Recent developments of Fe-based metal-organic frameworks and their composites in photocatalytic applications: fundamentals, synthesis and challenges

The ever-increasing consumption of fossil fuels has led to a serious global energy crisis due to high demands on these energy resources.1 Moreover, burning of these fuels generates massive amounts of toxic pollutants including greenhouse gases (GHGs).2,3 On the other hand, solar energy is considered to be one of the best choices as an alternative and renewable energy resource due to its abundance and availability. Therefore, tremendous efforts were made to utilize solar light through various technologies. Among these solar light-driven technologies there is a photoreaction process known as photocatalysis. This process is conducted over the surface of a semiconductor material called a photocatalyst, when light having an equal or greater bandgap of that material is exposed to its surface, generates a huge population of electron-hole pairs (e^–/h⁺), after which these photogenerated charge carriers undergo a series of reduction and oxidation (Redox) reactions to produce the final solar products.4,5

In 1972, the first attempt of using TiO₂ as a photocatalyst was reported by Fujishima and Honda,6 who highly promoted the field of solar energy applications. Many approaches were then designed using various semiconductor materials in all photocatalysis fields. These semiconductors include various nanostructured materials such as zeolites, zinc oxide (ZnO), zirconium oxide (ZrO₂), zinc sulfide (ZnS), magnesium oxide (MgO), graphitic carbon nitride (g-C₃N₄), perovskites and layered double hydroxides (LDHs).7,8 However, the performance of these materials is generally modest. Therefore, there is a significant need for the development of more efficient photocatalytic materials for energy production and environmental remediation applications. According to previous studies, metal oxide photocatalysts such as TiO₂ have shown good photocatalytic performance under UV light. Unfortunately, the UV light represents only 3% of the solar light energy compared to 43% that of the visible light.9,10 At the same time, the noble metal nanoparticles such as Pt, Au and Ag demonstrated great response to visible light and hence were applied to solve this issue. However, their high cost and low stability greatly limit their use in thisfield.11,12

Recently, new crystalline and low-cost materials known as metal-organic frameworks have been introduced to this field. These MOF materials have many advantages including high specific surface area, high porosity, well-ordered structures, visible light response and exceptional photocatalytic functionalities.13--16 Among the most commonly studied MOFs, Fe-MOFs have been extensively investigated for many photocatalytic applications such as oxygen and hydrogen production, CO₂ reduction and photodegradation of pollutants.17--19

Iron is considered as a low toxic and inexpensive element due to its abundance in the earth's crust.20 In comparison with the oxo-clusters in Zr- and Ti-containing MOFs, the Fe–O clusters in Fe-MOFs can be directly excited by visible light irradiation making it an excellent visible-light-responsive material. Moreover, the Fe-MOFs has a bandgap ranging from 1.88 to 2.88 eV, which increases its photocatalytic activity under visible light, compared to wide bandgap energies exhibited by other types of MOFs such as MOF-­5 (3.4 eV), MIL-125(Ti) (3.6 eV) and UiO-66(Zr) (3.9 eV).21--23 All the above-mentioned merits of Fe-containing MOFs, in particular, make them promising photocatalysts and good competitors with other materials for energy production and environmental remediation applications.

Recently, Joseph et al.24 reported the current advances in the applications of Fe-MOFs in wastewater treatment and presented a detailed discussion on the structural instabilities of these materials which limits their utilization in this field. However, the use of Fe-based MOF photocatalysts involves a wider range of applications that need to be presented. In 2017, Wang and Li²⁵ investigated Fe-MOFs as photocatalysts in various types of photocatalytic reactions with more focus on their visible light-harvesting properties. However, since then there have been many research studies that need to be summarized in a comprehensive review. Therefore, this review aims to summarize the recent studies reported over the past decade on the use of Fe-MOFs and their composites in the main photocatalytic applications.

This review provides a brief insight into the photocatalytic fundamentals of metal-organic framework materials and their classifications. Then, the recent developments of the main synthesis techniques used for the preparation of Fe-MOFs with the perspectives for industrial scale-up production are discussed in depth. Furthermore, the major photocatalytic applications of various Fe-MOFs in oxygen and hydrogen production, CO₂ reduction and pollutant photodegradation are summarized and deliberately explained. Finally, this review is concluded with the future recommendations and challenges associated with the applicability of Fe-MOF photocatalysts for energy production and environmental remediation applications.

In recent years, a new type of promising materials known as metal-organic frameworks has attracted researcher's interests mainly due to their excellent optoelectronic properties, ordered crystalline structures, tunable porosity and high mechanical and thermal stability. In catalysis, MOFs are considered as good materials, in which the partially-occupied metal sites can provide active sites act as Lewis acidic sites for both gas- and liquid-phase reactions.26 Therefore, MOFs can catalyze a wide range of reactions including cycloaddition of CO₂ and epoxides,27 cyanosilylation of aldehydes,28 Friedel-Crafts reaction,29 various organic processes30 and isomerization reactions.31 Moreover, stable MOFs can also be modified as redox catalysts by metalo-functionalization of their organic linkers. However, when MOFs were synthesized for the first time, they were studied mainly for gas storage and separation.32--34 Later, they were extensively investigated for other uses including photocatalytic applications such as oxygen production,35,36 hydrogen production,37 CO₂ conversion38,39 and degradation of pollutants.40

Metal-organic frameworks can act as efficient charge carrier transportation systems through a photoexcitation process in organic linkers or metal clusters.41 Moreover, the optical properties of pure MOFs can easily be tuned by functionalizing the organic linkers (ligands) with other groups such as amino groups which can greatly enhance the visible light harvesting and hence, increase the photocatalytic performance. As shown in Fig. 1, these 3D hybrid materials are constructed from metal nodes (ions or clusters) and organic linkers using different synthesis methods including solvothermal, microwave-assisted, sonochemical, mechanochemical and electrochemical synthesis.

