of antibiotic accumulation in aquatic environments, posing a threat to human survival.^{195,253–255} Current technological solutions to pollution, such as adsorption,²⁵⁶ membrane separation,²⁵⁷ advanced oxidation processes,²⁵⁸ and photocatalytic degradation have been widely applied.^259–262 Among these, photocatalytic technology, which utilizes light energy, has gained significant attention in recent years.

MOFs demonstrated significant advantages in water treatment, offering an effective strategy for pollutant removal. The use of semiconductors to degrade organic pollutants under visible light has proved to be an effective means of mineralizing harmful substances.^263–265 In this process, pollutants are oxidized into compounds such as CO₂, H₂O, NO₃⁻, or other oxides, phosphates, sulfates, and halides. This mineralization method, compared with others like ozonation, chlorination, and adsorption, does not use strong oxidizers or toxic substances, thereby not producing solid waste or polluting the atmosphere.^266,267 The degradation of organic pollutants depends on the ability of MOF-based photocatalysts to generate ROS (such as ˙O₂⁻ and ˙OH) under visible light. Thus, modifying MOFs in various ways can facilitate the separation of photoinduced electrons and holes within the MOFs, thereby enhancing their photocatalytic performance.²⁶⁸

Within these MOFs, d-block transition metal-based MOFs (e.g., Zn(II), Fe(II)/(III), Co(II)/(III), Zr(IV), Ti(III)/(IV), Ni(II), Cd(II) and Cu(II)) demonstrate significant potential in photocatalytic activities aimed at environmental cleanup. Ligand regulation can effectively modify the band gap, which in turn boosts the efficiency of photocatalysis.²⁶⁹

According to reports, UiO-66 as a catalyst for degrading pollutants still suffers from low electron–hole separation efficiency. The mismatch in pore sizes also makes it difficult to efficiently adsorb and degrade pollutants.^270–272 Kevin C.-W. Wu et al. reported a Zr-based MOF photocatalyst named VNU-1 that was synthesized using a solvothermal method.²⁷³ VNU-1(VNU represents Vietnam National University), a Zr-based MOF, was constructed using the bidirectional linker H₂CPEB (1,4-bis(2-[4-carboxyphenyl]ethynyl)benzene). The designed MOF has a longer linker that enlarges the pore size and enhances the optical properties. The surface areas and pore size distributions of various Zr-based MOFs were analyzed using N₂ adsorption–desorption isotherms and NLDFT simulation, demonstrating that larger linkers enhance pore dimensions. Fig. 20A(a) showed the widest light absorption spectrum, as measured by UV-Vis, signifying superior light-harvesting efficiency. TDDFT calculations confirmed that introducing amino functional groups to UiO-66-NH₂ expands its light absorption range to the visible spectrum through sp² hybridization of carbon and nitrogen in the aromatic rings. In Fig. 20A(b), the bandgap values decreased sequentially from UiO-66 (3.97 eV) to UiO-67 (3.63 eV) and further to VNU-1 (2.28 eV), indicating that longer organic linkers enhance charge separation. This phenomenon can be attributed to an increase in the carbon states within the organic linkers, raising the valence band maximum (VBM) and reducing the bandgap. Fig. 20A(c) clearly shows that the lifetime of photoinduced electron–hole pairs in VNU-1 increases from 0.47 ns to 0.63 ns. Compared with UiO-66 MOF, VNU-1 exhibited a 7.5 times higher adsorption capacity. It achieved 100% photodegradation of sulfamethoxazole within 10 minutes. Furthermore, in systems mixed with antibiotics of varying sizes and humic acids, it can selectively adsorb small-molecule antibiotics, leaving other natural organic components (such as humic acids) in the water. Ti₃C₂ is a typical 2D MXene material and can serve as an electron trap for the effective transfer of photogenerated charges.²⁷⁴ Li et al. synthesized bimetallic Sn-Bi-MOF (ligand = H₂BDC) nanoparticles using the solvothermal method and grew them on the surface of Ti₃C₂, forming a Sn-Bi-MOF/Ti₃C₂ Schottky junction.²⁷⁵ When the layered structure of Ti₃C₂ contacts the bimetallic Sn-Bi-MOF nanoparticles, the resulting Schottky junction accelerates electron transfer to Ti₃C₂, effectively promoting electron transfer and carrier separation, thereby generating free radicals and enhancing the photocatalyst performance in the photocatalytic degradation of tetracycline.

Fig. 20 (A) (a) UV–vis spectra, (b) Tauc plot, and (c) time-resolved PL analysis of Zr-based MOFs from ref. 273. Copyright 2023, Elsevier B.V. (B) Schematic representation for the fabrication of UiO-66/MIL-125/g-C₃N₄ hybrid structure. (C) Schematic representation of the reaction mechanism of UiO-66/MIL-125/g-C₃N₄ photocatalyst from ref. 285. Copyright 2022, Elsevier B.V. (D) The possible photo-Fenton degradation mechanism from ref. 292. Copyright 2022, Elsevier B.V.

MOF-on-MOF composites represent a versatile and promising series of new materials. MOF-on-MOF is commonly constructed using the epitaxial growth strategy.^276,277 Within the reaction system, two types of MOF with matching lattice structures are used; one MOF acts as a “seed” upon which the other grows epitaxially. Hou et al. used the epitaxial growth strategy in which PCN-134 (ligand = 1,3,5-tris(4-carboxyphenyl) benzene (BTB), TCPP) is used as an MOF seed to grow rod-like PCN-222 secondary MOFs on its surface.²⁷⁸ The prepared lines of P21-X exhibited excellent visible photocatalytic performance for the degradation of NZT (nizatidine). Among them, P21–3 exhibits the best catalytic activity, achieving a removal rate of 90.81% within 120 minutes of light exposure. MOF-on-MOF configurations can additionally create heterojunctions that enable the reverse migration of electrons and holes via the CB and VB, effectively enhancing charge separation and carrier mobility.^279–281 Combining advanced oxidation processes (AOPs) that utilize visible light to generate reactive oxygen species (ROS) under sunlight can degrade pollutants in a more efficient and environmentally friendly way.^282–284 A new generation of double Z-scheme UiO/MIL/CN heterojunctions was designed for ofloxacin photodegradation by Mohammad Ali Zolfigol et al.²⁸⁵ NH₂-MIL-125 was first synthesized using the solvothermal method as a host MOF, followed by the growth of UiO-66 crystals on the surface of NH₂-MIL-125 with the same technique. Then g-C₃N₄ nanosheets were modified on the surface of Zr-MOF-on-Ti-MOF as shown in Fig. 20B. The photogenerated electrons on the CB of NH₂-MIL-125 spontaneously combine electrostatically with holes in the VB of g-C₃N₄ and UiO-66. The CB potentials of g-C₃N₄ (−1.08 V) and UiO-66 (−0.6 V) are more negative than the O₂/˙O₂⁻ potential (−0.33 V vs. NHE), enabling the photogenerated electrons to reduce O₂ to ˙O₂⁻. Additionally, as the VB of both NH₂-MIL-125 and UiO-66 is more positive than H₂O/˙OH (1.99 V vs. NHE), the photogenerated holes in the VBs of NH₂-MIL-125 and UiO-66 (3.17 V and 3.22 V) can activate water molecules to form ˙OH. Eventually, these ROS (˙O₂⁻ and ˙OH) can effectively degrade and mineralize the ofloxacin (Fig. 20C). The degradation rate of ofloxacin is approximately 1.79 times higher than that of the original UiO-66-on-MIL-125. Zhang et al. developed a nanoreactor composed of iron-doped carbon dots (Fe-CDs) and MOF-808.¹⁹⁵ Fe-CDs/MOF-808 and Fe-CDs@MOF-808 were synthesized for cascade degradation and the detection of paraoxon and parathion, respectively.

In typical advanced oxidation processes (AOPs), hydrogen peroxide (H₂O₂) is used to generate hydroxyl radicals (˙OH).²⁸⁶ However, the instability of hydroxyl radicals limits their application. Recently, sulfate radicals (SO₄˙⁻, SR), which have a longer half-life than ˙OH, have gained increasing attention and solid persulfate (Na₂S₂O₈, PS) is more convenient for storage and transportation compared with liquid H₂O₂.²⁸⁷ Combining AOPs with photocatalysis and using materials containing metals such as iron, cobalt and manganese as photocatalysts is considered an ideal method to enhance degradation efficiency. Zhou et al. synthesized an Fe-based MOF MIL-88A (ligand = fumaric acid) via hydrothermal methods,²⁸⁸ which, when combined with sulfate-based advanced oxidation processes (SR-AOPs), effectively degrades tetracycline hydrochloride under visible light. The integration of photocatalysis with SR-AOPs significantly enhances the degradation capacity of tetracycline. Scavenging experiments and ESR analysis indicate that SO₄˙⁻ and ˙O₂⁻ radicals are the primary radicals responsible for the degradation of tetracycline. Similarly, Li et al. showed that sulfonamide antibiotics in water are effectively removed through the coupling of photocatalysis and persulfate advanced oxidation technology using Fe-doped UiO-66.²⁸⁹

Fenton technology is a widely used advanced oxidation process in wastewater purification. Using heterogeneous catalysts and combining photocatalysis with the Fenton method not only avoids the issues of traditional technology but also achieves efficient degradation. The photo-Fenton technique develops a fast electron transfer channel, enhancing the swift breakdown of H₂O₂ to produce reactive oxygen species, ensuring the efficiency of pollutant removal.^290,291 Wang et al. designed a self-assembled CuS/MIL-Fe catalyst to effectively degrade Acetaminophen (APAP) through the photo-Fenton reaction (Fig. 20D).²⁹² Using UV–vis and Tauc curve analyses, the band gaps (E_g) of CuS, MIL-101(Fe) and CuS/MIL-Fe were determined to be 1.80, 2.71 and 2.18 eV, respectively. MIL-101(Fe) and CuS are typical n-type and p-type semiconductors, respectively. When combined, these two types of semiconductor form a p–n heterojunction, improving the APAP degradation effectively. R Geetha Balakrishna et al. have developed a low-cost and simple method to enhance the performance and stability of Fe-MOFs through amine (NH₂) functionalization and in situ integration of metal oxides.²⁹³ The dual photoexcitation pathway induced by NH₂ functional groups facilitates electron transfer to the Fe center, directly exciting Fe-o clusters (Fe³⁺ to Fe²⁺), endowing the system with multifunctionality. The NH₂-MIL-101(Fe) and α-Fe₂O₃ composite materials obtained were subsequently embedded into PAN and electrospun nanofibers, used for enhancing the self-cleaning properties of the membranes and the simultaneous separation and photocatalytic degradation of various pollutants. This heterostructure composed of metal oxides and MOF composites exhibits excellent photocatalytic activity, effectively facilitating carrier separation. Integrating the composite materials into nanofiber membranes for water treatment achieved an overall removal rate of 94%, with 20% of the MB being degraded simultaneously, making it a valuable reference for emerging processes that involve simultaneous degradation and separation. The summary of different MOF-based photocatalysts for pollutant degradation was shown in Table 1.

