Rare earth-based MOFs for photo/electrocatalysis

Abstract

Metal–organic frameworks (MOFs) are porous crystalline materials that have been studied and applied in the fields of energy storage, catalysis and separation. Rare earth MOFs have the advantages of MOF materials in addition to the rare earth metal ion 4f electronic layer, large orbital coupling effect, and internal magnetic anisotropy. Rare earth MOFs have higher coordination numbers and richer coordination geometries than transition metal ions as functional metal centers of MOFs. Since the 4f electron layer endows rare earth MOFs with special optical and electrical properties, they have the potential for application in photocatalysis and electrocatalysis. This review reports the current research progress on rare earth MOFs, including synthesis methods and photocatalytic and electrocatalytic applications of rare earth MOFs. We divided the different ligands into 1,4-benzene dicarboxylic acid (BDC), 1,3,5-benzenetricarboxylic acid (BTC), the zeolite imidazole framework (ZIF), and some uncommon rare earth MOFs materials. The photocatalytic and electrocatalytic applications include the oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), oxygen evolution reaction (OER), CO₂ reduction reaction (CO₂RR), and other applications. Finally, we summarize the progress and existing challenges of rare earth MOF materials in the fields of photocatalysis and electrocatalysis.

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Senyao Meng

Senyao Meng was born in 1996 in Henan province. He obtained his Master's degree from the College of Science, China University of Petroleum (2022). Currently, he is pursuing his PhD degree at the College of New Energy and Materials, China University of Petroleum (Beijing). His research interest is in the applications of rare earth MOF in photocatalysis.

Ge Li

Ge Li was born in 1999 in Hebei Province. She received her bachelor's degree from Shijiazhuang University and is currently studying in Professor Zhenxing Li's research group at the College of New Energy and Materials, China University of Petroleum (Beijing). Her research is on the applications of polymetallic layered hydroxides in electrooxidation.

Ping Wang

Ping Wang received her Master's degree from the China University of Petroleum (Beijing) in 2022. She is a PhD student under the supervision of Prof. Zhenxing Li at the College of New Energy and Materials, China University of Petroleum (Beijing), China. Her current research interests are in the synthesis and application of functional nanomaterials.

Miao He

Miao He was born in 1996 in Heilongjiang province. She received her Master's degree from the China University of Petroleum (Beijing) in 2021. She is a PhD student under the supervision of Prof. Zhenxing Li at the College of New Energy and Materials, China University of Petroleum (Beijing), China. Her current research interests are in the synthesis and application of functional nanomaterials.

Xiaohua Sun

Xiaohua Sun obtained her PhD (2014) at the University of Cambridge. She is currently a Professor at the State Key Laboratory of Heavy Oil Processing and College of New Energy and Materials at the China University of Petroleum (Beijing). Her recent research interests focus on renewable energy, electrolytic hydrogen production, and nano self-assembly and imprint technology and its applications.

Zhenxing Li

Zhenxing Li obtained his PhD (2011) at the Peking University of China. He is currently a Professor at the State Key Laboratory of Heavy Oil Processing and College of New Energy and Materials at the China University of Petroleum (Beijing). His recent research interests focus on the design and synthesis of porous materials and rare earth-based nanomaterials for application in electrocatalysis, photocatalysis, and organic synthesis.

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1. Introduction

MOFs are a new type of porous crystal material, whose structure is mainly composed of organic ligands and metal ions. In the past years, MOFs have emerged as a novel class of porous materials that have received increasing attention for their applications in fields such as heterogeneous catalysis, biomimetic mineralization, intelligent sensors, gas storage and separation, etc.^1–4 Compared with traditional catalysts, the synthesis of MOFs can be flexible and there are many possibilities for the combination of metal and organic joint points. MOFs have many advantages, such as large surface area, adjustable pore structure, and multiple catalytic sites. Therefore, MOFs can provide great potential in the design of new catalyst materials.^5–7 The rare earth metals include 15 lanthanides as well as yttrium and scandium.^8,9 The unique luminescence and catalytic properties of rare earth metals make them irreplaceable and widely used in various fields. The introduction of multifunctional metal centers is a key function for MOF materials. Since rare earth metals easily coordinate with ligands containing oxygen¹⁰ and nitrogen,¹¹ especially carboxylic acid ligands,¹² and have elevated coordination numbers and diverse coordination modes, it is possible to obtain new structure MOFs. However, due to steric hindrance, rare earth elements cannot completely coordinate with ligands, leaving one or more coordination sites to coordinate with small solvent molecules to form end groups. When the synthesized rare earth MOFs are heat treated in a high vacuum for a certain period, these small molecules are discharged from the rare earth MOFs skeleton and the coordinatively unsaturated sites are exposed in the rare earth MOFs structure, which can be used as the active Lewis acid centers. Rare earth metals have similar coordination properties^13,14 and can have a variety of coordination changes. The changes in coordination number and geometric configuration only represent a small difference in energy, which opens the door to new discoveries.^15,16

In this review, we provide the background on the current state of research on rare earth MOFs (Scheme 1). The synthesis methods of rare earth MOFs are summarized and are divided into 1,4-benzene dicarboxylic acid (BDC), 1,3,5-benzenetricarboxylic acid (BTC), zeolite imidazole framework (ZIF), and some uncommon rare earth MOFs materials, according to the different ligands. We report in detail on the applications of rare earth MOFs in photocatalysis and electrocatalysis, including HER, OER, CO₂RR, and other applications. Finally, we summarize the application studies of rare earth MOFs and present the existing challenges. Some proposed development directions to address the problems are presented. Although our research is only a small part of the numerous rare earth MOFs materials, we hope to provide ample representation and insights into the development of this research field.

Scheme 1 Schematic diagram of the structure, synthesis, and catalytic application of rare earth-based MOFs.

2. Synthesis

2.1. BDC & BTC

2.1.1 BDC. The compounds, dimensions, and morphologies of MOFs are the key factors affecting the photocatalytic properties.¹⁷ For this reason, scientists have been researching efficient synthetic routes to MOFs.^18,19 The structure or morphology of the raw materials used to synthesize MOFs plays a crucial role in the final product properties. Among the different MOF ligands, terephthalic acid, also systematically known as 1,4-benzene dicarboxylic acid (BDC), has been showing remarkable properties in enhancing catalytic performance.^20,21 Furthermore, various methods have been used to improve the catalytic properties of the MOFs, such as doping metal ions in MOFs,²² linking metal oxides,²³ sulfides²⁴ and phosphates²⁵ with MOFs, etc. Recently, the use of rare earth metals as modifiers or replacements for the doping of metal nodes in MOFs with BDC as ligands has attracted significant attention.^26–29 Over the past few years, many procedures, including hydrothermal and solvothermal reactions,^18,27,30 solution precipitation,³¹ sonication,²⁰etc., have been used to efficiently synthesize MOFs. In this section, we will describe some of the achievements.
2.1.1.1 Hydrothermal method. The hydrothermal reaction is one of the most commonly used methods for synthesizing nanomaterials,^18,21,32 and the reaction is carried out by placing the reactant solution in an autoclave at high temperature and high pressure. This reaction usually yields products with good dispersion and high degrees of crystallinity. The hydrothermal reaction is an easily controlled method for effectively synthesizing MOFs.¹⁸ However, the low synthesis yield and long reaction times are still challenging to overcome.³⁰ The reactions of MOFs synthesized by the hydrothermal method have not been fully studied. In 2008, with rare earth ions as the central sites and pyridine-2,3-dicarboxylic acid and benzene-1,4-dicarboxylic acid as the organic framework, a novel three-dimensional (3D) hybrid compound species was synthesized by a hydrothermal method. The structure was composed of dimer units connected by pyridine-2,3-dicarboxylate units to form a two-dimensional (2D) layered structure, supported by benzoate units to complete the formation of the 3D structure (Fig. 1a). The structure had the coordination of two reversibly adsorbed H₂O to form a one-dimensional (1D) spiral chain structure. These MOFs contain regularly conjugated π systems (benzene) and nitrogen-containing ligands (pyridine) bound to lanthanide metals. Since these two groups have strong absorption peaks in the ultraviolet (UV) spectrum, the lanthanide ions are sensitized by the antenna effect.^33–35 The transition of π → π* or n → π* electrons will lead to energy transfer in the ligand, which is manifested macroscopically as a high-intensity band observed at 300 nm in the UV-Vis spectrum.

Fig. 1 (a) The 3D structure formed by the connectivity between the 2D layers and BDC units, Copyright 2008, Wiley VCH. (b) View of the asymmetric unit of Y₁₀(C₈H₄O₄)₆(CO₃)₃(OH)₁₂. (c) View of the coordination mode of BDC ligands, Copyright 2019, American Chemical Society. SEM image of (d) Ce–BDC and (e) Ce–BDC–NH₂, Copyright 2021, American Chemical Society. (f) Molecular structure of the linkers used. Copyright 2019, American Chemical Society.