Fig. 1

The metal-organic framework structure constructed by metal ions/clusters linked by the organic linkers. Reproduced from Ref. 47 with permission from Elsevier.

The building blocks of MOFs are formed by linking metal clusters to organic ligands so that an open 3D framework structure is obtained. This geometrically well-defined framework structure endows the materials higher porosity and hence, with an extremely high specific surface area compared to other semiconductors. For instance, in our recent study42 we found that the titanium-based MOF, NH₂-MIL-125(Ti), exhibited more than 81 times higher specific surface area and 17 times higher pore volume than g-C₃N₄, which is considered as a benchmark photocatalyst in various energy and environmental applications.43 Moreover, organic linkers in MOFs can provide more visible light absorption and more feasibility for functionalization with other groups, e.g. amino group. In terms of synthesis, MOF materials can be grouped into different series including Materials Institute Lavoisier (MILs), Universiteteti Oslo (UiO), Porous Coordination Network (PCN), Northeast Normal University (NNU) and Zeolitic Imidazolate Frameworks (ZIFs).44

However, clusters of various metals (mainly transition) such as Ti, Zr, Fe and Zn in the parent MOFs, as shown in Fig. 2 can also be modified by introducing of extra active sites via some strategies of doping,45 metal addition46 and MOF-based composites47 (Fig. 3).

Fig. 2

Structures of selected Ti-based MOFs (a), Zr-based MOFs (b), Fe-based MOFs (c) and Zn-based MOFs (d). Reproduced from Ref. 47 with permission from Elsevier.

Fig. 3

Structural illustration of reduced graphene oxide (GO) nanoparticles doped on ZIF-8 MOF (a). Reproduced from Ref. 45 with permission from Wiley. The addition of Cr and Ga metal-monocatecholato (CAT) groups to the linkers of UiO-66 MOF (b). Reproduced from Ref. 46 with permission from the Royal Society of Chemistry. PSE is post-synthetic exchange. A metal-MOF composite consisting of Au, Pd, Pt and MOF-74 (c). Reproduced from Ref. 47 with permission from Elsevier.

These structural and tunable features provide MOFs with unique properties related to a high specific surface area and porosity which rank these materials among all other solids. As porous materials with a high surface area, MOFs are considered as the most ideal adsorbents for the storage and separation of gases (mainly CO₂ and H₂) due to their high physical adsorption capacity. However, the pore size and shape can be predicted, tuned and controlled by analogy with other related MOF structures relative to the ligands size.48

Among the studied MOFs, the Fe-based MOFs attracted much attention due to their semiconducting properties, low cost, visible light response, environmental safety, reliable chemical and thermal stability.49,50 Generally, the Fe-based MOFs consist of an oxocluster made up of the earth-abundant Fe element obtained from its salts and connected with organic linkers. A wide variety of Fe-based MOFs with different compositions and structures are available, making it easy to study the influence of the composition and structure on the Fe-based MOF photocatalytic activity. Previous limited results have revealed that the photocatalytic performance is affected not only by the nature of the Fe cluster but also the organic linker and the framework topology. For instance, both MIL-100(Fe) and MIL-88B(Fe) MOFs are constructed of similar Fe₃μ₃-oxo clusters and various organic linkers. Although these MOFs exhibited good light absorption in the visible range, MIL-­88B(Fe) displayed higher catalytic activity for rhodamine photodegradation compared to MIL-100(Fe).51

Generally, the Fe-MOFs based materials as photocatalysts can be divided into three types: Fe-based MOFs, Fe-MOF derivatives and their hybrid nanocomposites (NC). A synergistic effect between Fe and other metals such as Ni, Co, W and Mo can be obtained by the incorporation of Fe-based MOFs and other guest metal atoms, thus providing more metal active sites and hence, better photocatalytic performance.52 Fig. 4a shows the fabrication strategy of Fe-MIL MOF nanorods through a hydrothermal reaction followed by Ni-doping, which then exhibited an enhanced performance for photocatalytic oxygen production.53

Fig. 4

Scheme representation of Fe-MOF preparation followed by Ni-doping (a),⁵³ the synthetic strategy to prepare the Fe-MOF derived g-C₃N₄/α-Fe₂O₃ nanocomposite (b). Reproduced from Ref. 55 with permission from Wiley. Schematic representation of the synthesis of Fe-Ni@NC-CNTs (c). Reproduced from Ref. 56 with permission from Wiley.

Fe-based MOFs can also be used as precursors or templates in the thermal-assisted synthesis of functionalized materials with different structures and tailored compositions, including carbon-derived materials such as carbides or, e.g., phosphides in addition to hydroxides and metal oxides.54 As shown in Fig. 4b, nanoparticles (NP) of α-­Fe₂O₃-coupled g-C₃N₄ were synthesized from a well-known metal-organic framework (MIL-100(Fe)) using an ultrasonication treatment.55

Finally, hybridizing Fe-based MOFs with graphene or other functional materials, such as amino or hydroxide groups, is another option to overcome the issues related to Fe-based MOF photocatalysts. In 2018, Thomas and co-authors56 reported the formation of an efficient Fe-­Ni@NC-CNT (CNT is nanotube) hybrid photocatalyst via the pyrolysis of dicyandiamide-functionalized MIL-­88(Fe)/Ni MOF (Fig. 4c).

The choice for a synthetic approach is of great importance in the construction of MOFs since it determines the structure and affects the performance of the resulting MOFs. However, Fe-based MOFs can be synthesized through either conventional or non-conventional routes, where the latter can be carried out via six main procedures using various forms of energies and different conditions as illustrated in Fig. 5 and discussed in next sections.