Table 1 The summary of different MOF-based photocatalysts for pollutant degradation

MOF	Pollutant	C Pollutant (mg L^−1)	C MOF (mg L^−1)	ROS	Time (min)	Efficiency (%)	K (min^−1)	Ref.
PDI refers to pyromellitic diimide. PET refers to polyethylene terephthalate. PDA refers to polydopamine. NS refers to nanosphere. Psf refers to polysulfone. PAN refers to polyacrylonitrile. MOX refers to metal–organic xerogel. P21 refers to PCN-134 and PCN-222. VNU refers to Vietnam National University. N.R = Not reported.
MIL-88A(Fe)	Tetracycline hydrochloride	100	250	SO4^−, ˙O2^−	90	100	0.0216	288
g-C3N4/PDI@NH2-MIL-53(Fe)	Carbamazepine	50	400	˙OH	150	78	N.R	294
PET@PDA@NH2-MIL-53(Fe)	Phenol	20	N.R	˙OH, h^+	60	98	0.0519	295
g-C3N4/PDI@NH2-MIL-53(Fe)	Tetracycline	50	400	˙OH	60	90	N.R	294
CuS/MIL-101(Fe)	Acetaminophen	5	200	˙OH, ^1O2	30	99.8	0.2099	292
MIL-100(Fe)@ZnO NS	Phenol	5	200	˙OH	120	92	0.0339	296
g-C3N4/PDI@NH2-MIL-53(Fe)	p-Nitrophenol	50	400	˙OH	30	100	N.R	294
MIL-101(Fe)/g-C3N4	Cr(VI)	20	50	˙O2^−, h^+	60	92.6	0.0432	297
PSf/MIL-101(Fe)	Methyl orange	10	N.R	˙OH, ˙O2^−	120	71.03	N.R	298
rPAN@ (NH2-MIL-101(Fe) + α –Fe2O3)	Methylene blue	10	200	˙OH, ˙O2^−, h^+	120	93.63	N.R	293
P21-X(PCN-134,PCN-222)(Zr)	Nizatidine	40	100	^1O2	120	90.81	0.01345	278
VNU-1(Zr)	Sulfamethoxazole (80 ppb)	0.08	100	˙O2^−, h^+	5	100	0.478	273
UiO-66/NH2-MIL-125/g-C3N4	Ofloxacin	20	100	˙OH, ˙O2^−	50	>99.1	0.07	285
NH2-MIL-53(Al)/CdS	Cr(VI)	10	400	h^+, ˙OH	180	99.23	0.0276	124
Ag-MOF	Methyl orange	10	50 mg	O^2−, h^+	120	74.5	0.0123	299

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3.2. Carbon dioxide (CO₂) reduction

The excessive use of fossil fuels has led to the accelerated emission of carbon dioxide, becoming a key factor in adverse climate and environmental changes.^300,301 Converting CO₂ into useful chemicals (such as CO, HCOOH, CH₃OH and C₂H₄) is imperative.³⁰² Despite significant efforts and progress over the past decade, challenges in reaction activity and product selectivity persist in this research field.³⁰³ Photocatalytic methods can efficiently utilize solar energy while also reducing the concentration of CO₂ in the air.^304–311 The porous structure of MOF-based photocatalysts stands out among many materials by providing more reaction sites and abundant CO₂ capture sites during the catalysis process.^312–315 MOF photocatalysts need a LUMO level higher than the redox potential of the CO₂ reduction half-reaction, which depends on the potential of various products. For example, the redox potentials for CO₂ reduction to CO, formic acid, methanol and methane are 3.50, 3.42, 3.65 and 3.79 eV respectively (Fig. 21A).³¹⁶ Bai et al. used a 2D Ni-based MOF NMF ([Ni(phen)(oba)]_n·0.5nH₂O, phen = 1,10-phenanthroline, oba = 4,4′-oxybis(benzoate)) composite with all-inorganic perovskite CsPbBr₃; when the aspect ratio of CsPbBr₃ nanocubes, nanorods and nanowires increases, the energy difference (ΔE) at the CB of CsPbBr₃ and the NMF interface increases, promoting the transfer of photoelectrons from CsPbBr₃ to NMF and enhancing the photocatalytic activity for CO₂-to-CO conversion.³¹⁷

Fig. 21 (A) The process of photocatalytic CO₂ reduction and the redox potentials for the reduction of CO₂ into CO, HCOOH, CH₃OH and CH₄ from ref. 316. Copyright 2020, Elsevier. (B) (a and b) Free energy profile of CO₂RR toward the production of CO and free energy diagram of HER for PCN 250-Fe₃ and PCN-250-Fe₂Co, respectively; (c) photocatalytic routes for CO₂ reduction from ref. 319. Copyright 2020, Royal Society of Chemistry. (C) (a) Time-dependent HCOOH evolution with InTCP-Co, FeTCP-Co and FeTCP-OH-Co as catalysts respectively, (b) detection of H₂O₂ by HPLC after CO₂, (c) the HCOOH yield in 3 cycles, and (d) the ¹³C NMR spectra of reaction solutions with ¹³CO₂/¹²CO₂ and over FeTCP-OH-Co from ref. 320. Copyright 2021, John Wiley and Sons. (D) The construction of Zr-MBA-Ru/Re-MOF via post-synthetic linker exchange and immobilization of the molecular photosensitizer and catalyst toward CO₂ reduction from ref. 321. Copyright 2021, Royal Society of Chemistry.

In most cases, rare metals such as Ru, Ir and Co are key components of photocatalysts, including photosensitizers and catalysts. Constructing CO₂ photoreduction systems using abundant and non-toxic elements on Earth aligns with the principles of sustainable chemistry. Daisuke Tanaka et al. reported a type of Sn^II-based MOF [Sn^II₂(H₃ttc)₂·MeOH]_n(KGF-10; H₃ttc = trithiocyanuric acid, MeOH = methanol) that converts CO₂ into formate ion (HCOO⁻) under visible light.³¹⁸ This represents the first example of using Sn-MOF to drive CO₂ reduction with visible light. Even in the absence of precious-metal cocatalysts, KGF-10 catalyzes the conversion of CO₂ to HCOO⁻ with high selectivity (>99%) and an AQY of 9.8% at 400 nm.

Feng et al. enhanced CO₂ conversion activity by varying the types of metal in MOF cluster nodes, achieving the highest-performing MOF photocatalyst for CO₂ conversion.³¹⁹ PCN-250-Fe₃ (ligand = 3,3′,5,5′-azo-benzene tetra-carboxylic acid (H₄abtc)) was made with Fe^III₂Fe^II metal-cluster nodes and open metal sites. Varying the type of MII metal ions in the PCN-250-Fe₂M (M = Mn, Zn, Ni, Co) clusters exhibited better catalytic activity and selectivity for CO₂ conversion. Among them, PCN-250-Fe₂Mn achieved the highest photocatalytic activity under visible light, reaching 21.51 mmol h⁻¹ g⁻¹, making it the most effective MOF-based CO₂ conversion photocatalyst under similar reaction conditions to date. DFT was utilized to further analyze the specific effects of the bimetallic PCN-250-Fe₂Co on CO₂ reduction performance (Fig. 21B). Introducing the second M^II metal ion enhances the CO₂ reduction pathway, inhibiting the formation of H₂ evolution intermediates. This promotes the migration of photogenerated electrons to active sites and improves the adsorption and activation of CO₂.