2.1.1.2 Solvothermal method. The synthesis of MOFs with BDC as the raw material has generally been carried out by the solvothermal method.^21,36–39 The steps of the solvothermal method are basically the same as the hydrothermal method. The hydrothermal method is a chemical reaction in a solution above the boiling point of water, but the solvothermal method uses a non-aqueous liquid as a solvent at a relatively high temperature.²⁷ Chen et al. used a mixed solution of water and N,N-dimethylformamide (DMF) to perform a solvothermal reaction to introduce Eu³⁺ into the lattice of Y₁₀(C₈H₄O₄)₆(CO₃)₃(OH)₁₂. The formed structure consists of four Y³⁺ ions, two deprotonated BBC²⁻ and a hydroxyl group (Fig. 1b) formed by a carbonate radical in the P3c1 system to make asymmetric units. BDC ligands have two coordination modes due to the influence of the two coordination modes of the carboxyl group (i.e. bridging bidentate and linking/bridging tridentate) (Fig. 1c). Inorganic plates connected by BDC organic ligands as framework structures, and the products prepared solvothermally have a hexagonal layered morphology. The material substituted by Eu³⁺ exhibits fluorescence and selective quenching of Fe³⁺ makes it potentially valuable in fluorescence detection materials.³⁷ In 2021, Das et al. synthesized Ce-MOFs with (Ce–BDC–NH₂) and without (Ce–BDC) amine functionalization for CO₂ immobilization and the photocatalytic reduction of CO₂ with epoxides. Ce–BDC and Ce–BDC–NH₂ were synthesized by a solvothermal reaction, Ce–BDC was synthesized by complexation synthesis, and amine-functionalized Ce–BDC–NH₂ was synthesized in almost the same way but with different doses and solutions. Ce–BDC was a light-yellow precipitate, and after cooling to room temperature in the oven, the Ce–BDC–NH₂ product was a brown solid (Fig. 1d and e).³⁹ The nano-crystallite diameters of Ce–BDC-66 and Ce–BDC–NH₂ were calculated by the Scherrer formula to be 58.83 and 22.22 nm, respectively.⁴⁰

Apart from the hydrothermal and solvothermal methods, an almost solvent-free strategy was proposed to obtain Ce-UiO-66. The initial steps were the energetic grinding of the metal salts and linkers in the presence of a small amount of acetic acid. This method controlled the liquid/solid ratio of the reactants between 0.1 and 1, similar to liquid-assisted grinding.⁴¹

The BDC linkers used for the synthesis of Ce-MOFs were named Ce-UiO-66-X (with X = H, Br, NO₂), Ce-UiO-66-4F, and Ce-MIL-140A-4F (Fig. 1f). Among these functional MOFs, the most stable were Ce-UiO-66-H and Ce-UiO-66-Br.⁴²

2.1.2 BTC.
2.1.2.1 Hydrothermal method.
Synthesis of La-BTC. Jeyaseelan et al. studied the preparation of the hydroxyapatite (HAP) lamellar lanthanum–phenyl tricarboxylic acid–base organic skeleton template for water defluorination (Fig. 2a).⁴⁴ The nucleation and growth of La-based MOFs require layer-by-layer (LBL) techniques in HAP hard templates. HAP-La-BTC MOFs were synthesized by a template-directed LBL technique. Ethanol was removed by the centrifugation of a HAP-containing ethanol dispersant. The morphology of HAP-La-based MOFs was a rod-like porous structure, and the size was not uniform at 5 μm.

Fig. 2 (a) The possible synthesis of rare earth metal ions-based MOFs, Copyright 2021, Elsevier. (b) Preparation of Eu(BTC)-MIP, Copyright 2019, Royal Society of Chemistry. (c) Perspective view showing the layered structure of Sm-BTC. (d) A space-filling model of Sm-BTC, Copyright 2019, Royal Society of Chemistry. (e) The mechanistic illustration of increased electrochemiluminescence by the antenna effect and the dual enhancement effect of CoS₂ triple shelled nano boxes, Copyright 2021, Elsevier. (f) Coordination environment of the Tb atom. (g) 2D frame diagram of Tb-MOFs, Copyright 2022, Royal Society of Chemistry.

Synthesis of Ce–BTC. Zhang et al. prepared Ce–BTC using (NH₄)₂Ce(NO₃)₆ as the Ce precursor.⁴⁵ Ce–BTC showed a large number of irregular and symbiotic nanoparticles. The particle sizes ranged from 0.10 to 0.36 μm, with an average diameter of 0.23 μm.
Synthesis of Sm-BTC. Lee et al. prepared Sm-BTC by a hydrothermal method to solve the problem of dye-containing wastewater in the textile industry.⁴⁸ The Sm-BTC was a rod-shaped crystal, and the size was about 5 μm to 25 μm (Fig. 2c). In terms of adsorption and the absorption of Bengal rose molecules, it is superior to existing MOFs. The adsorption capacity remained high even after three adsorption–desorption cycles, and a membrane adsorber was prepared based on the growth of Sm-BTC crystals in the matrix of three porous polymer materials (nylon,⁴⁹ polyethersulfone⁵⁰ and polytetrafluoroethylene⁵¹). The Sm-BTC supported by nylon showed the highest adsorption performance, which may be due to the uniform distribution of crystals in the scaffold (Fig. 2d).
Synthesis of Eu-BTC. Eu-BTC was synthesized by Zhao et al. and had a cauliflower appearance made up of many nanorods with a width of about 500 nm, and the dense arrangement of nanorods resulted in a rich active site. A large amount of SO˙⁴⁻ reacted with Eu-BTC˙⁻ in the holes of Eu-BTC to produce more abundant EuBTC*, thus improving the ECL efficiency (Fig. 2e).⁵²
Synthesis of Gd-BTC. Garg et al. synthesized Gd-BTC by a solvothermal method.⁵³ BTC and gadolinium nitrate hexahydrate were dissolved in a mixture of DMF, ethanol and water. After the reactants were dissolved, the reaction mixture was heated for one day. The product was then cooled to room temperature and allowed to air dry. Gd-BTC had a rod-like structure, and the length of the rods was 20–30 μm. After the GD-BTC was activated, the coordination unsaturation point of Gd(III) in the organic framework formed a 1D ionic chain, and the pore size could store hydrogen. Therefore, the GD-BTC can be further applied in hydrogen adsorption measurements⁵⁴ and humidity sensing.⁵⁵
Synthesis of Dy-BTC. Guo et al. prepared Dy-BTC crystals and Dy-BTC nanocrystals. Two basic parameters for the reduction of the size of the Dy-BTC to the nanoscale were revealed by studying the parameters using sodium acetate as a modulator.⁵⁸ One was to control the deprotonation of the organic bonds through the alkaline environment, which controlled the nucleation process. Another method was to use capping groups to inhibit the growth of microcrystals. Dy-BTC nanocrystals were prepared by HAc (acetic acid) + TEA (triethyl amine) and 2–5 equal amounts of acetic acid relative to BTC were added to the standard solution. After mixing, the pH value of the solution was adjusted to 5.9 with TEA. The tuning of the crystal size of Dy-BTC from micrometers to nanometers can be accomplished by adding different amounts of sodium acetate to the synthetic solution. Sodium acetate accelerated the crystal growth rate. The enhanced effect was related to the alkalinity of sodium acetate, which can increase the rate of deprotonation and thus the rate of nucleation.
Synthesis of Tm-BTC. Chinnapaiyan et al. reported a method for the synthesis of Tm-BTC via a unipolar solvent using a hydrothermal method.²⁵ The electrocatalytic sensing performance of edible roxarsone was investigated by using the prepared Tm-BTC-modified electrode. The Tm-BTC was white in appearance and about 1 micron in size, and the particles were dumbbell and needle-shaped.
2.1.2.2 Solvothermal method.
Synthesis of La-BTC. BTC is a rigid ligand with moderate size, which easily forms a compact structure and interpenetrating structures in the coordination process with metals. Prabhu et al. synthesized La-BTC using lanthanum chloride as the lanthanum precursor,⁴³ and the shape of the La-BTC was an irregular distribution with a size of 5 μm.
Synthesis of Ce–BTC. Peng et al. used a solvothermal method to synthesize cerium-based organic skeletons, and the Ce–BTC was in the form of needle-like particles with a size of 10–20 microns and small broken particles.⁴⁶ Tian et al. developed a way to synthesize organic frameworks for catalysis and pH-dependent selective tumor chemotherapy, and used this method to synthesize Ce-based organic frameworks. The cerium nitrate and phthalic acid were dripped separately into a flask containing H₂O through a two-channel syringe pump, and the Ce–BTC prepared by the flow process had a diameter of about 40 nm and a length in the range of several hundred nanometers.⁴⁷
Synthesis of Eu-BTC. The Eu-BTC with high luminosity was synthesized by Wu et al. at room temperature. Then, using lincomycin (LCM) as a template molecule, molecularly imprinted polymer (MIP) was formed on the surface of Eu-BTC by copolymerization (Fig. 2b). The fluorescent probe based on Eu-BTC coated with MIP was used for the selective determination of LCM. The rough surface of the synthesized Eu-BTC contained smooth rod-like crystals due to the formation of MIP layers on the surface of the Eu-BTC. The probe utilized both Eu-BTC and MIPs, and the recognition site of the probe can be selectively adsorbed by LCM to quench the fluorescence of Eu-BTC. Eu-BTC can also be prepared by 1,2,4-benzene tricarboxylic acid linked with Eu³⁺ ions and the co-precipitation method.⁵⁵ The morphology of Eu-BTC changed from a dense spherical shape to an irregular honeycomb shape by adjusting the pH of the reaction solution. With the increase in pH value, the morphology of Eu-BTC changed from a dense spherical shape to an irregular honeycomb shape. The crystal structure and thermal stability of Eu-BTC MOFs were consistent, the luminescence characteristics of Eu-BTC were related to the surface morphology, and the compact spherical structure had stronger emission intensity.
Synthesis of Tb-BTC. Yang et al. synthesized the hollow Tb-BTC using a one-pot solvothermal method (Fig. 2f and g).⁵⁶ The shape was a hollow sphere with a size of 5 μm, and Tb-BTC can detect Fe³⁺ by the luminescence quenching effect. The method had the advantages of simple operation, mild conditions, high yield, uniform morphology and good stability, and can be used for the determination of trace Fe³⁺.⁵⁷
Synthesis of other rare earth-BTC. The Nd-BTC was prepared by Asgari et al. using a solvothermal method with neodymium nitrate and DMF as the metal precursor and solvent.⁵⁹ Surble et al. prepared Pr-BTC in deionized water.⁶⁰ Pr-BTC consisted of nine-coordinated rare earth polyhedral chains linked by phenyl tricarboxylic acid groups. The chain was composed of square antiprisms with shared sides, and the coordination of rare earth atoms was ensured by oxygen atoms of the carboxylates and a terminal water molecule. Ren et al. grafted three different diamines to the coordination instability of Er(III) ions into the channel of the de-solvated Er-BTC.⁵⁷ Er-BTC is an effective heterogeneous catalyst for condensation reactions due to its high thermal stability⁶¹ and permanent porosity.⁶² Yb-BTC with Yb³⁺ as the metal center and phthalic acid as the ligand was constructed by a solvothermal method.⁶³ Yb-BTC is a typical microporous material whose high specific surface area and pore structure are conducive to the diffusion of reactant molecules. The unsaturated Yb metal coordination site on the skeleton can serve as the active Lewis acid center.⁶⁴