Fig. 5

Schematic representation of the main synthesis methods of Fe-based MOFs.

This method is the most widespread technique used for the preparation of MOFs. In this approach, the reactants (organic linker, metal salt and solvent) are mixed in certain ratios and heated at temperatures (353-453K) above the boiling point of the used solvent.57 This can be carried out in an autoclave, which can provide required reaction conditions for this process, namely, both high temperature and high pressure, where both liquid and gas phases exist simultaneously and thus, converting the reactants into supercritical fluids for boosting the mixing of compounds and promoting the crystal growth.58 The most commonly used reaction media are organic solvents such as dimethylformamide, diethylformamide (DEF), acetone, methanol and ethanol. However, a mixture of two or more solvents can be used to overcome the problem of insolubility.59 Fig. 6a demonstrates the solvothermal synthesis process. Whitefield et al.60 reported a successful synthesis of an Fe-­based MOF by a solvothermal method using DMF as the solvent. The crystalline MIL-53(Fe) MOF was obtained after 48 h at 180 °C. Also, Feng et al.61 employed this approach to prepare the PCN-332(Fe) MOF. A certain amount of FeM (15 mg) was ultrasonically dissolved in a mixture of acetic acid (1 mL) and DMF (2 mL), the mixture was then heated in an oven (12 h, 140 °C). After cooling down to room temperature, dark brown crystals were obtained through filtration. This method yielded an extremely stable and highly crystalline Fe-based MOF. Table 1 summarizes the recent advances in the field of Fe-based MOFs prepared by solvothermal and other non-conventional methods.

Fig. 6

The Fe-based MOF synthesis through the solvothermal method (a), the Fe-based MOF synthesis through the sonochemical method (b). Reproduced from Ref. 47 with permission from Elsevier.

Table 1

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Recent developments of Fe-based MOFs synthesized by different methods under various conditions

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This ultrasound-assisted synthesis method is based on sonochemistry and is used when chemical reactions need to be carried out quickly under ambient conditions. The sonochemical treatment is conducted by subjecting a substrate solution to very high ultrasonic frequencies (20 kHz-10 MHz) to create localized hot spots with extremely high temperature (5000K) and pressure (1000 bar), which facilitates the chemical process.83,84 Compared to the conventional solvothermal approach, this method can provide highly crystalline Fe-MOF materials with a smaller particle size in a short period of time.85 Fig. 6b illustrates the procedure for the Fe-based MOF synthesis by sonochemical treatment.

An Fe-MOF known as MIL-88A (Fe) was synthesized through a sonochemical approach by Chalati et al.66 with various reaction parameters being analyzed. A comparative study of the MIL-88A MOF synthesis via solvothermal, hydrothermal and microwave techniques was also conducted. However, the best results were achieved using the ultrasonic approach, which provided the rapid synthesis with a smaller particle size but in lower yields.

It is also worth mentioning that there are some factors that can greatly affect the properties of the resulting MOF material, such as the applied power, the power supply mode and the ultrasonic generator type. The sonication time has also a significant impact on the crystal growth and nucleation rate, since highly crystalline MOF nanoparticles are formed at short sonication times, while amorphous MOF structures are obtained at longer times.86,67

The microwave-assisted synthesis is considered as an important method that can be employed for the rapid synthesis of Fe-based MOFs.87 In the microwave-assisted mode, the solution is heated at a temperature ranging from 303 to 373K under high-frequency microwave irradiation.88 A uniform rise in the reaction temperature results in MOF crystallization within a few minutes with controlled crystal shape and size. Compared to the solvothermal synthesis, this method provides much faster preparation of high-quality MOFs. For example, the MIL-53(Fe) MOFs can be synthesized by microwave heating in as little as 5 to 10 min, while the solvothermal method takes 15 h.89 The microwave-assisted process is illustrated in Fig. 7a. A very significant study on the synthesis and functionalization of the MIL-101(Fe) MOF using the microwave procedure was reported by Taylor-Pashow et al.68 The obtained product displayed an unusual octahedron morphology and crystalline nanostructure with a high specific surface area reaching up to 4535 m2 g^–1. Using a microwave-assisted hydrothermal technique, Guo et al.69 investigated the effect of the reaction time on the morphology and properties of the resultant MIL-53(Fe) MOF samples. It was found that the porosity and morphology of the products vary with the microwave irradiation time (0.5, 1, 3, 6 and 12 h). As shown in Fig. 7b-f, various morphologies of spindle, uniform spindle, solid octahedron, yolk-shell octahedron and nanorods were obtained by changing the irradiation time.

Fig. 7

Scheme of the synthesis of Fe-based MOFs through a microwave method (a); TEM images of MIL-53(Fe)-0.5h (b), MIL-53(Fe) 1h (c), MIL-53(Fe)-3h (d), MIL-53(Fe)-6h (e) and MIL-53(Fe)-12h (f). Reproduced from Ref. 69 with permission from the American Chemical Society.

The electrochemical synthesis method is based on the conversion of electrical energy to chemical energy through an electrochemical set-up shown in Fig. 8a under mild operating conditions of temperature, pressure and pH. Controlling the applied voltage and current also helps to carry out a controlled reaction in a short period of time making this method one of the industrial scale-up methods for the production of Fe-based MOFs. Generally, the electrochemical method involves two main variants, namely, the anodic dissolution method and the cathodic deposition. In the anodic dissolution, the desired metal is employed as an anode while the organic linker is dissolved in the solvent (reaction medium).75 In contrast, in the cathodic approach both the organic linker and the metal ion are dissolved in the reaction medium, which then interacts with a cathode.90

Fig. 8

Scheme of the synthesis of Fe-based MOFs through an electrochemical method (a); SEM images of MIL-100(Fe) MOF synthesized at 130 °C (b), 150 °C (c) and 190 °C (d). Reproduced from Ref. 73 with permission from the Royal Society of Chemistry.