The preparation of hydroxylated MOFs with multiple active centers as catalysts achieved high efficiency and selectivity in artificial photosynthesis. Lin et al., by modifying the types of metal on the nodes and introducing excess hydroxyl groups on the ligands, prepared hydroxylated and heterometallic MOFs.³²⁰ Artificial photosynthesis was achieved without the addition of sacrificial agents and photosensitizers. Acid–base resistant frameworks of (In/Fe)TCP-Co (ligand = 5,10,15,20-tetrakis(3,4-dihydroxyphenyl)porphyrin-Co (3,4-TDHPP-Co)) and (In/Fe)TCP-OH-Co (ligand = 5,10,15,20-tetrakis(2,3,4-trihydroxyphenyl)porphyrin-Co (2,3,4-TTHPP-Co)) based on catechol-metalloporphyrin were prepared. The HCOOH evolution rate of FeTCP-Co far exceeded that of InTCP-Co by 3.7 times. The iron-oxo chain in FeTCP-Co is more favorable than the indium-oxo chain in InTCP-Co for photoinduced electron–hole separation, according to the experimental and DFT calculations findings. The reactivity of CO₂ to HCOOH conversion was further enhanced by incorporating uncoordinated -OH into the (In/Fe)TCP-Co framework. Their HCOOH yields were respectively 5.6 and 4.3 times those of uncoordinated non-hydroxylated compounds. FeTCP-OH-Co achieved a selectivity for HCOOH of 97.8%. Ion chromatography (IC), gas chromatography (GC) and high-performance liquid chromatography (HPLC) were used to detect the reduced products HCOOH, trace CO and oxidized product H₂O₂, respectively (Fig. 21C(a and b)). The results showed that under the same conditions, FeTCP-Co had the highest photocatalytic activity, with the photocatalytic efficiency increasing in turn from InTCP-Co, FeTCP-Co and InTCP-OH-Co to FeTCP-OH-Co. Repeated experiments also demonstrated the good stability of the catalysts (Fig. 21C(c)). To validate the carbon source of the evolved products, isotope-labeling experiments were checked (Fig. 21C(d)). The enhancement in performance was due to the synergistic effects of strong visible light absorption, suitable band structure, ideal CO₂ affinity and multiple evenly distributed redox active sites.

Molecular catalytic active sites, including photosensitizers, can also be grafted onto MOFs through post-synthetic modification (PSM), expanding the optical absorption range and enhancing the catalytic activity. Here, Tapas Kumar Maji et al. constructed a multifunctional Zr-MBA-Ru/Re-MOF (MBA = A [2-(50-methyl-[2,20-bipyridine]-5-yl)acetic acid])assembly using post-synthetic ligand exchange (PSE) and PSM.³²¹ MOF-808 (Zr-MOF, ligand = H₃BTC) was selected as the host due to its enclosed cage structure and robustness in water, first, formates were exchanged with MBA ligands through PSE to enter the chelating sites within the MOF unit. MBA was covalently grafted onto Zr-MOF, utilizing the N,N′ chelating center of the MBA ligands to chelate with [Ru(bpy)₂]²⁺, and subsequently metalated with [Re(CO)₅Cl] to obtain Zr-MBA-Ru/Re-MOF. The synthesized Zr-MBA-Ru/Re-MOF exhibited effective CO₂ reduction without requiring other sacrificial electron donors, and the selectivity for CO was >99%, with a maximum yield of 440 μmol g⁻¹ h⁻¹. Additionally, using BNAH and TEOA as sacrificial electron donors, the selectivity for CO was 69%, with a maximum rate of 180 μmol g⁻¹ h⁻¹. To simulate natural photosynthesis, the catalytic activity under direct sunlight was studied in water, producing 210 μmol g⁻¹ of CO within 6 hours (Fig. 21D).

The summary of different MOF-based photocatalysts for CO₂ reduction is shown in Table 2.

Table 2 The summary of different MOF-based photocatalysts for CO₂ reduction

MOF	Production	Sacrificial electron donor	Reduction product	Selectivity (%)	Ref.
MBA refers to 2-(50-methyl-[2,20-bipyridine]-5-yl)acetic acid. BNAH refers to 1-benzyl-1,4-dihydronicotinamide. TEOA refers to triethanolamine. TIPA refers to tri-isopropanolamine. Fc refers to ferrocene carboxylic acid. MOL refers to metal–organic layers. HHTP refers to 2,3,6,7,10,11-hexahydroxytriphenylene. BVO refers to BiVO4. TCPP refers to tetra(4-carboxyphenyl)porphyrin. BTC refers to 1,3,5-benzene tricarboxylate. CM refers to copper mesh. KGF-10 refers to H3ttc = trithiocyanuric acid, MeOH = methanol. BIH refers to 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole. SAs refers to single-atom catalysts. N.R = not reported.
Zr-MBARu/Re-MOF	CO	BNAH, TEOA	180 μmol g^−1 h^−1	69	321
PCN-250-Fe2Mn	CO	TIPA	21.51 μmol g^−1 h^−1	82.17	319
PCN-250-Fe2Zn	CO	TIPA	19.45 μmol g^−1 h^−1	82.30	319
PCN-250-Fe2Ni	CO	TIPA	15.86 μmol g^−1 h^−1	81.84	319
PCN-250-Fe2Co	CO	TIPA	14.01 μmol g^−1 h^−1	83.53	319
Ni-MOF(H2O)	CO	TEOA	9.61 mmol h^−1 g^−1	95.24	322
Ni-MOL-100	CO	TEOA	569.00 mmol g^−1/18 h	100	323
Co-MOF/Cu2O	CO	None	3.83 μmol g^−1 h^−1	∼100	324
SiW12@CdMOF	CO	TEOA	4.35 mmolCO molCd^−1 h^−1	∼99	325
PW12@CdMOF	CO	TEOA	3.6 mmolCO molCd^−1 h^−1	∼99	325
NH2-UiO-66-Fc	CO	TEOA	90.65 μmol g^−1 h^−1	100	326
Fe-soc-O	CO	TIPA	1804 μmol g^−1 h^−1	85.6	327
Fe-soc-M	CO	TIPA	1594 μmol g^−1 h^−1	90.7	327
Fe-soc-C	CO	TIPA	1550 μmol g^−1 h^−1	94.7	327
Ru@Cu-HHTP	CO	TEOA	156 mmol gcat^−1 h^−1	92.9	328
FeSx@ZnS@CoSx(MIL-88A@ZIF-8@ZIF-67)	CO/CH4	(Water vapor)	69.90 /2.01 μmol g^−1 h^−1	N.R	329
Ag/NH2-MIL125(Ti) (T3-Ag)	CO/CH4	TEOA	26.7/63.3 μmol g^−1 h^−1	90.5 (CH4)	330
Ni@MOF/BVO(NTU-9)	CO/CH4	TEOA	44.5/0.625 μmol g^−1 h^−1	99.2	330
Cu-ZnTCPP/g-C3N4	CO/CH4	N.R	92 /11.3 μmol g^−1 h^−1	N.R	331
CuTCPP & UiO-66/TiO2	CO/CH4	N.R	31.32 μmol g^−1 h^−1/0.148 μmol g^−1 h^−1	N.R	332
Cu2O@Cu3(BTC)2/CM(copper mesh)	CH4	N.R	0.73 mmol/8 h	N.R	333
FeTCP-OH-Co	HCOOH/CO	None	17.72/0.396 μmol g^−1 h^−1	97.8	320
FeTCP-Co	HCOOH/CO	None	4.14/0.088 μmol g^−1 h^−1	N.R	320
InTCP-Co	HCOOH/CO	None	1.125/0.041 μmol g^−1 h^−1	N.R	320
KGF-10	HCOOH	BIH	59 μmol, 4 mg, 5 h	>99	318
PCN-136(Zr)	HCOO^−	TIPA	0.294 μmol g^−1 h^−1	N.R	334
Cu SAs/UiO-66-NH2	Methanol/ethanol	N.R	5.33/4.22 μmol h^−1 g^−1	N.R	335

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3.3. Nitrogen (N₂) fixation

Nitrogen is essential for life, constituting important components of amino acids, nucleotides and other essential biomolecules. Nitrogen fixation stands as one of the most important natural processes in the Earth's nitrogen cycle, converting atmospheric N₂ into nitrogen-containing compounds accessible to organisms, including humans.^336–340 Although nitrogen gas constitutes 78 vol% of the Earth's atmosphere, its high electron affinity and large activation energy barrier (941 kJ mol⁻¹) prevent direct utilization. Effective storage and conversion into nitrogen-containing compounds, namely ammonia (NH₃) and nitrate (HNO₃), are crucial for promoting the nitrogen economy.^341–345

Currently, the most extensively studied system is the Mo-dependent nitrogenase from the nitrogen-fixing bacterium Azotobacter vinelandii, consisting primarily of Fe protein and MoFe protein, where the MoFe protein contains two metal clusters (P-cluster and M-cluster).^346,347 But relying solely on naturally occurring fixed nitrogen (primarily ammonia) generated through natural cycles is insufficient to meet the needs of industrial production and human life. The Haber–Bosch (HB) process, as one of the most significant inventions of the 20th century, greatly increased crop yields and the global economy. Until now, this remains the sole method for industrial ammonia production. The key chemical reaction of HB process can be expressed by the reaction N₂ (g) + 3H₂ (g) → NH₃ (g).^348–350 Each step of the reaction requires high temperatures and pressures (673–873 K, 15–25 MPa),^351,352 resulting in significant energy consumption and greenhouse gas emissions.

Photocatalytic nitrogen fixation offers environmental friendliness and energy efficiency, making it an ideal approach in the context of carbon neutrality and environmental protection.^353,354 Up to now, extensive research has been conducted to develop efficient heterogeneous photocatalysts for nitrogen fixation, ranging from inorganic to organic semiconductors, such as Fe₂O₃, CdS, ZnO, BiOBr and g-C₃N₄.^355,356 However, these catalysts similarly suffer from issues such as low efficiency. The kinetic diameter of N₂ (3.64 Å) is smaller than the pore sizes of multi-level porous MOFs (most of which are >4 Å), and MOFs possess an ultra-high surface area of up to 7000 m² g⁻¹, giving us reason to believe that MOFs can efficiently adsorb and store N₂.^357–361

This is the first example of N₂ fixation driven by visible light using MOF materials. Ye et al. first reported a series of targeted visible-light-active MOF catalysts, namely CH₃-MIL-125 (Ti), OH-MIL-125 (Ti) and NH₂-MIL-125 (Ti), with visible-light-assisted photocatalytic N₂ fixation Fig. 22A(a).³⁶² The structural schematic of NH₂-MIL-125 (Ti) is shown in Fig. 22A(b). Its 3D pseudocubic structure is composed of cyclic octameric inorganic subunits of edge and corner-sharing TiO₅(OH) octahedra, connected by aminoterephthalate ligands. MOFs with different functional groups but the same structure demonstrated effective photocatalytic N₂ fixation under visible light, thanks to the interaction between the Ti nodes and functionalized organic linkers. The results indicate that NH₂-MIL-125 (Ti) exhibits the best light absorption performance at 550 nm, with the highest ammonia evolution rate of 12.3 μmol g⁻¹ h⁻¹. The Ti³⁺ sites generated by LMCT electron transfer act as active sites, with ligand functionalization extending the MOF light harvesting to the visible spectrum, facilitating visible-light-assisted photocatalytic N₂ fixation.