Rare earth-based MOFs have a variety of remarkable properties, and bimetallic MOFs show synergistic effects and improved performance as compared with similar monometallic materials. MOFs containing bimetals represent the active frontier of heterogeneous catalyst development, which increases the dispersion of the two metals on the carbon composite support. Bimetallic MOFs can be used to create efficient, robust, non-noble metal catalysts to develop sustainable chemical processes. The synergy of the bimetallic centers can have unexpected effects on the effect of the catalyst. Therefore, the MOF of some bimetallic centers containing rare earth elements are described. Kong et al. developed the bow-shaped Zr/La bimetallic organic skeleton, which had the adsorption capacity of harmful oxygen anion, and the ratio of Zr to La in Zr/La-BTC influenced the performance.⁶⁵ Zr/La-BTC MOFs were prepared by the hydrothermal method, and the mixture of lanthanum nitrate, zirconium nitrate and BTC was dispersed in the DMF solution. In the synthesis of Zr/La-BTC composites, La and Zr were combined to form bow-like products. The ratio of La/Zr played a crucial role in the final structure. The bow-knot morphology disappeared with the increasing of the Zr component, and 0.1 Zr/La-BTC had the best adsorption performance. Foratirad et al. prepared tubular and secondary sea urchin-structured bimetal–organic frameworks (ST-MOFs) by cathodic electrodeposition on nickel foam in aqueous samarium nitrate and tellurium nitrate containing organic linkers. A bimetallic lanthanide MOF composed of samarium and thulium elements was directly grown on the surface of nickel foam by a simple electrochemical deposition method.⁶⁶ Besides the influence of the ligand and coordination, there are many ways to adjust the catalytic performance of MOFs. The required nanodots can be formed by means of interface etching and precipitation.^67–69

2.2. ZIF

The structural design of MOFs also gives them great application potential in the fields of gas separation and storage, catalysis, thin films, biomedical imaging, and sensors. As a branch of MOF materials, zeolite imidazole frameworks (ZIFs) are nanoporous materials formed by chemical bonding between Zn²⁺ and Co²⁺ as metal ligands and imidazole, and imidazole and other compounds as organic ligands.^70,71 ZIFs are new porous materials with great application prospects, which combine the advantages of MOFs and zeolites, and possess high specific surface area and structural and functional tunability, as well as high chemical and thermal stability.⁷² The excellent characteristics of ZIFs lay a good foundation for their application in catalysis,⁷³ gas storage and separation⁷⁴ and other fields. Therefore, how to improve the application performance of ZIFs has become a hot topic. Ce is one of the most attractive rare earth metals because of its free conversion between Ce(III) and Ce(IV) oxidation states.⁷⁵ Cerium-based nanomaterials can jump between the III and IV valence states, causing Ce to play a role in the activation of oxygen in multiphase catalysis, such as CO oxidation,^76,77 solid oxide fuel cells (SOFCs),^78,79 photocatalytic oxidation,^80,81 soot oxidation,^82,83etc.

Solution method. Inspired by this practical application, Zhao et al. dissolved cerium nitrate and zinc nitrate in methanol at the same time, and then added 2-methylimidazole in methanol solution, followed by stirring and washing to prepare Ce-ZIF-8 particles. The final product was obtained after carbonization and acid washing, with Ce sites embedded in graded macro-meso-microporous N-doped carbons (Ce SAS/HPNC). Ce SAS/HPNC with a unique 3D layered and ordered porous structure had small pore sizes and high specific surface area.⁸³ Chen et al. dissolved different contents of rare earth complexes and dimethyl imidazole in deionized water and methanol solution, and added a mixed solution of hexyl trimethyl ammonium bromide and zinc nitrate containing methanol into the system to obtain ZIF-8 particles loaded with rare earth complexes (Ln complex@ZIF-8) by an in situ method. The rare earth complexes were loaded into ZIF-8 pores through the electrostatic effect and π–π stacking. A stopper with silane at one end was selectively inserted into the pores of the surface layer of ZIF-8 particles to plug the Ln complex@ZIF-8 surface layer pores.⁸⁵ The Zn²⁺ and 2-methylimidazole-formed ZIF-8 are sensitive to a weak acid environment,⁸⁶ which helps to release drugs into cancer cells. In addition, Gd has more single electrons in its outermost layer, while Tm³⁺ makes its radiative deexcitation process easier due to its diamagnetization, thus enhancing fluorescence.⁸⁷ Based on this, Xu et al. dissolved potassium ferricyanide, gadolinium nitrate and thulium chloride in deionized water, respectively, followed by mixing and centrifuging. By adjusting the doping ratio of Gd³⁺ and Tm³⁺, a series of double rare earth-doped Prussian blue nanoparticles (Gd/TM-Pb) were synthesized. ZIF-8, an acid-responsive metal–organic framework, was then coated on the surface of Gd/TM-Pb, and polydopamine (PDA) was modified on the surface to form a composite nanomaterial (Gd/Tm-Pb@ZIF-8@PDA). The Gd/Tm-Pb@ZIF-8@PDA had photothermal stability and the ability to release drugs due to a pH/glutathione (GSH) dual response.⁸⁸ Guo et al. prepared La-doped ZIF-8 material with a feed ratio of 3 at% by a solution method, and further prepared La-doped ZnO nanoparticles through a subsequent high-temperature calcination process. La doping was beneficial for obtaining sphere-like nanostructures with smaller particle size,⁸⁹ but La doping did not change the crystal structure of ZIF-8 and the derived ZnO nanomaterials.⁹⁰

Vapor phase method. Ce-Doped ZIF-8 was also studied by Wu et al. by using a steam phase method to dissolve a certain amount of zinc nitrate and dimethylimidazole in deionized water, followed by filtering and drying the obtained solid ZIF-8 as zinc source precursor, and some of the zinc nitrate was replaced with cerium nitrate (Ce@ZIF-8). The Ce@ZIF-8 sample was obtained by in situ doping with different concentrations of Ce.⁸⁴

As a branch of metal–organic framework materials, ZIF-67, with the shape of dodecahedron, is an important organic framework material through which various cobalt-based functional materials can be expanded. ZIF-67 has an open skeleton structure, large specific surface area and regular pore structure, and is widely used in electrochemical energy storage, electrocatalysis, supercapacitors, electroanalytical chemistry, flexible electrodes, and flexible electronic sensing.⁹¹ ZIF-67 has been widely used as a self-sacrificing template to construct hollow porous materials due to its simple synthesis and high specific surface area.^92–94 Rare earth elements have a special 4f shell electron configuration and exhibit special physical and chemical properties. Li et al. dispersed 2-methylimidazole, samarium nitrate, and cobalt nitrate in deionized water to prepare Sm-ZIF-67, and then Sm-ZIF-67 was pyrolyzed to obtain N-doped carbon-coated Sm₂O₃-Co nanoparticles. The introduction of Sm₂O₃ improved the electrocatalytic performance of the oxygen reduction reaction (ORR). The synergistic effect between Sm₂O₃ and Co species increased the effective potential, half-wave potential, and limiting current density.⁹⁵