Campagnol et al.73 used an electrochemical approach to prepare the MIL-100(Fe) MOF. The study of the influence of the temperature change (110-190 °C) on the crystals shape and size revealed that lower temperatures (110-130 °C) led to the formation of polydisperse crystals ranging in size from 50 nm to 1 μm, while only small monodisperse crystals were obtained at higher temperatures (170-190 °C) as shown in Fig. 8b-d. Also, some other parameters such as the applied current, the deposition time and the presence of tributylmethylammonium methyl sulfate (MTBS) as a conductive compound were investigated. In 2020, Pourfarzad et al.74 fabricated a novel bimetallic benzenetricarboxylic (BTC) metal-organic framework named as Ni/Fe-BTC MOF via an electrochemical approach. The newly synthesized composite was used as a bifunctional oxygen electrocatalyst, which exhibited exceptional electrical conductivity and chemical adaptability.

In 2006, Pichon et al.91 pioneered in the mechanochemical synthesis of MOFs. This technique is a solvent-free approach providing high yields and waste-free production, which makes it a potential competitor for industrial scale-up productions compared with other methods. In the mechanochemical synthesis route, the reaction is carried out by applying the mechanical energy, which induces the breakage of the intramolecular bonds of the reactants resulting in a chemical transformation within a short period of time (from 10 min to 1 h).92 Generally, the mechanochemical approaches can be divided into two groups: (a) neat grinding (NG), in which the process is conducted in the complete absence of solvent and (b) liquid-assisted grinding (LAG), which involves the addition of minor amounts of solvent into the solid reaction mixture.93 However, it was found that in the LAG process, 1D, 2D and 3D-­coordination polymers can be obtained from the same reaction mixture by varying the amount of the solvent.94--96 Fig. 9a shows the procedure for Fe-based MOF synthesis through a mechanochemical method.

Fig. 9

Theprocedure of Fe-based MOF synthesis through the mechanochemical method (a), dry sol-gel method (b). Reproduced from Ref. 47 with permission from Elsevier (Fig. 9a).

The synthesis of the MIL-100(Fe) MOF through a liquid-assisted grinding method was reported by Pilloni et al.76 In this study, an aqueous alkaline solution of tetramethylammonium hydroxide (TMAOH) was used to increase the yield and avoid the presence of unreacted H₃BTC acid in the final product. The mechanochemically synthesized composite displayed a high surface area, enhanced crystallinity and thermal stability.

Recently, a comparative study has been carried out on the synthesis of the MIL-88A(Fe) MOF using various types of solvents (triethylene glycol, H₂O, solution-phase and solvent-free) in the mechanochemical process.77 It was found that the presence of a medium during the synthesis of Fe-MIL-88A(Fe) has a great impact on the morphology formation and the crystal growth. However, the MIL-­88A(Fe) MOF prepared via a solvent-free mechanochemical procedure displayed a rod-like microparticle structure with the highest surface area (108.42 m² g^–1) compared to that of the other MIL-88A(Fe) MOF samples prepared under other mechanochemical conditions.

Also, He et al.78 reported the synthesis of a Fe-MOF-derived nanocomposite named as (Fe–Pd@C) using a fast and simple mechanochemical approach. The pyrolysis of the Fe-based MOF precursor afforded an efficient carbonized catalyst with an exceptional catalytic performance, good stability and extended catalyst life time.

The idea of dry-gel conversion (DGC), also known as steam-assisted conversion (SAG) was first discovered by the zeolite research group of Xu et al.97 In this technique, the Fe-MOF is formed by the contact of the MOF precursor with water vapours (Fig. 9b).98 Compared to other conventional MOF synthesis methods, the DGC has some advantages such as minimum waste disposal, higher concentrations of reactants in the reaction medium, the controlled shape and structure of the resulting MOFs and the possibility of continuous production.99 It is also worth mentioning that the used solvent in DGC can also be reused in multiple synthesis cycles without affecting the yield and quality of the formed MOF crystals.79

Ahmed et al.80 used the dry-gel conversion to prepare the MIL-100(Fe) MOF. In this study, a mixed gel of Fe/­H₃BTC was placed on a porous holder placed inside an autoclave containing water. The autoclave is then heated in an oven at 165 °C so that the dry gel is treated with steam to give a crystalline MOF material with a high surface area and pore volume. It was also observed that the crystallinity of the product increases with increasing the heating time up to a certain value.

Tannert et al.81 also synthesized the MIL-100(Fe) MOF through a novel and facile microwave-assisted DGC technique. Compared to the above-mentioned study,80 the MOF production was achieved in a shorter reaction time at a lower temperature. Moreover, the obtained MIL-100(Fe) MOF exhibited a Brunauer–Emmett–Teller (BET) surface area reaching up to 1287 m² g^–1.

In 2020, Luo et al.82 investigated the effect of absence of a solvent (water) on the crystallinity, morphology, pore structure, adsorption and desorption capacity of the dry-gel synthesized MIL-100(Fe) MOF. The prepared Fe-BTC dry gel was placed in a sealed glass jar and heated at 165 °C for 24 h. It was found that the BET specific surface area gradually decreases with the decrease in the reactive sol concentration. However, the maximum specific surface area of 1736 m² g^–1 was obtained with zero a sol concentration.