Fig. 22 (A) (a) Schematic diagram of the targeted visible light active MOFs for N₂ fixation, (b) the structure of NH₂-MIL-125 (Ti) from ref. 362. Copyright 2020, Elsevier. (B) A proposed mechanism for a U(0.5Hf)-2SH sample in nitrogen reduction from ref. 363. Copyright 2021, Elsevier. (C) (a) XRD patterns of UiO-66 before and after UV-Vis light and visible light irradiation, (b) the concentrations of Zr in the first and second photocatalytic solutions determined by ICP-MS, (c) N₂ adsorption–desorption isotherms for UiO-66-fresh and UiO-66-UV-vis, and (d) plot of the scan rates against the differences in the double-layer charging current of UiO-66-fresh and UiO-66-UV-vis from ref. 364. Copyright 2021, Royal Society of Chemistry. (D) (a–c) Structures of N₂ adsorbed on different sites of catalysts, (d) difference charge density of Ru₁/d-UiO-66 with the adsorption of N₂, (e) free energy diagrams for N₂ reduction on Ru₁/d-UiO-66 through distal and alternating mechanisms with corresponding reaction energy from ref. 365. Copyright 2023, Wiley-VCH GmbH.

Inspired by nitrogenase, Jiang et al. designed a series of modular UIO-66-type metal–organic frameworks, consisting of bimetallic Zr–Hf nodes and MOF photocatalysts U(Zr–Hf)-X with various functional ligands.³⁶³ In the Zr–Hf bimetallic clusters, the elements zirconium (Zr) and hafnium (Hf) selectively adsorb nitrogen. Due to the potential difference between the Zr–O and Hf–O clusters, there was an MMET pathway from Hf species to Zr species, acting as an electron buffer optimizing electron transfer, and Zr species served as the catalytically active site, synergizing electron movement and utilization in photocatalysis. In the organic ligands, introducing targeted functional groups effectively generates electrons, enhancing the MOFs’ light absorption and utilization, with organic ligands containing two thiol (-SH) groups extending the absorption edge to the visible light region. The nitrogen fixation rate of the photocatalyst reaches up to 116.1 μmol g⁻¹ h⁻¹ under visible light, which is attributed to the LMMET pathway and Zr-based π-backbonding mechanism. By independently manipulating the metal nodes and organic ligands, this nitrogenase-inspired MOF material strategy paves a new pathway for designing dual synergistic catalysts for chemical, photochemical and biochemical conversions in energy and environmental fields (Fig. 22B).

Defects can capture electrons and then inject these electrons into antibonding π-orbitals, thereby weakening and activating the adsorbed N₂. Li et al. reported the first instance of UiO-66 featuring photoexcited cluster defects and linked defects for photocatalytic nitrogen fixation.³⁶⁴ Linker defect-formed unsaturated metal nodes exhibited greater contribution to photocatalytic nitrogen fixation than cluster defects, which defined as the PSE process. Notably, in an air atmosphere and pure water without a sacrificial agent, the full-wavelength performance increased from 131 to 196 μmol h⁻¹ g⁻¹ compared with fresh UiO-66. UiO-66 was synthesized using a straightforward solvothermal method and various tests confirmed the presence of linker defects. Following UV-Vis irradiation, UiO-66 exhibited an increased Brunauer–Emmett–Teller (BET) specific surface area and pore volume. These results are attributed to the formation of defects, as demonstrated by the impact of illumination on the material electrochemical performance through electrochemical surface area (ECSA) measurements (Fig. 22C(a–c)). As shown in Fig. 22C(d), the C_dl of UiO-66-UV-vis (0.428 mF cm⁻²) is higher than the UiO-66-fresh C_dl (0.202 mF cm⁻²), indicating a higher ECSA for UiO-66-UV-vis. The larger BET surface area and ECSA provide more active sites, which is beneficial for enhancing the performance of photocatalytic nitrogen fixation. This paper presents a novel and effective strategy for designing high-performance nitrogen-fixing photocatalysts.

Similarly, Meng et al. proposed a photochemical strategy for creating defects,³⁶⁵ further depositing Ru single atoms on the UiO-66 (Zr) framework. After treatment with nitrogen gas and ultraviolet light, UiO-66 was centrifuged, washed, and vacuum-dried to yield defect-laden UiO-66 (d-UiO-66). Covalent bonding between the Ru single atoms and the support establishes electron-metal–support interactions (EMSI). EMSI facilitates accelerated charge transfer between Ru SAs and UiO-66, conducive to enhanced photocatalytic activity. After introducing defects to UiO-66, the photocatalytic yield of ammonia increased from 4.57 μmol g⁻¹ h⁻¹ to 16.28 μmol g⁻¹ h⁻¹, and following the loading of Ru SAs, it further rose to 53.28 μmol g⁻¹ h⁻¹. DFT calculations were employed to investigate the strong EMSI effect in Ru₁/d-UiO-66 and further elucidate the impact of Ru single-atom anchoring on UiO-66 for local electronic structure tuning and NRR pathways. For the pristine UiO-66, N₂ molecules are terminally adsorbed at Zr sites with a N–N bond length of 1.112 Å, closely approximating the bond length of free N₂ molecules at 1.11 Å. Bader charge analysis shows that a transfer of 0.05 e⁻ to N₂ molecules makes their activation challenging, indicating the difficulty of NH₃ formation on the pristine UiO-66 surface (Fig. 22D(a)). Subsequently, the adsorption results of N₂ at different sites in Ru₁/d-UiO-66 indicate a preference for adsorption at Ru sites on UiO-66 nodes. Bader charge analysis indicates that the 0.174 e⁻ transferred to N₂ molecules at Ru sites is 35 times that transferred at Zr sites (Fig. 22D(b and c)). Due to EMSI, electron mobility within active sites enhances the activity of single atom metal centers. Concurrently, the N–N bond length extends from 1.112 to 1.171 Å, showing an efficient activation of N₂ molecules. Charge density differential analysis also reveals the formation of strong EMSI at the active sites of Ru₁/d-UiO-66. Fig. 22D(d) showed visible charge accumulation around Ru, further evidencing the modulation of the electronic structure of the metal center following the introduction of Ru single atoms. Fig. 22D(e) displayed the free energy diagram for the reduction of N₂ on Ru₁/d-UiO-66 through a distal and corresponding reaction energy alternating mechanism. The summary of different MOF-based photocatalysts for N₂ reduction is shown in Table 3.

Table 3 The summary of different MOF-based photocatalysts for N₂ reduction

MOF	Light source	Raw materials	Nitrogen fixation activity	Scavenger	Ref.
abtc refers to 3,3′,5,5′-azomellitic acid (H4abtc). PSE refers to post-synthetic ligand exchange process. DFC3N4 refers to defected film C3N4. N.R = Not reported.
NH2-MIL-125 (Ti)	Xenon lamp (300 w) λ ≥400 nm	N2/H2O	12.3 μmol g^−1 h^−1	None	362
OH-MIL-125 (Ti)	Xenon lamp (300 w) λ ≥400 nm	N2/H2O	5.04 μmol g^−1 h^−1	None	362
CH3-MIL-125 (Ti)	Xenon lamp (300 w) λ ≥400 nm	N2/H2O	1.39 μmol g^−1 h^−1	None	362
MIL-53(FeII/FeIII)	Xenon lamp (300 w) λ ≥420 nm	N2/H2O	306 μmol g^−1 h^−1	K2SO3	366
4U(0.5Hf)-2SH (UiO-66)	Xenon lamp (300 w)	N2/ultrapure water	116.1 μmol g^−1 h^−1	K2SO3	363
Fe-abtc	Xenon lamp (300 w)	N2/H2O	49.8 μmol g^−1 h^−1	K2SO3	367
Zr-abtc	Xenon lamp (300 w)	N2/H2O	35.7 μmol g^−1 h^−1	K2SO3	367
MIL-125(Ti)-250 (250 °C)	Xenon lamp (300 w)	N2/H2O	156.9 μmol g^−1 h^−1	N.R	368
Ru-MOF-74(Zn)	Xenon lamp (300 w) λ >400 nm	N2/H2O	70.9 μmol g^−1 h^−1	None	369
UiO-66-PSE	Xenon lamp (300 w)	N2/ultrapure water	196 μmol g^−1 h^−1	None	364
ZCS(ZnCdS)@CAU-100(Bi)	Xenon lamp (300 w)	N2/ultrapure water	46.96 μmol g^−1 h^−1	None	370
CAU-17@Bi2S3	Xenon lamp (300 w)	N2/ultrapure water	14.89 μmol g^−1 h^−1	None	370
UiO-66(SH)2-200(200 °C)	Xenon lamp (300 w)	N2/H2O	32.4 μmol g^−1 h^−1	None	371
Ru1/d-UiO-66	Xenon lamp (300 w)	N2/H2O	53.28 μmol g^−1 h^−1	N.R	365
CeZr5-UiO-66	Xenon lamp (300 w)	N2/ultrapure water	200.13 mmol g^−1 h^−1	N.R	372
ZIF-67@PMo4V8	Xenon lamp (300 w)	N2/H2O/ethanol	149.0 μmol L^−1 h^−1	N.R	373
ZIF-67@PMo112	Xenon lamp (300 w)	N2/H2O/ethanol	39.4 μmol L^−1 h^−1	N.R	373
ZIF-67@PMo11V	Xenon lamp (300 w)	N2/H2O/ethanol	70.0 μmol L^−1 h^−1	N.R	373
ZIF-67@PMo10V2	Xenon lamp (300 w)	N2/H2O/ethanol	74.8 μmol L^−1 h^−1	N.R	373
ZIF67@PMo9V3	Xenon lamp (300 w)	N2/H2O/ethanol	134.6 μmol L^−1 h^−1	N.R	373
MOF-74(Zn)@DFC3N4	Xenon lamp (300 w)	N2/H2O/methanol	2.32 mmol g^−1 h^−1	Methanol	374