2.3. Other rare earth MOFs

2.3.1 Hydrothermal method.
Synthesis of Tb-MOFs. Tb(TCA)(NDC)·H₂O, a Tb-MOF, was prepared by a hydrothermal method using 4,4,4-triphenylamine tricarboxylic acid (H₃TCA) and 2,7-naphthalenedicarboxylic acid (H₂NDC) as ligand.⁹⁷ Doping Eu³⁺ in Tb(TCA)(NDC)·H₂O resulted in better optical performance.⁹⁸ By using pyrazine-2,3-dicarboxylic acid as the ligand and terbium nitrate as the metal precursor, Tb-MOF was synthesized in a DMF and water mixture solution.⁹⁹ Tb-MOFs have the advantages of frequent detection in styrene vapor and as luminescent sensors. With trimesic acid as the ligand and terbium nitrate as the metal precursor, another Tb-MOF was synthesized in ethanol for the detection of volatile organic compounds (VOCs).¹⁰⁰ Physical quantity temperature is an important indicator in industrial production, but it is difficult to measure the temperature of sub-micron objects or fast-moving objects with conventional sensors or thermometers. [Tb₂(bpydc)₃(H₂O)₃]·nDMF was the ratiometric luminescence thermometer material. Terbium nitrate, 2,2′-bipyridine-5,5′-dicarboxylic acid and DMF were mixed and stirred at room temperature and sonicated to ensure adequate mixing. Afterwards, the solid was immersed in DMF solution and stirred to remove the unreacted linker, then washed with ethanol by Soxhlet extraction. The unique breathing behavior of [Tb₂(bpydc)₃(H₂O)₃]·nDMF is due to a structural transition during formation. The carboxyl groups connected the TbO_x (x = 7 or 8) polyhedral into 1D inorganic chain-like structures, and the bpydc groups joined these chains in series into a 3D backbone structure. 1D channels were solvent accessible when DMF was present in the structure. As the solvent was removed, the 1D channel shrank and the structure transitioned to non-porous, which was an irreversible process.¹⁰¹ Tb-MOFs can strengthen and quench the ligand central ions to produce many glowing colors for multi-colored luminescent materials. Ou et al. added terbium acetate tetrahydrate to DMF and dissolved it by ultrasound to get a terbium salt solution. Homophonic acid was added to DMF to obtain a homophonic acid solution, which was mixed with terbium salt solution to obtain a Tb-MOF powder. Then, the activated carbon particles were placed in terbium salt solution and immersed in DMF solvent overnight to obtain the Tb-MOF@activated carbon composite. The Eu-MOF material was also obtained by slowly adding all the homoparbenzoic acid solution to the europium salt solution, and the activated carbon particles were placed in the europium salt solution to obtain the Eu-MOF@activated carbon composite. The multifunctional Tb-MOF@activated carbon composite material had high stability and selective adsorption and identification effect on Fe³⁺ and Cr³⁺ heavy metal ions in aqueous solution. The Eu-MOF@activated carbon had a selective adsorption and identification effect on Fe³⁺ heavy metal ions in aqueous solution.¹⁰²
Synthesis of Ho-MOFs. Proton-conductive MOFs have attracted the interest of many scholars with the continuous development of MOFs synthesis techniques. Among them, Ho-MOF can be used as an excellent proton conductive material. Ho-MOF ([Ho(SIP)(H₂O)₅]·3H₂O) was synthesized with holmium as nodes and 5-sulfoisophthalic acid (SIP) as ligands under hydrothermal conditions by the reaction between Ho₂O₃ and NaH₂SIP without any additives.¹⁰³ The co-action of abundant water molecules and residual sulfo groups established a staggered pathway for proton conduction.
Synthesis of Tm-MOFs. Tm-MOF ([Tm(μ2-L) (μ4-L)0.5(H₂O)₂] 3H₂O) was synthesized by a hydrothermal reaction of thulium nitrate with 5-methylpyrazine-2-carboxylic acid. The Pd/Tm-MOF composites were obtained by incorporating small palladium nanoparticles into the Tm-MOF host material by impregnation. Pd/Tm-MOF has a certain catalytic activity in the hydrogenation of styrene. This study shows that metal nanoparticles can be immobilized on MOF materials using uncoordinated carbonyl groups, and the obtained composites can be used for heterogeneous and efficient catalysis.¹⁰⁵ Gd/Tm-MOFs have a great advantage in medical imaging. Thulium chloride, gadolinium chloride, polyvinylpyrrolidone (PVP), and sodium acetate were used to synthesize the rod-like Gd/Tm-MOF crystals in DMF and water.¹⁰⁶ Due to the interdependence of Gd³⁺and Tm³⁺, Gd/Tm-MOFs have important applications in biochemical engineering. Gd/Tm-MOFs with up-conversion luminescence showed minimal autofluorescence interference, deeper near-infrared (NIR) light penetration, and slight photo-damage for optional imaging and bioassay. When the concentration of the Tm³⁺ was about 6%, the up-conversion luminescence was the strongest, while the gadolinium complexes had advantages in better drug loading and pH-responsive drug release. These special performances were the results influenced by Tm³⁺ and Gd³⁺ mutually.
Synthesis of Lu-MOFs. Lu-MOFs were prepared by the solvothermal method with lutetium as nodes and H₃L as a ligand in DMF solution.¹⁰⁷ The Eu³⁺/Tb³⁺@Lu-MOF ratiometric sensor was constructed by introducing Eu³⁺ and Tb³⁺ cations into the Lu-MOF. This sensor can recognize ornidazole and ronidazole with a low detection limit and high quenching efficiency. MOFs-based fluorescence sensing platforms have different effects on different biomarkers, which helps explore the association effects between other biomarkers and MOFs-based sensing platforms.

2.3.2 Solvothermal method.
Synthesis of Gd-MOFs. Gd₂(btec)₂ was prepared by solvothermal reaction with 1,2,4,5-phenyltetracarboxylic acid (H₄btec), 4,4′-azopyridine and gadolinium nitrate under the conditions of a DMF/H₂O/acetic acid volume ratio of 1 : 1 : 2. The spatial basis of Gd₂(btec)₂ is the tri-oblique space group P1, whose asymmetric unit consists of a nine-coordinated Gd center, two half btec nodes, and half a 4,4′-azopyridine. The 4,4′-azopyridine ion is close to a specific location in the Gd₂ (btec)₂ channel.⁹⁶
Synthesis of Er-MOFs. The dual-emission ThT@Er-MOF ratiometric fluorescence sensor was a first-synthesis-then-assembly method that introduced the fluorescent dye thioflavin T (ThT) into Er-MOF. Through the synthesis of Er-MOF ([Er(L)(DMF)_1.27]_n) (H₃L = terphenyl-3,4′′,5-tricarboxylic acid) by solvothermal and ultrasonic methods, Alzheimer's disease (AD) biomarkers in cerebrospinal fluid can be detected by ThT@Er-MOF using three methods.^99,104
Synthesis of Yb-MOFs. 2D Yb-MOFs were prepared with ytterbium as nodes and 4,4′,4′′,4′′′-(21H,23H-porphine-5,10,15,20-tetrayl) tetrakis-benzoic acid (H₂TCPP) as the ligand in DMF solution.¹⁰¹ 2D Yb-MOFs had strong electrochemiluminescence (ECL) behavior because of the larger specific surface area, better electrochemical conductivity and higher productive ratio of fluorescence quantum yield. Ytterbium chloride and DDPDBCBIm(Br)₂ ionic liquid can be used to synthetized the Yb-MOFs in DMF solution. The Au nanoparticles were loaded on the Yb-MOFs to obtain the Yb-MOF@Au-NPs nanocomposites for the ultrasensitive detection of the disease-related biomarker carcinoembryonic antigen (CEA).¹⁰³

3 Photocatalysis

In semiconductor photocatalysis, the photogenerated holes and electrons produced by the semiconductors after absorbing light give the semiconductor catalysts photocatalytic properties, effectively avoiding the disadvantages of high temperature, high pressure and cumbersome steps in traditional processes, and this has attracted widespread attention.^108,109 At present, in addition to inorganic materials being used in photocatalysis, organic materials are also being widely reported in the field. The porous structures of MOFs and metal nodes enable the catalyst to act as an island of semiconductor quantum dots.^110,111 Organic structures or light absorption in MOFs can excite these quantum dots.^108,112,113 Rare earth atoms easily gain or lose electrons to enhance charge transfer due to the partial filling of 4f orbital in their electronic structure. Moreover, aqueous solutions of rare earth metal ions are commonly Lewis acids, and most rare earth ions exhibit the trivalent state due to the 4f orbital being unsaturated.¹¹⁴ Rare earth MOFs also have great advantages in catalyst recycling. They can be separated from the reaction system by basic operations such as filtration and centrifugation, which is beneficial to the next step of recycling, while some homogeneous catalysts are difficult to recover in the reaction system. Therefore, rare earth MOFs are considered potential photocatalysts.

3.1 Hydrogen evolution reaction

The already reported excellent photocatalysts of MOFs (UiO-66-NH₂ and Ti-MOF-NH₂) have a utilization efficiency of only 5% for sunlight as they only respond to UV light. Therefore, it is necessary to develop catalysts with an excellent response to visible light energy to increase the efficiency of optical utilization.¹¹⁵