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Among all MOF types, Fe-based MOFs are considered to be extremely attractive materials, since, firstly, iron is an earth-abundant element, secondly, the presence of large iron oxo clusters make almost all Fe-based MOFs visible-light-responsive materials compared to other MOFs such as Ti- and Zr-containing MOFs, this property being attributed to the direct excitation caused by the Fe–O metal clusters.65 All these advantages make the Fe-containing MOFs, in particular, promising materials in the field of photocatalysis. In Fe-based MOFs, the photocatalytic oxygen evolution reactions (OER) can be represented by the following equations:

Shah et al.100 reported a successful encapsulation of two different cobalt-functionalized polyoxometalate (POM) anions in the MIL-100(Fe) MOF as shown in Fig. 10a. The POMs anions were referred to as Co2 for ­[Co^IICo^IIIW₁₁O₃₉(H₂O)]^7– and Co4 for ­[Co₄(PW₉O₃₄)₂(H₂O)₂]^10-. Interestingly, these two ­POM-­MOF composites exhibited greater enhanced photocatalytic water oxidation ability compared to their individual components. This was explained by the integration of the MIL-100(Fe) MOF which addressed the issue of POM solubility. The synergic effect of electrostatic interactions between the guest units (POM) and the host material (Fe-­MOF) also played an important role in improving the overall photocatalytic performance. The proposed mechanism for photocatalytic oxygen evolution is illustrated in Fig. 10b. When light is applied to the photocatalyst surface, it undergoes an excitation and generates a population of photogenerated charges; these electrons (e^–) are transferred from the LUMO of the MIL-100(Fe) MOF to its HOMO, leaving holes (h⁺) in LUMO, which then oxidize water to produce oxygen. The POM units provide an additional pathway for the transfer of photogenerated charges by accepting electrons from the MOF HOMO into the conduction band (CB) of POM. The POM units are most likely to exist in close proximity to the MIL-100(Fe)_μ3-O bridged Fe₃ units due to electrostatic interactions between the anionic POM and the Lewis-acidic Fe₃ units, thereby promoting more efficient electron transfer from LUMO of the MIL-100(Fe) MOF to its HOMO and hence improving the photocatalytic water oxidation process.

Fig. 10

The synthesis of two POMs encapsulated in the MIL-100(Fe) MOF (a), the proposed reaction mechanism for photocatalytic oxygen production under visible light irradiation (b). Reproduced from Ref. 100 with permission from Elsevier. Types of linkers employed to prepare the MIL-88B(Fe) MOFs (c), amount of O₂ produced by thedifferent iron-based MOFs after 2 h of visible light irradiation (d). Reproduced from Ref. 103 with permission from the American Chemical Society.

Qu et al.101 reported the preparation of a Fe-based MOF known as MIL-53(Fe). This photocatalyst exhibited good photocatalytic evolution for oxygen production under visible light irradiation. However, higher O2 evolution of 120 μmol was also obtained by functionalizing the MOF with an amino group (NH₂). The NH₂-MIL-53(Fe) composite displayed an O₂ evolution of 120 μmol after 140 min; this improvement was attributed to the final narrower bandgap and to the more efficient separation of photogenerated electron-hole pairs.

A new MOF-derived nanotube composite (Fe–Ni‒P) was synthesized by Li et al.102 using bimetallic Fe-Ni-MIL-88(Fe) nanorods as the template. The resulting nanotubes were integrated with two different dyes based on tris(bipyridine) ruthenium(II) [Ru(bpy)₃]²⁺ and S₂O₈^2- as a both visible light absorber and an electron sacrificial agent, respectively.

The maximum O₂ amount (900.3 μmol g^–1 h^–1) was achieved; however, the photocatalytic activity for the O₂ production depended on the Fe/Ni ratio in the Fe–Ni–P nanotubes. The same composite, was further tested for the H₂ production under similar conditions but using different dyes and sacrificial agents and was demonstrated to exhibit the production of 5420 μmol g^–1 h^-1.

In 2019, Lionet et al.103 synthesized nine different functionalized iron-based MOFs using the MIL-88B(Fe) MOF structure with multiple linkers (–4H, –Br,–NO₂, –NH₂, –OH, –4F, –4Me, –2Me, –2OH) as shown in Fig. 10c. Clearly, the tetrafluoroterephthalic-based MOF exhibited the highest photocatalytic O₂ production (30 μmol) which is 5 times higher than that of the pristine MIL-88B(Fe) MOF as depicted in Fig. 10d. This enhancement in the photocatalytic activity was found to be attributed to the hydroxylation rate of the organic linkers that can be altered by introducing activating or deactivating groups into the benzene ring.

A series of Fe-based MOFs with three different topological structures bearing the same functional ligands, specifically, MIL-53(Fe), MIL-88B(Fe) and MIL-101(Fe), were investigated for visible-light-driven photocatalytic oxygen evolution with the use of [Ru(bpy)₃]²⁺ as a photosensitizer and Na₂S₂O₈ as an electron acceptor.104 Also, the performance of these three pristine photocatalysts was compared to that of their amino-functionalized derivatives. However, the highest O₂ evolution of 36.5 μmol was achieved using the MIL-101(Fe) MOF. The effect of water pH on the photocatalytic oxygen evolution was also studied and found to increase with the pH increase (up to pH 10), since higher pH values are thermodynamically favourable for water oxidation reactions. However, at pH 11, a significant decrease in O₂ evolution was observed, which may be due to the degradation of the [Ru(bpy)₃]²⁺ photosensitizer at higher pH values.