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3.4. Hydrogen (H₂) production

The depletion of traditional fossil fuel resources has generated a substantial demand, prompting the exploration of sustainable fuels. In this context, hydrogen is regarded as the most promising emerging fuel for the future due to its zero carbon emissions and high energy density (142 MJ kg⁻¹).^375–378 Hydrogen energy is considered a green and renewable alternative to traditional energy sources. However, H₂ production has been somewhat limited due to its high production costs.³⁷⁹ The hydrogen evolution reaction (HER) is significant in the field of photocatalysis as it converts solar energy into hydrogen. To enhance the efficiency of HER, researchers are developing efficient photocatalysts such as those based on MOFs and their composites.³⁸⁰ Due to their large porosity, which provides a substantial surface area and numerous active sites, MOFs have tremendous potential in hydrogen production. Currently, an increasing number of MOF-based photocatalysts are being used for hydrogen production.^381–383 In 2009, the first instance of an MOF being used as an active site for photocatalytic water reduction to hydrogen molecules was reported by Wasuke Mori et al.³⁸⁴

Considering the remarkable efficiency of platinum photocatalysts in hydrogen evolution from water, growing research has diverted its focus towards investigating Pt and other noble metal catalysts.³⁸⁵ Lai et al. synthesized ultrathin Zr-TCPP(Pd) nanosheets.³⁸⁶ Uniform anchoring of Pt nanoparticles onto smaller nanoparticles enables the synthesis of 2% Pt/Zr-TCPP (Pd) hybrid nanosheets with a homogeneous distribution. Under visible light irradiation, the hydrogen production rate of 2% Pt/Zr-TCPP(Pd) reaches 3348 μmol h⁻¹ g⁻¹. The AQE of 2% Pt/Zr-TCPP(Pd) at 420 nm is approximately 1.56%. This outstanding photocatalytic H₂ production performance is due to the ultrathin nanosheets’ ability to rapidly transfer long-lived electrons from pd-porphyrin photosensitizers to PtNPs, obviously enhancing the efficiency of photogenerated electron–hole separation.

In the early stages, noble metal co-catalysts (Pt, Pd, Au) were used to enhance photocatalytic hydrogen evolution; however, the scarcity and high cost of these metals limit the HER process. Therefore, developing high-density active sites and high-efficiency non-noble metal photocatalysts remains a significant challenge. In recent years, research on non-noble metals has increasingly grown.³⁸⁷ Huang et al. prepared a conductive MOF-based catalyst,³⁸⁸ Ni₃(HITP)₂/TiO₂ (ligand = 2,3,6,7,10,11-hexaaminotriphenylene (HATP)), whose corresponding HER demonstrated a significant enhancement in photocatalytic performance when TiO₂ was combined with Ni₃(HITP)₂. Systematic photoelectrochemical experiments indicated that Ni₃(HITP)₂ effectively facilitated charge transfer and reduced charge recombination. Furthermore, using the Ni-N₄ active sites in Ni₃(HITP)₂ as the reaction centers, it successfully achieved noble-metal-free photocatalysis without relying on the typical co-catalyst Pt. This provides a new direction for subsequent MOFs in the field of non-noble metal photocatalytic production of H₂.

Moreover, Shi et al. synthesized a series of trimetallic MOF-based photocatalysts containing uniformly dispersed photosensitive Cd-NS and photothermal Ni-NS single-sites at a molecular level.³⁸⁹ By balancing the photosensitivity and photothermal effect, the optimized Ho₆-Cd_0.76Ni_0.24-NS (ligand = 3-fluoropyridine-4-carboxylic acid (HFNA)) exhibited an outstanding hydrogen evolution rate of 40.06 mmol g⁻¹ h⁻¹, with an AQE of 29.37% at 420 nm. Owing to the ultrashort distance from the uniformly dispersed Cd-NS single-sites to Ni-NS single-sites, combined with the photothermal effect, the transport of photoinduced electrons for proton reduction was greatly enhanced.

To investigate the impact of electrons on photocatalytic activity, Zhang et al. proposed a scheme to modulate the rate of electron transfer and the corresponding photocatalytic HER activity by adjusting the size of CdS quantum dots (QDs).³⁹⁰ Using a partial sulfidation strategy, they constructed a series of sensitized semiconductors composed of monodisperse CdS QDs with controlled sizes and cadmium tetra(4-carboxyphenyl)porphyrin (Cd-TCPP) nanosheets. By controlling the size of the CdS to adjust the bandgap, as shown in Fig. 23A, it was observed that as the average particle diameter increased from 3.3 nm to 8.1 nm, the bandgap of the CdS decreased from 2.72 eV to 2.40 eV. Larger CdS quantum dots exhibited lower conduction band positions, as shown in Fig. 23B. CdS/Cd-TCPP composites with larger CdS quantum dot sizes are expected to exhibit stronger photoresponsivity. A variety of experiments revealed that CdS/Cd-TCPP composites with medium-sized CdS QDs and optimal conduction band positions achieved the highest photocatalytic activity, with HER activity nearly five times that of pure CdS. This may be because larger CdS quantum dots facilitate electron injection from Cd-TCPP to CdS, although their synthesis consumes more Cd centers in the Cd-TCPP substrate during the sulfidation process. Thus, medium-sized CdS quantum dots exhibited the greatest photoresponse and photocatalytic activity. Fig. 23C illustrates the electron transfer process within its heterostructure.

Fig. 23 (A) Size-dependent bandgaps and energy-band positions of CdS QDs with various sizes. (B) Schematic diagram of the CB position in relation to −ΔG. The spheres denote photoinduced electrons. (C) The exciton dissociation and transfer in the CdS/Cd-TCPP. The spheres denote photoinduced electrons and holes, respectively from ref. 390. Copyright 2022, Elsevier B.V.

In addition to hydrogen production via water electrolysis, hydrogen can also be generated from hydrogen storage compounds, such as acetic acid and formic acid. Formic acid is widely regarded as a highly advantageous liquid organic hydrogen carrier, primarily due to its facile synthesis from CO₂ and its notable solubility in water, eliminating the need to recover byproducts that deplete H₂. Hermenegildo Garcia et al. used a titanium-based MOF, MIP-177-LT (ligand = 3,3′,5,5′-tetracarboxydiphenylmethane (H₄mdip), LT = low temperature).³⁹¹ MIP-177-LT was an efficient photocatalyst that catalyzed the release of H₂ from formic acid without the need for neutralizing acidity, or the additional use of sacrificial agents or noble metals. Notably, the photocatalytic H₂ release has a quantum efficiency of 22%, which is an order of magnitude higher than the decomposition activity of benchmark MOF photocatalysts such as MIL-125(Ti) and UiO-66(Zr) and over 30 times that of TiO₂P₂₅. This is particularly significant for a non-toxic, precious metal-free photocatalyst.

Moreover, Jiang et al. first reported on the coupling of photocatalytic hydrogen production with a value-added oxidation reaction. The photogenerated hydrogen production of MOF composite materials can be combined with selective benzylamine oxidation under light irradiation.³⁹² Using PCN-777 (formulated Zr₆O₄(OH)₁₀(H₂O)₆(TATB)₂) with a conjugated H₃TATB (4,4′,4′′-s-triazine-2,4,6-triyl-tribenzoic acid) ligand, upon MOF photoexcitation, electrons quickly reduced protons to generate H₂, while holes promoted the oxidation of benzylamine (Fig. 24). Li et al. successfully synthesized three types of MOF: Ni–Ni PMOF, Zr PMOF, and Zr–Ni PMOF; all of them were based on TCPP using the solvothermal method.³⁹³ The study found that among these MOFs, Zr–Ni PMOF exhibited excellent photocatalytic coupling properties, which can couple the photoreaction of aromatic alcohols with enhanced H₂ evolution (or CO₂ reduction).

Fig. 24 Illustration of simultaneous proton reduction and selective benzylamine oxidation over Pt/PCN-777 by photocatalysis from ref. 392. Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

We summarized examples of photocatalytic hydrogen production in the past five years, as shown in Table 4.