Recently, some applications for extending photosensitive MOF catalysts have been successively reported.^116,117 MOFs have higher charge mobility compared to inorganic semiconductors, as MOFs can promote the electron diffusion of reactants to disperse them to active sites, thereby improving the catalytic performance of photocatalysts.¹¹⁸ Therefore, the interaction of metal-linked orbitals is closely related to the choice of inorganic metal junctions.^119,120 The electrons in the 4f orbital of rare earth ions, due to the existence of unpaired electrons, can promote the interaction between the ligand and the cluster charge transfer (LCCT).^121,122 Through the LCCT reaction, rare earth MOFs can limit the recombination of the separated photogenerated electrons and holes.^123–125 Zhang et al. chose dye-like azocarboxylic acids as organic linkers to construct a Gd³⁺-based metal–organic framework [Gd₂(abtc)(H₂O)₂(OH)₂]₂H₂O (Gd-MOF) (H₄abtc = 3,3′,5,5′-azobenzenetetracarboxylic acid), and this Gd-MOF catalyst exhibited a broad absorption band under UV light irradiation. When the Gd-MOF photocatalyst was used, the hydrogen production rate was 7.71 μmol h⁻¹. When the co-catalyst Ag was added in the form of deposition, the catalytic activity was superior. When the Ag deposition amount was 1.5%, the catalytic activity increased by 1.5 times (Fig. 3a and b). In Fig. 3c, compared with Gd-MOF, the addition of Ag presented a lower semicircle at high frequency, and the addition of Ag reduced the charge transfer resistance in the solid-state interface.¹²⁶ Yu et al. synthesized a MOF of [Dy₂(abtc)(H₂O)₂(OH)₂]2H₂O (H₄abtc = 3,3′,5,5′-azobenzenetetracarboxylic acid), in which the photosensitizer of the MOF is integrated into the MOF through the reaction of Dy³⁺ ions with the dye ligands in the MOF. The absorption edge showed a broad absorption band at 570 nm. The hydrogen evolution rate of Dy-MOF under photocatalysis was 21.53 μmol h⁻¹ g⁻¹, and the activity of the catalyst did not decrease after three cycles (in the case of H₂ recovery) (Fig. 3d–f).¹²⁷ Wu et al. calculated the electronic structure of UiO-66, and explored the electronic structure of the MOF doped with different metal elements, and explained the impact. The low-position nature of the 4f orbital resulted in a negative efficient ligand–metal charge transfer (ELMCT), which enhanced the photocatalytic HER activity of this catalyst, and the UiO-66 node had a stable and efficient LMCT excitation due to the introduction of Ce⁴⁺. Furthermore, Ce-MOF has a large and negative LMCT energy, and there is a high photon absorption energy in this process, which limits the application of Ce-MOFs as visible light catalysts. The doping of Zr and Ti in UiO-66(Ce) has been studied in detail, and doping with Zr and Ti improved the ELMCT of the catalyst and helped to reduce the adsorption energy, thus improving the photocatalytic properties of Ce-UiO-66.¹²⁸ Huang et al. synthesized UiO-67, and then used bispyridine of 2,2′-bipyridine-5,5′-dicarboxylic acid (bpydc) to substitute biphenyl-4,4′-dicarboxylic acid (bpdc) and introduced Ce⁴⁺ ions into UiO-67 to obtain the UiO-67-Ce. UiO-67-Ce can weaken the energy transfer between bpdc and bpydc-Ce, so that the recombination of carriers is suppressed. The LMCT process of UiO-67 was weakened with suppressed carrier recombination. The UiO-67-Ce was ten times more than that of UiO-67, and the HER efficiency of UiO-67-Ce was also better than that of most Zr-based MOF photocatalysts.¹²⁹ Zhuang et al. synthesized hinge-like CeO₂ by thermally decomposing a Ce-based MOF, and formed a N, S co-doped C-coated structure. Compared with commercial CeO₂, the newly synthesized catalyst has a superior light absorption range because the hinge structures allow light to undergo multiple reflections after being incident. The hinge structure can enhance the charge transfer ability of the porous structure on the C layer, and the low charge transfer resistance in the porous coating, so the catalyst has higher activity in photocatalytic HER and hydrogen production. The modified CeO₂ has a higher mass-normalized velocity than the commercial CeO₂ (555 > 195 μmol h⁻¹ g⁻¹).¹³⁰ Zhang et al. designed a fibrous graphitic carbon nitride(g-C₃N₄) derivative prepared from Ce³⁺(Ce–C₃N₄)-doped MOFs as a raw material, which can be used as a photocatalyst. The MOF-derived after thermal polymerization and further heat treatment showed superior photocatalytic activity as compared with Ce–C₃N₄ doped with cerium melamine, benefiting from the fibrous morphology (Fig. 3g and h). Changes in pore structure and larger surface area promote surface charge interactions. The catalytic activity showed that the morphology and structure played a major role in the catalytic performance of the samples. The finer fiber structure provides abundant active sites and therefore has higher photocatalytic activity. Under visible light irradiation, the photocatalytic activity of 0.015 Ce–C₃N₄ went through four cycles within 24 h (Fig. 3i). The researchers further modified the sample with NH₄F to make the fiber morphology of the sample finer and the dispersion better, which further improved the active site and enhanced the absorption of solar energy.¹³³

Fig. 3 (a) Schematic diagram of Gd-MOF and Ag/Gd-MOF catalytic hydrogen production reaction under light conditions. (b) Schematic diagram of Gd-MOF structure. (c) EIS Nyquist plots of the prepared electrodes covered with Gd-MOF and Ag(1.5)/Gd-MOF in 0.5 M Na₂SO₄ aqueous solution, Copyright 2016, Royal Society of Chemistry. (d) Schematic diagram of the energy level transitions of Dy-MOF. (e) Hydrogen evolution intensity of a Dy-MOF series. (f) The catalytic activity of Dy-MOF after 15 cycles of visible light irradiation, Copyright 2018, Royal Society of Chemistry. (g) Comparison diagram of metal-C₃N₄ photocatalytic hydrogen evolution intensity. (h) Comparison of Ce–C₃N₄ hydrogen evolution with different doping ratios. (i) 24 hour cycle hydrogen evolution measurement of Ce–C₃N₄ with a doping ratio of 0.015, Copyright 2019, MDPI.

3.2 Carbon dioxide reduction reaction

As a greenhouse gas, the continuous emission of CO₂ leads to global warming. Converting CO₂ into valuable products such as formic acid, ethanol and methane is therefore important to address both environmental and resource concerns. At present, there are many reactions for CO₂ conversion, such as the use of thermal catalysis, photocatalysis and electrocatalysis to convert CO₂ into renewable resources.^131,132,134 For this reason, light-driven CO₂ conversion is currently an effective solution for light energy utilization. In recent decades, more and more materials have been developed and used in the carbon dioxide reduction reaction, such as molecular compounds, semiconductor materials, hybrid materials and so on. In recent years, the ideal capture and conversion concept of photocatalysis has attracted much attention because photocatalysis can promote the absorption of solar energy by materials into usable energy storage. Therefore, the study of efficient catalysts that can react under mild conditions in the photocatalytic process has attracted wide attention. MOFs are ordered porous crystalline materials that use organic ligands to connect metal ions or ion clusters to a network structure. Because of their high porosity, regular pore channels and adjustable pore size, MOFs are promising photocatalysts. The advantages of rare earth MOFs are mainly attributed to the abundant energy level structures of rare earth cations. MOFs with the multi-functional coordination structure of rare earth clusters can provide more possibilities for new rare earth MOFs (Fig. 4a). Rare earth MOFs have strong acid catalytic sites, such as Lewis acid sites, which help to introduce rare earth metals and improve catalytic efficiency (Fig. 4b and c).¹³⁵ They also have basic catalytic activity centers from the carefully selected organic ligands.¹³⁶ They also have a platform suitable for host-guest interactions in the post-synthesis modification process that helps to produce materials that provide synergistic catalytic performance for specific reactions.^127,137 Rare earth MOFs have the characteristics of enhanced charge transfer mobility¹³⁸ and are stable under catalytic conditions, so they can be recycled.¹³⁹ Therefore, rare earth MOFs have great research and application potential in the field of photocatalysis.¹⁴⁰ Lee et al. found that Eu ions showed good catalytical activity in reductive transformation,^141,142 and Zheng et al. used EuRu(phen)₃-MOF as a photocatalyst to study the catalytic reaction of CO₂ reduction to formate.¹⁴³ They introduced the Eu cluster as a metal connection node into MOFs, and the resulting independent Eu active cluster sites can be used to reduce CO₂. The EuRu (phen)₃-MOF showed the selective photoreduction of CO₂ driven by visible light, and the Eu cluster became the active catalytic center for CO₂ photoreduction after accepting the photoexcited electrons of metal oligomers.¹⁴⁴ Under the irradiation of visible light, Ru(II)(phen)₃ was excited into a triplet state, and then an electron was transferred to [Eu(III)–H₂O–Eu(III)] to synthesize [Ru(III)(phen)₃]. Therefore, [Eu(III)–H₂O–Eu(III)] achieved the active site [Eu(II)–H₂O–Eu(II)] by accepting two electrons, and CO₂ can be selectively reduced to HCOOH in the dual-electron process.¹⁴⁵ Wu et al. reported the research progress on the photocatalytic reduction of CO₂ by Ln-MOF. They used the amino linker (H₃TCA) to synthesize a MOF based on 3D-Gd, which was labeled as Gd-TCA (Fig. 4d). When compound 3 was added to CH₃CN/H₂O (1 : 1), the quenching coefficient K_SV of the Gd-TCA suspension was 2.65 ± 0.2 × 10³ M⁻¹. In the presence of compound 3, the emission band lifetime of Gd-TCA was reduced from 2.38 ns to 1.58 ns. These results confirm the occurrence of photoinduced electron transfer from the Gd-TCA excited state to model compound 3. When Gd-TCA (1 mg) and compound 3 (50 μM) were added to 5 mL CH₃CN/H₂O (1 : 1) solution, the hydrogen evolution efficiency was the highest when the concentration of the sacrificed-electron donor triethylamine was 10.0% (v/v) at pH = 11.0 (Fig. 4e and f). This MOF was composed of numerous Gd-basic chains that were mainly connected to a network through TCA and stacked through π–π interactions. Gd-TCA is a microporous structure that is suitable for the selective capture of CO₂ and conversion of CO₂. Gd-TCA was deposited on the film of the α-Al₂O₃ plate, which extended the service life of the artificial system, provided a platform for practical application, and made the device easy to operate and recyclable.¹⁴⁶ Wang et al. used ligand–metal and metal–ligand charge transfer processes to prepare Eu-MOFs. This activity was due to the high electron transfer in the Ru(Phen)₃-based connection network forming a double interpenetrating framework (Fig. 4g and h), which caused a red-shift in the absorption of the photocatalytic MOF system. Combined with the redox process of Eu(III) to Eu(II) and LMCT, the band energy of the MOF system was suitable for CO₂ further reduction, thus activating the C=O of the CO₂ molecule for reduction. After 10 h of illumination, the HCOO⁻ content was very low in the precursor and without catalyst, and the amount of HCOO⁻ was about 47 μmol when the catalyst was added, indicating good catalytic performance (Fig. 4i).¹⁴³ Zhao et al. synthesized Th-MOF by a solvothermal reaction with isonicotinic acid and determined that the adsorption capacity of the MOF of the transition metal thorium (TM-Th MOF) for CO₂ was 42.44 cm³ g⁻¹. At the same time, this catalyst was an excellent catalyst for CO₂ capture and reduction, mainly attributed to the Lewis acidic sites provided by Th₆ clusters. TM-Th MOF showed catalytic ability for the CO₂ cycloaddition reaction.¹⁴⁷ Zhang et al. used PCN-900(Eu) to conduct CO₂ cycloaddition with epoxides. They immersed the synthesized PCN-900(Eu) in DMF solution containing Co²⁺ to prepare CoTCPP-CoBPYDC. The Co of this catalyst can provide acidic sites to promote the conversion of epoxides. The crystalline phase of the MOF was cubic, and the metal sites were located at the vertices, edges and surfaces of the cubic cage, which were highly adjustable, resulting in the high porosity of the MOFs.¹⁴⁸ Zhou et al. added a linear linking group to PCN-905-SO₂ to achieve a tandem protection-aldol reaction. It formed large cuboid crystals with open metal sites and low connectivity, making MOF stable in water, and realizing the accurate placement of functional groups in the pore environments to achieve functionalization. They used the rare earth coordinatively unsaturated cluster of PCN-905-SO₂ to place the BDCSO₃H linker with Brønsted acid sites on the rare earth cluster post-synthesis. This MOF combined with NH₂ groups on the main linker of PCN-905-SO₂ can be used as an acid and alkali bifunctional catalyst, but also avoids the neutralization reaction of acid and alkali.¹⁴⁹