Various Fe-based MOF photocatalysts such as MIL-­101(Fe), MIL-53(Fe), MIL-88(Fe), MIL-100(Fe) and MIL-126(Fe) were also tested for photocatalytic water oxidation by Horiuchi et al.105 Among all tested samples, the MIL-­101(Fe) MOF displayed the highest O₂ evolution (14.7 μmol, 9 h visible light irradiation) with the use of AgNO₃ as a sacrificial agent. The photocatalytic reactivity can be attributed to the limited recombination rate of charge carriers resulting from the formation of finely dispersed Fe-oxo clusters embedded as nodes of the porous framework and also due to the largest pore diameter exhibited by the MIL-101(Fe) MOF. The photocatalytic O₂ production performances of the typical Fe-based MOFs and composite photocatalysts based thereon are summarized in Table 2.

Table 2

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Recent developments of Fe-MOFs and Fe-MOF composites for photocatalytic O₂ production

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The environmental pollution, fossil fuels depletion and growing energy demand have turned hydrogen generated by photocatalytic water splitting into one of the cheapest and cleanest alternatives to energy productions. Huge efforts have been directed towards the synthesis of heterogeneous photocatalysts for photocatalytic H₂ evolution with high efficiency and stability. However, it was established that the photocatalytic H₂ production depends entirely on visible light harvesting, energy bandgap positions, charge recombination rate and the availability of active sites.106 In 2009, MOFs were first applied in the photocatalytic production of hydrogen from water under visible light irradiation. 107 Subsequently, Garcтa and co-authors108 reported two Zr-based MOFs (UiO-66 and NH₂-­UiO-66) for the photocatalytic production of hydrogen in methanol or water/methanol mixture under visible light irradiation. Since then, the use of MOFs as photocatalysts, including Fe-based MOFs, for the photolysis of water to produce hydrogen has become a research hotspot.

Iron-based MOFs consist mainly of iron-oxo clusters, which act as efficient active moieties for photocatalytic water splitting due to the uniform and highly dispersed small clusters. These clusters shorten the migration pathways between the photogenerated charge carriers and reactant molecules thereby improving the photocatalytic performance. Moreover, the porous structure of Fe-based MOFs greatly contributes to the improved diffusion of reactants.105

However, Fe-based MOF photocatalysts have insufficiently negative conduction band positions compared to the redox potential of water reduction,109 which is considered to be one of the main obstacles to their application. Nevertheless, to solve this issue, a dye-sensitization strategy can be implemented, leading to a more efficient H₂ production, in which energetic electrons can be effectively injected into Fe-MOFs via dye sensitization thereby lifting up the MOF quasi-Fermi level to overcome the kinetic barrier. This was recently proved by Li et al.62 through sensitizing three different Fe-based MOFs (MIL-101(Fe), MIL-88B(Fe) and MIL-53(Fe)) with Eosin Y dye for the efficient photocatalytic H₂ production.

Dai et al.63 synthesized a novel hybrid Fe-based MOF nanocomposite named ZnIn₂S⁴@NH₂-MIL-53(Fe/Co_0.75) with high photocatalytic performance of water splitting for hydrogen production under visible light irradiation (26954.13 μmol g^–1 h^–1). Different amounts of 1.875, 3.75, 7.5 and 15 mg of the Co-doped amino-functionalized MIL-53(Fe) MOF were coupled with ZnIn₂S₄ (referred to as 0.25, 0.5, 1 and 2 respectively, in Fig. 11a) to improve the visible light response. This efficiently promoted the transfer and separation of photogenerated charges and thus increased the hydrogen production rate. As shown in Fig. 11b, a typical n-type heterojunction photocatalyst was formed with a more negative valence band (VB) and CB of ZnIn₂S₄, upon light irradiation, the electrons are excited in both NH₂-MIL-53(Fe/Co_0.75) and ZnIn₂S₄. The photogenerated electrons are then transferred from the ZnIn₂S₄ CB to the NH₂-MIL-53(Fe/Co_0.75) CB and reduce H⁺ to produce H₂. However, the photogenerated holes on the MOF VB migrate to the VB of ZnIn₂S₄.

Fig. 11

UV-Vis diffuse reflectance spectroscopy (DRS) spectra of NH₂-MIL-53(Fe/Co_0.75), ZnIn₂S₄ and coupled ZnIn₂S₄@NH₂-MIL-53(Fe/Co_0.75) nanocomposites (a), the mechanism of photocatalytic H₂ production over the ZnIn₂S₄@NH₂-MIL-53(Fe/Co_0.75) (ZIS@NM(Fe/Co_0.75)) nanocomposite (b). Reproduced from Ref. 63 with permission from Elsevier. The deposition of Pt nanoparticles onto the MIL-100(Fe) MOF illustrated by a TEM image(c), the amounts of H₂ evolved over (1) pure MIL-100(Fe); MIL-100(Fe) with different Pt loadings of (2) 0.5 wt.%, (3) 0.8 wt.%, (4) 1 wt.%, (5) 3 wt.%, (6) 5 wt.%, (7) pure Pt nanoparticles and (8) 1 wt.% Pt/MIL-100(Fe)-NaBH₄ (d). Reproduced from Ref. 110 with permission from the Royal Society of Chemistry.

A photocatalytic hydrogen production of 5.9 mmol g^–1 h^–1 was reported for the MIL-100(Fe) Fe-­based MOF under visible light irradiation in a methanol-water mixture.110 However, an enhanced photocatalytic hydrogen evolution (109 mmol g^–1 h^–1) was observed by depositing Pt nanoparticles onto the pure MIL-100(Fe), which acted as a co-catalyst (Fig. 11c). As shown in Fig. 11d, the optimum H₂ evolution was achieved with Pt nanoparticle loading of 0.8 wt.%.