Table 4 Examples of photocatalytic hydrogen production in the past five years

Catalyst	H2 production rate	Sacrificial reductant	Light source	H2 source	Ref.
TEOA refers to triethanolamine. PMOF refers to porphyrin-based MOF. ZSTU refers to Zhejiang Sci-Tech University. TCPP(Pd) refers to Pd-metalized tetra(4-carboxyphenyl)porphyrin. mPT refers to mixed phenanthroline dibenzoate. MIP refers to Materials from Institute of Porous Materials of Paris. LT refers to low temperature. BIH refers to 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole.
Co0.74Fe0.26-MOF-74	1214.32 μmol h^−1 g^−1	TEOA	Visible	H2O	394
PMOF	8.52 mmol g^−1 h^−1	Ascorbic acid	Visible	H2O	395
MoO3/MIL-125-NH2	399.0 μmol h^−1 g^−1	Triethylamine	Visible	H2O	396
V2O5/MIL-125-NH2	298.6 μmol h^−1 g^−1	Triethylamine	Visible	H2O	396
Ni3(HITP)2/TiO2	350 μmol h^−1 g^−1	Ethylene glycol	UV–vis	H2O	388
Zn 0.5 Cd 0.5S/M-TiO2	180.4 mmol g^−1 h^−1	NaS·9H2O and Na2SO3	Visible	H2O	397
0.5(Co/Zr)-UiO-66-NH2	29.94 μmol h^−1 g^−1	CH3 OH	Visible	H2O	398
NH2-ZSTU-2	431.45 μmol h^−1 g^−1	TEOA	Visible	H2O	399
2% Pt/Zr-TCPP(Pd)	3348 μmol h^−1 g^−1	Ascorbic acid	Visible	H2O	386
Au@NH2-UiO-66/CdS	664.9 μmol h^−1 g^−1	Lactic acid acetonitrile	Visible	H2O	400
In-MOF	777.65 μmol h^−1 g^−1	TEOA	UV–vis	H2O	401
ZnIn2S4-NiO@MOF	4970.9 μmol h^−1 g^−1	TEOA	Visible	H2O	402
CdS/Cd-TCPP	3150 μmol h^−1 g^−1	TEOA	Visible	H2O	390
mPT-Cu/Co	TON: 18 700	BIH	Visible	Acetic acid	403
MIP-177-LT	650 μmol h^−1 g^−1	—	Solar	Formic acid	391

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3.5. Organic transformations

Presently, there is a substantial interest in MOFs as solid catalysts for organic reactions. The attractiveness of photocatalytic reactions lies in their ability to proceed under mild conditions, aligning well with the principles of green chemistry and sustainability for environmental conservation. MOFs offer a combination of heterogeneous catalysis advantages, such as easy recovery and high stability, along with homogeneous catalysis benefits, including selectivity, controllability, efficiency and mild reaction conditions. In recent times, significant progress has been made in the research on MOFs for photocatalytic organic transformations.^404,405 We provide a review of the types of photocatalytic reaction emerging in the past five years as shown in Fig. 25 and below is a summary of typical photocatalytic organic reactions.

Fig. 25 Scheme of the photocatalytic oxidation reaction with MOF.

3.5.1. Oxidative coupling of benzylamines. Considering the inherent properties of ketones or aldehydes, the conventional approach to imine generation via amine and carbonyl dehydration condensation necessitates expensive dehydration reagents and often presents challenges in product control. The amines-to-imines photocatalytic oxidative coupling represents a significant functional group transformation. This method offers not harsh conditions, environmental friendliness and economic efficiency, circumventing the excessive use of Lewis acids, toxic catalysts and expensive dehydrants typical in conventional thermal catalysis processes. Considering the significance and the straightforwardness of the reaction, the photocatalytic oxidation coupling of benzylamine has emerged as a prevalent basic reaction for assessing novel photoactive MOFs and COFs in the field of photocatalysis.⁴⁰⁶

Li et al. synthesized a hybrid structure comprising ultra-small Pd nanocrystals and MOFs, evaluating its photocatalytic performance using benzylamine coupling as a model reaction.⁴⁰⁷ The findings revealed that the Pd/MIL-125-NH₂ composite material demonstrated a notably enhanced photocatalytic performance compared with MIL-125-NH₂, attaining a conversion efficiency of 94.08% and an impressive benzylamine conversion rate of 3136 μmol h⁻¹ g_cat⁻¹. This enhancement is attributed to the Pd/MIL-125-NH₂ superior molecular activation capability, with ˙O₂⁻ and ¹O₂ identified as the primary oxidizing species. Thus, the Pd/MIL-125-NH₂ structure facilitates the transfer and separation of photogenerated electrons, providing catalytic sites for molecular activation and thereby enhancing the activity of the reaction.

Li et al. synthesized a series of UiO-68 (ligand = 4,4′-(benzo[c][1,2,5]thiadiazole-4,7-diyl)dibenzoic (H₂L)) with different metals to study the impact of various metals on catalytic activity.⁴⁰⁸ They incorporated an excellent photosensitizer, D–A–D type H₂L, into MOFs, which has been proved to significantly promote charge separation. Four 3D MOFs were synthesized (isostructural 1-Co/1-Zn with Co₂/Zn₂ units) and 2-Gd/2-Tb (Gd/Tb cluster chains) as photocatalysts. The structure–performance relationship indicated that the photocatalytic activity of benzylamines and phenylsulfides can be influenced by various transition metal ions, but not by lanthanide ions. Among them, 1-Zn is a robust multiphase catalyst with excellent performance, effectively oxidizing and removing benzylamines and phenylsulfides at room temperature (Fig. 26A).

Fig. 26 (A) Schematic showing the synthesis of UiO-68s from ref. 408. Copyright 2024, Royal Society of Chemistry. (B–D) The synthesis for UiO-66s and dye@UiO-66s; band structure figure of dyes, UiO-66s and dye@UiO-66s; our work for synthesis of [1,2,5]thiadiazole[3,4-g]benzoimidazoles from ref. 435. Copyright 2023, American Chemical Society. (E) MOFs with different metals catalyze the oxidation and removal of harmful benzylamines and phenylsulfides under visible light from ref. 436. Copyright 2023, American Chemical Society. (F) Schematic diagram of 3D MOF-2(Co) and 2D-MOF-1(Co) photocatalytic organic transformation from ref. 437. Copyright 2023, Elsevier B.V.

Zhang et al. synthesized a multifunctional heterostructure photocatalyst Cu-NH₂-MIL-125/TpPa-2-COF, with Cu-NH₂-MIL-125/TpPa-2-COF (4 : 6) exhibiting a high rate of visible light-driven hydrogen evolution. This photocatalyst demonstrated both high conversion (91.2%) and selectivity (>99%), effectively oxidizing amines to imines.⁴⁰⁶ Meanwhile, Jiang et al. utilized porphyrin MOF (PCN-222) to promote the visible-light-catalyzed oxidative coupling of various amines.⁴⁰⁹ Research has shown that PCN-222 exhibited excellent photocatalytic activity, which is attributed to the synergistic interactions between energy transfer-generated ¹O₂ and charge transfer-induced electrons and holes that promote the reaction.

3.5.2. Oxidation of sulfides to sulfoxides. Sulfoxides serve as widely used synthetic reagents in both industrial production and pharmaceutical applications. Oxidation of thioether stands as a common method for producing corresponding sulfoxides. Conventional approaches often rely on heavy metal ions or harsh oxidants like tert-butyl hydroperoxide, hydrogen peroxide and trifluoroacetic acid, which can lead to environmental pollution. Conversely, the photocatalytic oxidation of sulfides utilizing visible light and oxygen exclusively as oxidants presents a promising opportunity for environmentally friendly sulfoxide synthesis, characterized by its simplicity, cost-effectiveness, mild reaction conditions and high efficiency and stability. Typically, sulfoxides emerge as primary products, with overoxidation products as byproducts. Notably, the photocatalytic oxidative degradation of chemical warfare agents exemplified a significant application of sulfide oxidation, addressing the persistent threat posed by such agents in global communities over decades. Among these agents, bis (2-chloroethyl) sulfide (mustard gas) (HD) stands out as one of the most potent, and notorious for its blistering effects. Initially, HD and its less toxic analogue, 2-chloroethyl ethyl sulfide (CEES), were studied due to CEES's reduced toxicity compared with HD while maintaining the sulfide portion.⁴¹⁰ Researchers have developed various porous photocatalysts, including MOFs, to detoxify HD analogues such as CEES owing to their exceptional capacity to generate reactive oxygen species (ROS).⁴¹¹

Tang et al. prepared sandwich-like nanocomposites of UiO-66@Au₂₅@UiO-66 by sandwiching Au₂₅(Capt)₁₈ (abbreviated as Au₂₅, Capt = captopril) between UiO-66.⁴¹² The composite material exhibited notably improved catalytic efficiency and exceptional stability during the selective photocatalytic process of oxidizing sulfides to sulfoxides in an aerobic environment. Omar K. Farha et al. synthesized a microporous MOF, NU-400, which serves as an efficient photosensitizer, facilitating the production of ¹O₂ and subsequent oxidative degradation of chemical warfare agents.⁴¹³ The remarkable activity of NU-400 facilitates the photocatalytic conversion of the CEES into non-toxic sulfoxide derivatives in air, with a half-life of less than 15 minutes.

Most research on the detoxification of CEES and the role of defects in MOFs during detoxification has been conducted in the liquid phase. Teresa J. Bandosz et al. reported on studies dealing with gas–solid interactions. They synthesized UiO-66 composites with nanographite or oxidized graphitic carbon nitride nanospheres, using them for the purification of gaseous CEES.⁴¹⁴ The interactions between CEES vapor and UiO-66, its defective composites, nanographite, and oxidized graphitic carbon nitride nanospheres under ambient conditions were evaluated. The results indicate that these materials significantly enhance the detoxification capability of CEES vapor in terms of adsorption and surface reactivity. The composites can be used for the adsorptive degradation of gaseous CEES.

3.5.3. Synthesis of 2-phenyl-quinazlon-4(3H)-one. Quinazolin-4(3H)-one derivatives are among the most significant heterocyclic compounds containing nitrogen, widely used in biological and clinical applications. They exhibit biological activities including antibacterial, antiviral, antidiabetic, phosphorylation inhibition, anti-inflammatory and anticancer properties.⁴¹⁵ Therefore, besides the extraction of widespread present natural derivatives of quinazolinone, synthetic quinazolinones also hold significant research importance. As of now, there have been abundant reports documenting the synthesis of quinazolinone pharmacophores through reactions of alcohols with 2-aminobenzamide in catalytic systems such as Ru/Xantphos/Crotononitrile, Ir, and Pt under harsh conditions,⁴¹⁶ a-MnO₂/tert-butyl hydroperoxide (TBHP), Ru/Xantphos/Crotononitrile and Fe(BTC)/TBHP/DMSO,⁴¹⁷ and Ni(II) complexes/NaOtBu/xylene. However, these systems suffer from drawbacks such as high catalyst costs, difficult catalyst recovery, catalyst deactivation, the use of additional additives and complex reaction conditions. Therefore, there is an urgent need to introduce catalysts that are low-cost, recyclable, efficient and environmentally friendly.