Fig. 4 (a) Scheme of the photochemical CO₂ reduction catalysed by MOFs. (b) Scheme of the Prins reaction catalysed by MOFs. (c) Scheme of the Strecker reaction catalysed by MOFs, Copyright 2017, Royal Society of Chemistry. (d) Construction of Gd-TCA consolidated by the dinuclear units exhibits the opening of the pores along an axis and its application on the light-driven hydrogen production with the Fe–Fe hydrogenase model compound and carbon dioxide reduction with the Ni(Cyclam) complex, respectively. (e) Family of emission spectra of the Gd-TCA suspension (0.02%) upon the addition of complex 3 up to 1.1 × 10⁻⁴ M in 1 : 1 CH₃CN/H₂O. (f) ¹³C NMR spectra for the product obtained from reaction with (1) ¹³CO₂ under irradiation; (2)¹³CO₂ in the dark, and (3) ¹²CO₂ under irradiation, Copyright 2016, American Chemical Society. (g) Crystal structure of Eu-Ru(phen)₃-MOF. (h) Stick/polyhedra model structure of the metalloligand. (i) Time profiles of HCOO⁻ produced catalytically by Eu-Ru(phen)₃-MOF or H₃L, or without catalyst under irradiation with a Xe lamp, Copyright 2018, Springer Nature.

Functional linking groups with excitation and extended lifetimes of excited states can be added to MOFs, enabling the more efficient separation of ligand–metal photogenerated holes and electrons. The energy levels of vacuum alignment can also be adjusted by ligand functionalization However, due to the limited light absorption energy of Ce-UiO-66, it is difficult to absorb visible light. Sun et al. tried to modify the ligand and doped with Zr/Ti to improve the negative LMCT energy of Ce-UiO-66 and reduce the absorbed energy. The Ce-UiO-66 (Ti₄)-I had 1.65 eV and 2.5 eV light adsorption energy in the solar radiation range.¹¹⁴ Cai et al. constructed the heterostructure between Ce-MOF and Bi₂MoO₆ by electronic coupling. The structure of Ce-MOF is rod-like, and the structure of BMO is lamellar. The combination of the two gives the material a high surface area and promotes strong contact between them. The Ce-centered MOF was synthesized with benzene tricarboxylic acid as the ligand. The photoreduction performance and adsorption capacity of CO₂ were enhanced by the contact with the electron coupling interface. The products of CO₂ reduction in Ce-MOF/Bi₂MoO₆ composites were CH₄ and CH₃OH. The yields of CH₃OH and HCOOH increased with the increase of heterogeneous components but decreased with the increase of Bi₂MoO₆.¹⁵⁰ Zhang conducted research on rare earth cluster-based organic framework materials for photocatalytic CO₂RR. They assembled the catalytically active rare earth clusters with photosensitive ligands to synthesize the rare earth Eu-Ru(phen)₃-MOF catalyst with a macroporous structure and realized the efficient selective conversion of CO₂ to formate, driven by visible light. The photocatalytic reaction rate of formate reached 321.9 μmol h⁻¹ mmol MOF⁻¹. The binuclear Eu²⁺ units generated in situ under light conditions were the photocatalytic active sites.¹⁴³ Our group synthesized a MOF nanosheet structure for photocatalytic CO₂RR.¹⁵¹ The nanosheet structure has a high specific surface area, the photocatalytic reduction of CO₂ can be carried out efficiently,^151,152 and CO₂ can be efficiently adsorbed on MOF nanosheet structure.¹⁵³

3.3 Other reactions

The design of rare earth MOF photocatalysts can be performed using isometric or building block approaches.¹⁵⁴ This section introduces the reports on the photocatalytic treatment of organic chemicals by rare earth MOFs. Environmental pollutants such as dyes and antibiotics have become an important burden on energy and the environment.¹⁵⁵ Ce has doublet states of excitation and emission from 4f to 5d, which endow it with good energy conservation and no energy loss after the spin state change.¹⁵⁶ Cai et al. prepared Ce-doped sensitized Ag₃PO₄ composites by the in situ ion exchange deposition method.¹⁵⁷ The crystal structure of MOF was changed due to the addition of Ce. The specific surface area decreased with the increase in Ce but the pore size and pore volume increased to provide more sites for photocatalysis. Ce contains a large number of oxygen vacancies and has a high oxygen storage capacity, which leads to the generation of ˙O²⁻, and the Ce³⁺/Ce⁴⁺ redox pair in the system can promote photogenerated hole transfer to the electron donor (Fig. 5a). Wang et al. prepared an isostructural rare earth MOF [Eu_xTb_1−xL(DMF)₂(NO₃)] (H₂L = 2,2′-bipyridine-4,4′-dicarboxylic acid) in situ by doping a rare earth salt mixture into the Tb-MOF. Under UV irradiation, the degradation efficiency of Tb-MOF to catalyze the decomposition of rhodamine (RhB) was 96%, and that of Eu-MOF was 94% (Fig. 5b).¹⁵⁸ Wu et al. used the rigid dicarboxylic acid ligand H₂L as an organic ligand to synthesize Gd-MOF. Its energy band gap (e_g) was 2.68 eV, and it can generate ˙OH active groups at the same time. In the presence of this catalyst, the decomposition rate of RhB can reach 95%, which greatly improves the decomposition efficiency as compared with the condition without catalyst (15%).¹⁵⁹ Lian et al. synthesized a europium-functionalized material (MIL-125(Ti)-AM-Eu) based on MIL-125(Ti)-NH₂. The crystals formed by the material were truncated bipyramidal, having a high surface area, good luminescence, high conversion of photocatalytic oxidation of α-acetophenone, and strong selectivity. It had a fluorescence signal reaction to phenyl ethanol. The conversion rate of α-phenethyl alcohol to acetophenone was 20.4% for 6 hours in the presence of catalyst and oxygen (Fig. 5c and d).¹⁶⁰ Jiang et al. used a top-down synthesis method to synthesize porphyrin-based lanthanide MOFs without the action of surfactants (Ln-TCPP, TCPP = 4-carboxyphenylporphyrin). Under light conditions, the photocatalytic oxidation of 1,5-dihydroxynaphthalene to juglone by Yb-MOF outperformed other lanthanide MOFs. The possible reason is that the electron transfer between TCPP and Yb³⁺ ions can cause MOF to generate ˙O²⁻ and O₂, thus showing higher photocatalytic activity (Fig. 5e and f).¹⁶¹ Kaur et al. designed and synthesized Eu-MOF with CdTe-conjugated quantum dots (QDs). The rhodamine 6G dye can be completely degraded by QD/Eu-MOF within 50 min (Fig. 6a).¹⁶² Pan et al. reported that chalcone dicarboxylic acid ligands combined with rare earth elements to form MOFs, and reacted the ligand with lanthanide metals to generate seven kinds of rare earth MOFs.¹⁶³ Aguirre-Díaz et al. prepared a series of rare earth coordination polymers by solvothermal synthesis (RPF-30-Er, RPF-31-Ln (Ln = La, Nd, and Sm)). The photocatalytic conversion rates of the five MOF materials were 60–76% on the oxidative coupling of benzylamine and imine, and the selectivity exceeded 99% (Fig. 6b).¹⁶⁴ Jiang et al. constructed porphyrin-based 2D MB-/Yb-TPCC-SO₄ nanosheets. Such rare earth MOFs had the following characteristics (Fig. 6f). The chromophores of MOFs were close to each other but not in contact; the chromophores were highly ordered. 2D MOFs have a larger specific surface area and more accessible active sites, providing abundant active sites and a higher mass transfer rate for improving photocatalytic performance. These unique properties not only facilitate photon capture, energy transfer, and the increase of active centers but also inhibit the quenching of chromophores. In the photocatalytic synthesis of artemisinin on 2D MB-/Yb-TCPP-SO₄ nanosheets, the internal conversion rate of artemisinin was 99%, and the yield was 80% for 3 hours.¹⁶⁵

Fig. 5 (a) Schematic of the photodegradation and possible reaction mechanism for the MIL-101-NH₂/Ag₃PO₄ system under visible-light irradiation, Copyright 2020, Elsevier. (b) The photocatalytic degradation profiles of RhB under different conditions, Copyright 2019, Elsevier. (c) ⁵D₀ → ⁷F₂ transition intensity ratios from the emission spectra of MIL-125-AM-Eu introduced into α-phenethyl alcohol after UV radiation for different times. (d) Recycling experiments of MIL-125(Ti)-AM-Eu for sensing α-phenethyl alcohol. Red and black columns represent the luminescence intensity of MIL-125(Ti)-AM-Eu and the conversion of the photocatalytic reaction, respectively, Copyright 2016, American Chemical Society. (e) Illustration of the synthesis of 2D Ln-TCPP nanosheets and the thickness- and metal-node-dependent photocatalytic activity. (f) The possible electron and energy transfer paths in 2D Yb-TCPP-4 nanosheets during the photocatalytic process, Copyright 2020, Wiley VCH.