A series of noble metal-free NiSe₂ nanosheet-modified MIL-53(Fe) microrods with different NiSe₂ amounts were synthesized via a two-step solvothermal procedure and tested for the photocatalytic H₂ evolution.111 The highest H₂ production of 10.31 mmol h^–1 g^–1 was achieved with 1.0 wt.% NiSe₂ loading under visible light irradiation, which is more than 11 times higher compared to the pristine MIL-53(Fe) MOF. The NiSe₂ nanosheets acted as a co-catalyst and greatly boosted the photocatalytic activity of the MIL-53(Fe) MOF by enhancing the transfer and separation efficiency of photogenerated charge carriers in the NiSe₂/MIL-53(Fe) composite as it was revealed by the photoluminescence (PL) spectra and photocurrent analysis (Fig. 12a and b respectively).

Fig. 12

PL spectra (a), transient photocurrents (b) of pure MIL-53(Fe) and NiSe₂/MIL-53(Fe) samples. Reproduced from Ref. 111 with permission from Elsevier. SEM images of CdLa₂S₄ (c), MIL-88A(Fe) (d) and CdLa₂S₄/MIL-88A(Fe) (e). Reproduced from Ref. 112 wit hpermission from Elsevier.

A novel Z-scheme photocatalyst was fabricated hydrothermally with anchored CdLa₂S₄ nanoparticles on the microrod surface of the MIL-88A(Fe) MOF as illustrated in Fig. 12c-e.112 The CdLa₂S₄/MIL-88A(Fe) nanocomposite displayed remarkable photocatalytic H₂ evolution of 7677.5 μmol h^–1 g^–1 under visible light irradiation, which is about 8 times higher compared to pristine CdLa₂S₄. The enhanced photocatalytic activity was attributed to the formed Z-scheme heterojunction, which resulted in the efficient separation and transfer of photogenerated electron-hole pairs. In a Z-scheme photocatalyst system, electrons are transferred in a way similar to the Z-scheme, thereby faciliting the migration of electrons. This system consists of one oxidation photocatalyst (PC I) with a low CB position and another reduction photocatalyst (PC II) with a high VB position. During the light-induced excitation process, the electrons in the PC I are generated and transferred from the VB of PC I to its CB and recombined at the heterostructure interface with the holes in the VB of PC II and are again excited and transferred to the PC II CB to reduce the reactant molecules.

Another ternary-shelled, MIL-88A(Fe)-derived nanotube photocatalyst for photocatalytic water splitting was prepared by Zhao et al.113 Nanosheets of ZnIn₂S₄ were grown in situ on the surface of MOF-derived Ni-Fe LDH. The resulting ZIS@Ni-Fe LDH nanocomposite exhibited a photocatalytic H₂ production of 2035.81 μmol g^–1 h^–1 under visible light irradiation. The hierarchical heterostructure provided a larger surface area, more inner space, more active sites, increased light harvesting and better interfacial electron transfer. Table 3 summarizes the recent developments of Fe-based MOFs and their composites for the photocatalytic hydrogen production.

Table 3

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Recent developments of Fe-MOFs and Fe-MOF composites for photocatalytic H₂ production

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The conversion of CO₂ into valuable fuels and chemicals through the utilization of solar energy is considered one of the best strategies to solve the problems of global warming and fossil fuel depletion. Many attempts were performed to achieve this goal by employing various photocatalyst materials; however, a high BET surface area, available active sites and an appropriate porous structure are required for efficient sunlight-driven photocatalytic performance for CO₂ reduction. Among all other semiconductors, metal-organic frameworks (in particular, Fe-based MOFs) can provide all these properties in addition to high stability, high capacity for both CO₂ adsorption and visible light harvesting.114

Thermodynamically, the CO₂ gas is an extremely stable material and most of the tested pure MOFs, including Fe-containing ones, still show low activity for CO₂ reduction. Moreover, the obtained products were almost limited to formate (HCOO^–). Therefore, many strategies such as amine functionalization, dye sensitization and metal/non-metal doping can be applied to resolve these issues.115 From this point of view, the photocatalytic CO₂ reduction performance of a series of earth-abundant Fe-containing MOFs(MIL-53(Fe), MIL-101(Fe) and MIL-88B(Fe)) was investigated by Wang et al.65 Due to the direct excitation of the Fe-O clusters, which promoted the electron transfer, all tested Fe-based MOFs displayed a moderate photocatalytic activity for the reduction of CO₂ into formate under visible light irradiation. The effect of amino-functionalization on these Fe-containing MOFs was further studied. Compared to non-functionalized MOFs, all amino-functionalized derivatives exhibited increased photocatalytic performance for CO₂ reduction as illustrated in Table 4. This can be attributed to the dual excitation pathways created on the NH₂ functionality and Fe–O clusters.

Table 4

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Recent developments of Fe-MOFs and Fe-MOF composites for photocatalytic CO₂ reduction

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Table 5

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Table4 (continued)

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A significant photocatalytic CO production of 1128 μmol g^–1 h^–1 was also achieved under visible light irradiation by using a [Ru(bpy)]₃²⁺ dye-sensitized Fe-MNS (MOF nanosheets) system.116 The CO production was further improved to 1367 μmol g^–1 h^–1 with Co-doping by constructing an efficient dye/Co-Fe-MNS photocatalytic system. The [Ru(bpy)] ₃²⁺ dye sensitizer resulted in a negative shift of the Fe-MOF LUMO potential to fit for the theoretical thermodynamic reduction potential of CO₂/CO, while the Co doping regulated the MOF electronic ­structure.