Ali Reza Oveisi et al. synthesized and cluster-metalized a novel multifunctional MOF Fe@PCN-222(Fe) by PSM of a bioinspired Fe(III) porphyrinic Zr-MOF (PCN-222(Fe)) with FeCl₃.⁴¹⁸ This solid displayed catalytic activity when exposed to visible light in the presence of air or oxygen, all without the need for any additives, synthesizing quinazolin-4(3H)-one via an oxidation–cyclization–oxidation reaction in a one-pot tandem process with alcohols and 2-aminobenzamide. Moreover, its catalytic performance surpasses that of bare PCN-222(Fe) and comparable heterogeneous catalysts such as UiO-66, Fe(BTC) and mixtures of UiO-66 with FeTCPPCl and FeCl₃. This is the instance of using an MOF under an O₂ atmosphere with visible light (or sunlight) to synthesize quinazolin-4(3H)-ones under mild and environmentally sustainable conditions.

3.5.4. Trifluoromethylative difunctionalization of alkenes. Given the importance of CF₃ groups in small-molecule drugs, evaluating the catalytic performance of photoreactions through the trifluoromethylative difunctionalization of alkenes serves as a model reaction, holding profound research significance. Lin et al. reported a bifunctional metal–organic layer (MOL) Hf-EY-Fe constructed from Fe-TPY (TPY = 4′-(4-carboxyphenyl)[2,2′:6′,2′′-terpyridine]-5,5′′-dicarboxylate) ligands and Eosin Y (EY)-Hf₆ secondary building units (SBUs).⁴¹⁹ The organic dye EY, serving as a photosensitizer, with TPY-Fe(OTf)₂ acting as the catalytic core, enables Hf-EY-Fe to efficiently catalyze aminotrifluoromethylation, hydroxytrifluoromethylation and chlorotrifluoromethylation reactions of alkenes. Hf-EY-Fe can also catalyze the synthesis of CF₃-substituted derivatives of biologically active molecules, such as estrone.

3.5.5. aza-Henry reaction. The aza-Henry reaction stands out as one of the most prominently utilized photocatalytic model reactions, widely used to assess the photooxidoreductive activity of various catalytic systems. This reaction involves the functionalization of N-aryltetrahydroisoquinolines (THIQs) alongside other tertiary amines using various nucleophilic reagents. Many nucleophiles can act as coupling agents with oxygen serving as the terminal oxidant. Youngmee Kim et al. encapsulated the cations [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine),⁴²⁰ [Ru(phen)₃]²⁺ (phen = 1,10-phenanthroline) and [Ru(bpz)₃]²⁺ (bpz = 2,2′-bipyrazine) within the mesopores of 3D InBTB MOF (ligand = 1,3,5-benzenetribenzoic acid) to prepare three new types of RuL₃@InBTB MOF. The photocatalytic activity of RuL₃@InBTB MOF was assessed using the aza-Henry reaction. Concurrently, the research explores both the hydrogenation of dimethyl maleate and the visible-light-induced decomposition of methyl orange at ambient temperature. The results indicated that RuL₃@InBTB MOF exhibited high stability and recyclability.

3.5.6. Aromatization tandem reactions of glycine esters and styrenes. For materials with high stability and excellent performance, their photocatalytic activity can also be evaluated through the photooxidative cross-dehydrogenative coupling (CDC)/aromatization tandem reactions of N-(4-methoxyphenyl)glycine and styrene. Chen et al. developed a highly delocalized interpenetrated 3D MOF Zn-TACPA (ligand = tris(3-carboxybiphenyl)amine (H₃TACPA)) photocatalyst based on vinyl-functionalized triphenylamine and bipyridine ligands.⁴²¹ It possesses high stability and delocalization, serving as an ROS generator to catalyze the photooxidative CDC/aromatization tandem reaction of glycine esters and styrenes. The introduction of functional vinyl double bonds enhanced the conjugation of the MOF semiconductor, charge carrier separation and charge transfer efficiency, significantly increasing its catalytic yield.

3.5.7. The synthesis of β-thioacids. The synthesis of β-thioacids has garnered significant interest. However, traditional methods for synthesizing β-thioacids have numerous drawbacks, such as the need to use toxic H₂S, low yields and cumbersome multi-step processes. Therefore, developing a photoassisted redox method using stable and recyclable photocatalysts, without the need for additional co-catalysts/alkali, for synthesizing thio-carboxylate olefins with CO₂ into β-thioacids is of significant importance for green and economical synthesis. MOFs, with their aromatic donors and high oxidation state metals, facilitate photoinduced charge separation, and their distinctive porous structure and exceptionally high surface area have broad applications in photocatalysis.⁴²²

Suman L. Jain et al. reported an efficient, simple and economically feasible photoassisted redox-mediated method,⁴²² which utilizes the porous functionalized Fe-MIL-101-NH₂ as a photocatalyst, to synthesize important β-thioacids via the carboxylation of olefins with CO₂ and thiols at room temperature.

3.5.8. Selective alcohol oxidation to aldehydes/ketones. In industrial synthesis and laboratories, the selective oxidation of alcohols to aldehydes or ketones is a vital step in organic compound synthesis, as aldehyde or ketone derivatives are valuable fine chemical intermediates with wide applications in fields like medicine and the food industry.⁴²³ In the oxidation process of aromatic alcohols, the use of additional oxidizing agents (ClO⁻, KMnO₄ and CrO³⁻) is often expensive and dangerous, causing environmental pollution and resource wasting. In contrast, visible light photocatalysis is an effective strategy that facilitates the selective oxidation of alcohols to benzaldehyde under mild conditions, without the need for additional co-catalysts.⁴²⁴

In the presence of Bi(NO₃)₃, Yin et al. successfully synthesized BFO/MIL-53(Fe) (BFO = bismuth ferrite) nanocomposites at room temperature by partially etching the peripheral structure of MIL-53(Fe).⁴²⁵ The prepared composites provided more active sites for photocatalytic reactions. To evaluate the photocatalytic performance of the composites, Shuang-Feng Yin studied the photocatalytic activity of BFO/MIL-53(Fe) nanocomposites for the partial oxidation of aromatic alcohols under visible light. The study found that the highest conversion rate reached 58.5% when the mass ratio of BFO to MIL-53(Fe) was 2.0.

3.5.9. Oxidative hydroxylation of boronic acids. Phenol is a crucial raw material in the synthesis of fine chemicals. Traditional phenol synthesis involves complex and costly synthetic steps. Therefore, there is a need to develop green and efficient methods for producing phenol, particularly in synthesizing various multifunctional group-substituted phenols. Arylboronic acids and aromatic/heteroaromatic compounds are essential in organic synthesis.⁴²⁶ Many MOFs have been proved to be effective photocatalysts in the conversion of arylboronic acids to phenol. Generally speaking, phenol is the sole product of the oxidation of arylboronic acids. Additionally, using MOF photocatalysis for phenol production offers advantages such as good substrate adaptability, high yield, environmental friendliness and selectivity, with a well-defined reaction mechanism that can serve as a model reaction to preliminarily evaluate the photocatalytic activity of most MOFs catalysts.⁴²⁷

Li et al. reported the synthesis of a robust microporous Zn-MOF (JNU-204) with a pyrazole−benzothiadiazole−pyrazole (D–A–D type) photosensitizer as the organic linker, featuring a wuk topology.⁴²⁸ Using JNU-204 as a catalyst, three aerobic oxidation reactions were carried out via different oxygenation pathways, such as the oxidation of phenylboronic acid, demonstrating JNU-204's excellent photocatalytic activity. Recyclability tests confirmed its heterogeneous nature and structural integrity, with no significant loss of photocatalytic activity after three runs. The synthetic application of JNU-204 was further extended to the straightforward construction of N-heterocyclic derivatives. The pyrrolo[2,1-a]isoquinoline-containing heterocycles, a core framework of a series of marine natural products, can be synthesized via a series of photocatalytic oxidation/[3 + 2] cycloaddition/oxidative aromatizations.

3.5.10. Selective dehalogenation. Organic halides constitute a group of chemicals that are commonly employed as intermediates or solvents in the process of organic synthesis.⁴²⁹ Nonetheless, the majority of organic halides are classified as persistent organic pollutants, which pose significant threats to both environmental well-being and human health.⁴³⁰ A highly efficient and environmentally friendly solution is selective dehalogenation via photocatalysis. Previous studies have used transition metal complexes to catalyze the aforementioned reactions, but due to limitations in conditions, stability and reusability, these efforts have not successfully resolved the issues. However, employing MOFs as catalysts for these reactions can effectively overcome these problems.

Niu et al. synthesized an NDI-based polymer using POMs and amine catalysts, designing a novel MOF, CoW-DPNDI−PYI (DPNDI = N,N′-bis(4-pyridylmethyl)-naphthalenediimide, PYI = pyrrolidine-2-yl-imidazole).⁴³¹ Without the addition of any co-catalysts, a heterogeneous reduction of various aryl bromides and chlorides was achieved along with a significant impact on CO₂ cycloaddition reactions under mild temperatures and sub-atmospheric pressure.