Fig. 6 (a) Changes in the efficiency of the Rh 6G (concentration: 1 mg L⁻¹) samples with irradiation time after photodegradation with 5 mg L⁻¹ of QD/Eu-MOF, Copyright 2016, Elsevier. (b) Degradation rates of the MB, PH and RhB solutions in the presence of Ln–MOFs, Copyright 2018, American Chemical Society. (c) RhB degradation by NNU-15(Tb) and NNU-15(Dy) over a H₂O₂/Ln-MOF/RhB system. (d) Photocatalytic reactions at different premixing processes, (Curve 1) premixed NNU-15(Ce)/RhB, (Curve 2) premixed NNU-15(Ce)/H₂O₂, (Curve 3) premixed NNU-15(Ce)/H₂O₂/RhB, (1) Premixed NNU-15(Ce)/RhB, (2) premixed NNU-15(Ce)/H₂O₂, (3) premixed NNU-15(Ce)/H₂O₂/RhB. (e) Time-dependent absorption for the system of NNU-15(Ce)/H₂O₂/RhB/visible light. For all reactions: NNU-15(Ce), 30 mg; H₂O₂, 0.6 M, Copyright 2017, American Chemical Society. (f) Working principle of the constructed 2D ALHS with chloroplast bionic structure for artemisinin production, Copyright 2021, Royal Society of Chemistry.

In degrading water pollution into less toxic organics, the advanced oxidation process (AOP) of hydrogen peroxide (H₂O₂) has been widely used due to its stronger photocatalytic performance than multifunctional organics.^166–169 AOP not only has the characteristics of high efficiency and easy handling but it can also generate highly active substances in situ. Therefore, synthesizing new MOF photocatalysts and participating in the degradation of AOPs of H₂O₂ is a very promising research direction. Xia et al. used anthracene dicarboxylic acid as a ligand to prepare a series of lanthanide MOFs by thermal synthesis. In the presence of H₂O₂, about 99% of RhB in solution was degraded by Ce-MOF after 12 min of light irradiation. The efficiency of Dy-MOF to degrade RhB reached 95% at 60 minutes, and the degradation of RhB reached 90% at 60 minutes on Tb-MOFs (Fig. 6c). Ln-MOFs can enhance the photocatalytic performance by rapidly generating hydroxyl radical active species. Because H₂O₂ can produce hydroxyl radicals under light conditions, it is beneficial to the photocatalytic degradation of RhB by lanthanide MOFs (Fig. 6d and e).¹⁷⁰

Aguirre-Díaz et al. synthesized five rare earth catalysts, and 1D rare earth-based coordination polymers (CPs) were used as the photocatalyst for the oxidative coupling of amines. Five spirobifluorene-containing Ln-CPs, namely, [Er₃(Hsfdc)₃(sfdc)₃(H₂O)]·xH₂O (RPF-30-Er), [Ln(Hsfdc)(sfdc)(EtOH)]·S(RPF-31-Ln, where Ln= La, Nd, and Sm and S = H₂O or EtOH), and [Ho(Hsfdc)(sfdc)(H₂O)] (RPF-32-Ho) (RPF = rare-earth polymeric framework and H₂sfdc = 9,9′-spirobi[9H-fluorene]-2,2′-dicarboxylic acid), have been solvothermally synthesized for the oxidative coupling of amine. The selectivity of the oxidative coupling of amine can reach 92% and the conversion rate can reach 60% at the optimal ratio.¹⁷¹ Zhang et al. used the defection-induced nanomanufacturing strategy to prepare the nanostructured MOF-76(Ce){[Ce(BTC)(H₂O)] × DMF}_n for nitrogen reduction. The nitrogen was adsorbed and desorbed on the MOF catalyst surface by bubbling into the solution. The formation of NH₃ was catalyzed by the strong light, and the production rate of NH₄⁺ was 36.4 μmol g⁻¹ h⁻¹.¹⁷²

4. Electrocatalysis

Due to the unique pore structure of the MOF, it can provide more active sites for the catalyst to attach and provide a conductive medium for electron transport.¹⁷³ By synthesizing different configurations of MOF materials and embedding multifunctional groups on organic ligands, the catalytic activity can be improved without changing the topology of MOFs. Rare earth elements are very active metals with excellent electronic structures.^174–177 Doping with rare earth elements can change the performance of electrochemical catalysts.^178,179 Their electronic structures, including metals and non-metals, endow them with enhanced HER, OER, CO₂RR, etc. properties because of the unique properties of 4f electrons.

4.1 Hydrogen evolution reaction

Hydrogen is a form of clean energy with a high calorific value^180–183 and therefore, hydrogen may become an important energy source on the world energy stage in the foreseeable future. The technologies for the fetching, storage, transportation and application of hydrogen have received extensive attention.^184,185 Hydrogen production by the electrolysis of water has been the mainstream industrial hydrogen production technology since 1789.¹⁸⁶ However, the large overpotential and low electrochemical energy conversion efficiency involved in the hydrogen evolution reaction process have become the main obstacles.¹⁸⁷ Liao et al. synthesized the erbium-based MOF material with 1,3,5-benzenetricarboxylic acid as the ligand by the hydrothermal method. Er-MOF/MoS₂ was obtained by doping MoS into MOF material by adjusting the pH. There are a large number of folds on the surface of MoS₂, which are conducive to the exposure of catalytic active sites. Er-MOF/MoS₂ has many micropores that can transport electrolytes to the catalytic surface and improve hydrogen production efficiency. The overpotential of Er-MOF was 790 mV at 10 mA cm⁻² and that of MoS was 327 mV. It can be seen from Fig. 7a and b that the Tafel slope of Er-MOF/MoS₂ is lower than that of pure MoS₂ and pure Er-MOF. The improved catalyst activity is due to the rough surface formed by the lamellar structure between the MoS₂ and Er-MOF interfaces, which increases the area where the active sites can attach. The pore structure of MOF can reduce the gas-solid surface tension and increase the reaction rate. Er-MOF/MoS₂ and CV curves of MoS₂ are shown in Fig. 7c. Er-MOF/MoS₂ has a high current density and area. The REDOX peak of Er-MOF/MoS₂ is higher than that of MoS₂ under the same conditions. The higher the active surface area, the higher the catalytic activity.¹⁸⁸ Xiong et al. used ZIF-8 as a template to synthesize a ternary hybrid CeO₂–CoP–C catalyst by a high-temperature carbonization and low-temperature phosphating route. The carbonization of MOF not only increased the porosity of the catalyst, but the carbon skeleton could also achieve rapid electron transport and efficient mass transfer, which improved the stability of the catalyst. The co-ion exchange of Ce ions effectively doped the rare earth element into the MOF material. The rare earth catalyst has an overpotential of 71 mV at 10 mA cm⁻² (Fig. 7d).¹⁸⁹ Lan et al. reported a CeO_x-modified CoP@carbon material. Since rare earth metals changed the electronic structure of MOF, the HER overpotential was measured to be 127 mV (Fig. 7e).¹⁹⁰ Xue et al. reported a Cu₂MoS₄ nanosheet material grown vertically on Ce-MOF, exposing more active sites at the edges. When the HER was measured at a current density of 10 mA cm⁻², the overpotential was 350 mV, and the Tafel slope was 70 mV dec⁻¹ (Fig. 7f).¹⁷³

Fig. 7 (a) Linear sweep voltammograms of Er-MOF/MoS₂, MoS₂, and Er-MOF, in 0.5 M H₂SO₄ at a scan rate of 5 mV s⁻¹. (b) The corresponding Tafel plots of Er-MOF/MoS₂, MoS₂. (c) Cyclic voltammetry of Er-MOF/MoS₂ in the non-Faraday potential region at different scanning rates in 0.5 M H₂SO₄, Copyright 2022, Elsevier. (d) Polarization curves of carbon cloth (CC), ZIF-8/CC, CoP-C/CC, CeO₂-CoP-C/CC, and Pt/C in a N₂-saturated 0.5 M H₂SO₄ solution. Scan rate: 2 mV s⁻¹. Inset shows the corresponding overpotential of CoP-C/CC, CeO₂-CoP-C/CC, and Pt/C at the current density of 10 mV cm⁻² (η₁₀), Copyright 2018, Elsevier. (e) Polarization curves of HER, Copyright 2021, Wiley VCH. (f) The corresponding Tafel plots of different catalysts for HER, Copyright 2021, Wiley VCH.

4.2 Oxygen evolution reaction

Zhang et al. reported a cobalt-doped mixed lanthanum oxide and hydroxide heterostructure using La-MOF-NH₂. As an OER catalyst, the overpotential was 330 mV at a current density of 10 mA cm⁻² (Fig. 8a).¹⁹¹ Ma et al. reported a Fe-MOF doped with Dy (Dy_0.05Fe-MOF) by a one-step solvothermal reaction. The results showed that the catalyst was hydrangea shaped and composed of ultrathin nanosheets. The catalyst showed durability, activity, stability and constitutive OER activity. Dy_0.05Fe-MOF needs an overpotential of 319 mV to drive the OER at a current density of 500 mA cm⁻², while Fe-MOF can only drive a current density of 100 mA cm⁻² under the same overpotential. The less electronegative Dy doping can provide electrons to neighbouring Fe atoms as compared to Fe doping. The Dy and Ni spin ferromagnetic coupling can change the roughness of the substrate surface of the catalyst.¹⁹² Wan et al. reported a 3D rhombus nanostructure doped with Fe and Dy by MOF-Ni on carbon cloth. The FeDy@MOF-Ni catalyst required an overpotential of 251 mV to reach 10 mA cm⁻² and had a Tafel slope of 52.1 mV dec⁻¹. The unique 3D diamond structure and Fe–Dy–Ni coupling synergism gave it electrocatalytic performance (Fig. 8b and c).¹⁹³ Li et al. reported a MOF nanosheet doped with Y and Ce, which can be applied in the OER. The incorporation of Y and Ce atoms into Ni-MOF resulted in a bimetallic synergistic effect and the construction of rich active centers, thus improving the overall water decomposition performance of the material. At the same time, Y and Ce doping can control the electronic structure of NiYCe-MOF. For the OER, the overpotential at 10 mA cm⁻² was 245 mV, and the Tafel slope was 65 mV dec⁻¹, but for the HER, the overpotential at 10 mA cm⁻² was 136 mV. When NiYCe-MOF served as the anode and cathode electrocatalyst at the same time, a voltage of 1.54 V was needed when it reached 100 mA cm⁻² (Fig. 8d–f).¹⁹⁴ During the OER process, the surface of the MOF catalyst is reconstructed, in which metal oxides and hydroxides are the most common active sites. Most metal elements and metal oxides are thermodynamically stable endpoints of most chemical processes under a strong oxidizing environment. The adjustment and termination of the reforming process are key points in improving the catalytic efficiency.

Fig. 8 (a) Polarization curves of RuO₂, LaMOH@NC, and 3Co–LaMOH|OV@NC in 0.1 M KOH, 10 mV s⁻¹ (inset: the corresponding Tafel plots), Copyright 2021, Elsevier. (b) OER performance of the different catalysts in 1.0 M KOH. (c) LSV curves obtained at a scan rate of 5 mV s⁻¹, Copyright 2020, Elsevier. (d) Polarization curves. (e) Corresponding Tafel slopes of different samples for the OER. (f) Long-term testing of NiYCe-MOF/NF for 100 h at different current densities, Copyright 2022, American Chemical Society.

4.3 Carbon dioxide reduction reaction

The carbon dioxide reduction reaction (CO₂RR) is a very complex reaction process that involves a variety of products, reaction mechanisms, catalysts and electrolytic conditions.^195,196 The CO₂RR also has multiple reaction steps and intermediates, which mainly involve 2, 4, 6, 8 and 12-electron transfers.¹⁹⁷ The CO₂RR is the reason for the product diversity and can convert CO₂ into at least 16 different products through different reaction pathways.¹⁹⁸

These products can be divided into C1 and C2. The main representative products in C1 are carbon monoxide (CO), acid (HCOOH), methanol (CH₃OH), formaldehyde (HCHO) and methane (CH₄), and the main representative products in C2 are ethylene (CH₂CH₂), ethanol (C₂H₅OH) and acetic acid (CH₃COOH).¹⁹⁹ CO and HCOOH in C1 are the most efficient products in the CO₂RR.²⁰⁰ The process of C1 production from CO₂RR is relatively simple, including CO₂ adsorption, intermediate formation and product desorption. In contrast, the mechanism by which CO₂RR generates multi-C products is a complex process involving multiple electrochemical and chemical reactions. C2 is more difficult to produce than C1 because C2 is a multi-carbon product that has to transfer more electrons and protons.²⁰¹ For example, it takes 12 electrons to convert carbon dioxide into ethylene, and 2 electrons to produce carbon monoxide. In general, C2 and C3 multi-C products are more attractive than C1, but they also suffer from low selectivity, high energy consumption, and complex reactions. In the process of the CO₂RR, the change in the C=O bond is a necessary step; however, CO₂ with the C=O bond is very stable, and the destruction of the C=O bond requires a dissociation energy of about 750 kJ mol⁻¹.²⁰² Therefore, the CO₂RR requires higher energy. Rare earth MOFs are very important in the CO₂RR process to reduce energy loss. In the process of CO₂RR reaction, the C=O bond of CO₂ will coordinate with the core elements of the rare earth MOF catalyst, reducing the bond energy of C and O atoms, weakening the stability of the CO₂ molecular structure, thus reducing the energy barrier of the CO₂ molecular fracture reaction, and eventually catalyzing the whole CO₂RR process. Luna et al. studied the growth of MOF films on high-curvature gold nanostructured microelectrodes (AuNMEs). The porosity and crystal structure of MOF films are easy to change. Re-ndc-fcu-MOF (Re = Y, ndc = 1,4-naphthalenedicarboxylic acid, fcu = face-centered cubic) thin films grown on AuNME were prepared by solvothermal deposition, and the size of the Re-ndc-fcu-MOF octahedral crystal was 50 μm. For the CO₂RR electrocatalyst, the Faraday efficiency of CO was 18%. Because Re-ndc-fcu-MOF did not completely coat the nanoneedles, many exposed active sites of AuNME could contribute to CO formation (Fig. 9a–c). The adsorption capacity and Lewis acidity of catalysts can be changed by adjusting the type of organic ligands or metal junctions.²⁰³ Nam et al. used UiO-66 as the starting material, and changed the organic ligand of UiO-66 to NH₂BDC to enhance the adsorption capacity of CO₂ because the amino group can provide a stronger force with CO₂. The effect of acidity on CO₂RR was explored by replacing the central cluster from Zr to Y (Fig. 9d and e). At the same time, the active substance Ag nanoparticles were added to improve the selectivity, and the addition of Ag nanoparticles increased the local CO₂ concentration, which led to the enhancement of CO₂RR and inhibited the HER. The Faraday efficiency of CO was 60% at a current density of 30 mA cm⁻².²⁰⁴

Fig. 9 SEM images of RE-ndc-fcu-MOF on AuNME before (a) and after electrochemical reaction (b). (c) Linear sweep voltammetry (LSV) scans (0 to −1 V vs. RHE, 0.1 M KHCO₃) of different MOF@AuNMEs and AuNME control, Copyright 2018, American Chemical Society. (d) Microscopic diagram of fcu-MOF-BDC. (e) The metal node of MOF is replaced with Y to get Y-fcu-MOF-BDC, Copyright 2020, American Chemical Society.

5. Conclusion and prospects

In the past 30 years, MOFs have fascinated researchers with their structural diversity and tunability. Various MOFs have been synthesized from elements with metallic nodes, including elements in the s-, p-, d-, and f-regions. However, different types of MOFs have nearly similar functions, and MOFs synthesized by rare earth elements including scandium, yttrium and lanthanide have similar structures and functions to form an interesting series. Rare-earth MOFs can possess properties common to many MOF families, and they also display unique structures and properties due to the unique 4f electron shells of rare-earth elements. We have reviewed the progress of rare earth MOFs, including the synthesis of rare earth polymetallic MOFs materials, their structures, and current developments in the use of rare earth polymetallic MOFs in photo/electrocatalysis applications. Although there is much research on rare earth MOFs materials, the design of rare earth MOFs still needs to be improved, and the applications of MOFs still have great research potential (Fig. 10) For example, there is still no effective means of controlling the formation of tetranuclear, hexanuclear and nonanuclear cluster nodes, and there are many more cases where two or more clusters are in a network topology, which presents certain challenges. To improve the electrocatalysis properties of rare earth MOFs, the nuclear stability of MOFs is still a problem that needs to be solved. To improve the photocatalysis properties of rare earth MOFs, the problem to be overcome is to find multifunctional MOFs that have both photooxidation and photoreduction capabilities. To achieve the above goals, it is also necessary to find a substance that can span the energy bands of photocatalytic oxidation and reduction, while complying with the laws of thermodynamics. At the same time, in the narrow band gap structure, it can respond sensitively to light. Therefore, the topological structure of rare earth MOFs is correlated with photocatalytic performance. In the study of Re-MOF, MOF materials can not only complete conventional photocatalysis, electrocatalysis and other aspects but can also be used as materials for the development of battery electrodes to improve the working efficiency of batteries.^205–208 If the above challenges are overcome, rare-earth MOFs will have broader prospects and development in other fields. There are numerous unknown areas that need to be explored in the future.

Fig. 10 Schematic diagram showing the prospect of rare earth MOF.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the financial support from the National Nature Science Foundation of China (Grant No. 22122113) and National Key R&D Program of China (Grant No. 2022YFB3506200).

Notes and references

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