3.5.11. Amide bond green synthesis. The formation of amide bonds is a crucial step in many pharmaceutical chemical synthesis processes. Therefore, the synthesis of amide compounds is a central issue in the synthesis of natural products and drug development. Traditionally, amide bonds are constructed by condensing carboxylic acids or their derivatives with amines, requiring condensing agents and relatively harsh reaction conditions; furthermore, this synthetic method generates a large amount of by-products. Consequently, oxidative amidation reactions of aldehydes or alcohols with amines have been developed to replace traditional amide bond synthesis routes. Utilizing MOFs for the photocatalytic conversion of aldehydes or alcohols to amide bonds offers a promising approach.⁴³²

Cheng et al. synthesized three distinct Fe–O clusters, Lewis acid sites and morphologies of Fe-MOF through coordination structure engineering.⁴³³ The findings suggest that Lewis acid sites play a crucial role in photocatalytic performance. Pd/MIL-101(Fe) demonstrates outstanding photocatalytic performance, catalyzing the conversion of various aldehydes, alcohols and toluene with amine compounds into amide compounds at ambient temperature.

3.5.12. Other reactions. He et al. proposed embedding a copper(I) bimetallic photosensitizer in an economical and scalable zinc-based columnar layered MOF, for reactions involving both intra- and intermolecular radical-mediated N-acyloxy imidates and O-acyl oximes.⁴³⁴ Research has shown that anchoring a BINAP-containing copper photosensitizer onto the pillar-layered MOF supports significantly enhances its turnover frequency and recyclability in photocatalysis.

3.5.13. Synthesis of [1,2,5]thiadiazole[3,4-g]benzoimidazoles. Introducing dye molecules is a promising method to effectively enhance the activity of MOF photocatalysts. In 2023, Li et al. reported an efficient strategy to confine and stabilize photosensitive organic dyes within MOF frameworks,⁴³⁵ developing a series of dye-loaded photocatalysts, dye@UiO-66s (dye = fluorescein (FL)/rhodamine B (RhB)/Eosin Y (EY), UiO-66s = UiO-66 and Bim-UiO-66) (Fig. 26B). The results demonstrated that regardless of the functionalization of the MOF ligands, the encapsulated dyes effectively sensitize the MOF matrix, dominating the band structure and photocatalytic activity of dye@UiO-66s (Fig. 26C). Photocatalytic experiments indicated that dye@UiO-66s show enhanced activity compared with free dyes, with FL@Bim-UiO-66 efficiently catalyzing the green synthesis of novel carbon-bridged cycles [1,2,5]thiadiazole[3,4-g]benzoimidazoles at room temperature, achieving yields up to 98% with exhibited excellent stability and reusability (Fig. 26D).

3.5.14. The thioamide cyclization. Linker engineering is an effective strategy for imparting specific properties or new functionalities to targeted MOFs. Li et al. incorporated various benzothiadiazole units into the ligands to systematically regulate the activity of the MOF (UiO-68, UiO-68-Bs, UiO-68-Ph, UiO-68-Py) photocatalysts, and the light absorption and charge separation efficiency can be improved. Among these photocatalysts, UiO-68-Py composed of a fully conjugated benzothiadiazole acceptor and pyrene donor was proved to be an efficient and recyclable photocatalyst in facilitating the cyclization of thioamides under mild conditions (Fig. 26E).⁴³⁶

3.5.15. Synthesis for benzothiazoles. Highly active and selective visible light heterogeneous photocatalysts are crucial for the advancement of organic transformations, yet they remain a formidable challenge. Herein, Li et al. have developed a simple yet effective strategy to integrate tetrazine parts into a robust MOF.⁴³⁷ These include 2D MOF-1(M), (M = Co, Mn, Zn) and 3D MOF-2(Co). The MOF-1 series consisted of amorphous 2D porous frameworks, while MOF-2(Co) features a 3D porous structure. Due to the oxidative activity of ˙O₂⁻ species, these MOFs exhibited significantly enhanced photocatalytic activity in the direct condensation of o-aminothiophenol and aromatic aldehydes under visible white light irradiation. Notably, MOF-1(Co) demonstrated superior catalytic activity, exhibiting broad substrate adaptability, excellent steric hindrance tolerance, remarkable reusability, and outstanding stability (Fig. 26F).

4. Summary and outlook

In recent years, considering the non-renewability of fossil fuels and environmental pollution issues, new photocatalytic materials, MOFs, have gradually come into view. MOFs have highly ordered porous structures, large specific surface areas and abundant active and adsorption sites. Their inherent narrow bandgaps and thermal stability endow them with superior catalytic activity. Then, due to the structural diversity and design flexibility of MOFs, the catalytic reaction sites can be adjusted by designing bandgaps, controlling channels and regulating pore sizes. This review systematically summarises the optimization and application of visible light photocatalytic MOFs, focusing on the basic reaction types and specific reactions of photocatalytic organic transformations. It aims to provide a systematic guide for future researchers, from the design of MOFs to performance improvement and specific photocatalytic organic transformations. However, despite recent significant progress in MOF photocatalytic organic transformations, the following points may require further research to address the design challenges and performance enhancement of MOF-based catalysts, paving the way for future applications:

(1) The synthesis of MOFs typically involves complex conditions such as high temperatures, specific pH values, or the use of organic solvents, which limit their large-scale production and application. Therefore, optimizing the structure of ligands and central metal clusters, simplifying the synthesis process of MOFs and developing room temperature synthesis routes, aqueous synthesis methods, or green solvent systems, are currently important research directions.

(2) In many applications, the size and morphology of MOFs significantly affect their performance. Currently, precisely controlling the size and morphology of MOF crystals remains a technical challenge. By improving synthesis conditions, using templates, or adding growth regulators, the morphology and size of MOFs can be controlled to some extent, but further research is needed to achieve more precise control.

(3) Environmental factors, such as humidity, temperature and chemical corrosion, impact the stability of MOFs markedly, and the pore size of MOFs also significantly affects the stability.⁴³⁸ With the enlargement of pore size, its chemical stability and thermal stability usually decline. Developing MOF materials with high environmental stability and precision regulation of the pore structure of MOFs is crucial, and in-depth research on the correlation between the porosity and stability of MOFs, coupled with the continuous optimization of material design, will further expand the application prospects of MOFs in photocatalysis.⁴³⁹

(4) In practical applications the recyclability of MOFs has become an important factor influencing their photocatalytic performance and economic feasibility. To achieve MOFs with recyclable properties, future innovations in material selection, structural design, and post-treatment techniques can be explored, thereby reducing environmental pollution and achieving sustainable development.

(5) The mechanisms behind the formation of MOFs under various synthesis conditions are not yet fully understood. It is crucial to deeply understand the formation and catalytic mechanisms of MOFs and their composite materials. Therefore, conducting an in-depth study of their formation mechanisms through in situ characterization and DFT results is significant for future research on photocatalytic mechanisms.

(6) Although MOFs demonstrate numerous excellent properties, their commercial prospects are limited by high production costs and resource consumption. Finding more cost-effective materials, simplifying manufacturing processes and enhancing the multifunctionality of materials are key strategies to reduce costs and increase efficiency.

(7) The manufacture of functional materials with multifunctional applications has attracted increasing research interest. Although different types of MOF photocatalyst have rapidly developed applications in single fields such as HER, OER, NRR and CO₂RR, MOF-based composite materials as bifunctional or trifunctional photocatalysts have been rarely reported. Therefore, developing and designing bifunctional or trifunctional photocatalysts based on MOFs and their composites presents significant challenges.

(8) Meanwhile, the poor separation efficiency of photogenerated e⁻–h⁺ in photocatalysis severely limits the photocatalytic efficiency. How to improve photocatalytic efficiency is a hot topic among researchers. To cut down the recombination rate of photogenerated charge carriers, creating an internal electric field as a driving force in order to separate the charges has been considered an effective strategy. Therefore, using piezoelectric materials for piezo-photocatalysis, which simultaneously undergo light irradiation and mechanical stress, is of great significance.⁴⁴⁰

(9) In recent years, artificial intelligence technology has developed rapidly; AI can predict and design new MOFs with specific properties by learning the relationships between the structures and properties of existing materials, allowing us to bypass the experimental process and directly obtain results. In the future, with the continuous development of AI technology and the improvement of material databases, the application of AI in MOF research will become more extensive and in-depth, holding great potential.

In response to the challenges mentioned above, future research should focus on material design, synthesis optimization, application exploration and mechanism studies. Developing new synthesis methods, such as using biosynthesis or green chemistry approaches, can not only simplify the production process but also reduce the environmental impact. In application development, using AI technology to predict the structure and performance of MOFs, guiding their controlled design and synthesis and exploring their development in organic transformation will be an important direction for future research. Through these integrated efforts, the research and application prospects of visible-light MOFs will become even broader.

Author contributions

Xu-Sheng Li and Yu-Jie He: contributed equally; writing – original draft. Jiao Chen: writing – review & editing. Quan-Quan Li: writing – review & editing. Ping Liu: supervision, project administration. Jian-Li Li: supervision, project administration.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 22077099, 22171223 and 22307102), the Innovation Capability Support Program of Shaanxi (No. 2023-CX-TD-75 and 2024ZC-KJXX-040), the Technology Innovation Leading Program of Shaanxi (No. 2023KXJ-209 and 2024QCY-KXJ-142), the Key Research and Development Program of Shaanxi (No. 2024GH-ZDXM-22), the Natural Science Basic Research Program of Shaanxi (No. 2023-JC-YB-141), and Young Talent Fund of Association for Science and Technology in Shaanxi, China (No. SWYY202206), the Shaanxi Fundamental Science Research Project for Chemistry & Biology (No. 22JHZ010, 22JHQ080, and 23JHQ039), the Yan'an City Science and Technology Project (No. 2022SLZDCY-002), and the Key Laboratory of Hexi Corridor Resources Utilization (No. ZDSYS-2023-2).

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Footnote

† These authors contributed equally to this work.

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