In a recent study by Cheng et al.,117 the Fe-based MIL-­100(Fe) MOF was used to improve the CO₂ photoreduction activity of a cesium lead bromide (CsPbBr₃) perovskite composite by enhancing the visible light harvesting and increasing the specific surface area, which is beneficial for the adsorption of CO₂ molecules. The CsPbBr₃/MIL-100(Fe) nanocomposites showed significantly higher activity with a CO production of 20.4 μmol g^–1 h^-1, which is about five times higher compared to the pure CsPbBr₃ and MIL-100(Fe) materials. As shown in Fig. 13a, a type-II heterojunction is formed, in which photoinduced electrons in the CB of CsPbBr₃ migrate to the MIL-100(Fe) conduction band to reduce CO₂ gas to CO via a series of redox reactions, while the holes generated in the VB of MIL-100(Fe) are transferred to the VB of CsPbBr₃ to oxidize water.

Fig. 13

Proposed reaction mechanism for the CO₂ reduction over the CsPbBr₃/MIL-100(Fe) nanocomposite (a).¹¹⁷ The encapsulation procedure of MAPbI₃ QDs in the PCN-221(Fe_x) MOF (b), amounts of CO and CH₄ produced over PCN-221(Fe_x) and MAPbI3@PCN-221(Fe_x) in 80 h (c). MAI = MeNH₃I. Reproduced from Ref. 64 with permission from Wiley.

An iron MOF-derived nanocomposite was fabricated by heating the MIL-101(Fe) MOF in a two-step calcination process.118 The resulting Fe@C hybrid displayed a photocatalytic CO₂ conversion into CO and CH₄ of about 750 and 50 μmol, respectively, after 2 h of light irradiation. However, extremely good results were reported by using a photothermal approach, in which both photocatalytic and thermocatalytic CO₂ reductions are combined in one system. This extra enhancement was attributed to the thermal effect caused by the intense absorption of visible light and infrared radiation.

Iron-containing MOFs can also be used to increase the stability of other photocatalyst materials in aqueous reaction systems as investigated recently by Wu et al.,64 who encapsulated a low-cost perovskite of methyl ammonium lead iodide (MeNH₃PbI₃), abbreviated as MAPbI₃, into Fe-porphyrin-based MOF (PCN-221(Fe)) and tested the resulting composite for photocatalytic CO₂ reduction. The encapsulation process was carried out stepwise as depicted in Fig. 13b. Firstly, the PCN-221(Fex) MOF was synthesized with various Fe contents. The prepared samples were immersed in PbI₂ and MAI ethanol solutions, respectively, to encapsulate the MAPbI₃ quantum dots (QDs) in the MOF pores. The composite photocatalyst exhibited exceptional photocatalytic activity with CO and CH₄ productions of 530 and 10293 μmol g^–1, respectively. This enhancement was attributed to both improved stability and Fe catalytic sites, which increased the transfer of photogenerated electrons (Fig. 13c).

Metal-organic frameworks, particularly Fe-based MOFs, and their composites have been widely used for photocatalytic degradation of organic pollutants and related environmental applications. Iron-based MOFs are extremely attractive since they show strong absorption of visible light due to the existence of numerous iron-oxo (Fe–O) clusters that make them good Fenton-like catalysts.120 They also have strong coordination bonds, impacting high chemical and water resistance to such MOFs.121

When light strikes the surface of these Fe-based MOFs, they generate electrons in the valence band, which then move to the conduction band, leaving holes in the VB. Due to the strong reducing power of these photogenerated electrons, they reduce Fe(III) to Fe(II), which then react to produce a large amount of free radicals (e.g., ^•OH) that can decompose organic pollutants.47,115,122

Despite these advances in the field of photocatalysis, pristine Fe-based MOFs still have a problem with rapid recombination of photogenerated electron-hole pairs leading to low photocatalytic activity. To overcome this issue, many strategies can be applied including bandgap engineering through electron-donating or withdrawing species (e.g., –NH₂, –OH or –COOH),123,124 modifying these pristine Fe-MOFs by introducing metal/non-metals or semiconductor nanoparticles to form efficient heterojunctions,125,126 introducing sacrificial agents and electron acceptors using H₂O₂.127

In recent years, Guo et al.128 prepared a Fe-based MOF-derived composite and tested it for photocatalytic degradation of tetracycline, which is considered one of the most commonly used antibiotics, under visible light irradiation. A Fe-MOF-derived composite was synthesized through a co-calcination of melamine and MIL-53(Fe) MOF to obtain a Z-scheme heterojunction photocatalyst named α-­Fe₂O₃@g-C₃N₄. The resulting α-­Fe₂O₃ nanoparticles had a smaller particle size than spindle-shaped MIL-53(Fe) as seen from the SEM images in Fig. 14a and b. This might be attributed to the partial collapse in the MIL-53(Fe) inner structure caused by the calcination process. As depicted in Fig. 14c, the new photocatalyst composite promoted the degradation of ≈92% of tetracycline with the highest degradation rate of 0.042 min^–1, which is 6, 7 and 14 times higher than those using pure MIL-53(Fe), α-­Fe₂O₃ and g-C₃N₄, respectively.

Fig. 14

SEM images of MIL-53(Fe) (a) and a Fe-MOF derived-Fe₂O₃ nanocomposite (b), the degradation rate over the prepared samples under different conditions (c). Reproduced from Ref. 128 with permission from Elsevier. The UV-Vis DRS spectra of MIL-53(Fe), MIL-100(Fe) and MIL-101(Fe) (d). Reproduced from Ref. 120 with permission from Elsevier.

Three Fe-based MOFs (MIL-100(Fe), MIL-101(Fe) and Fe-MIL-53(Fe)) were also tested for photocatalytic degradation of tetracycline under visible light irradiation.120 The highest tetracycline removal performance (96.6%) was observed using the MIL-101(Fe) MOF compared to 57.4% and 40.6% for MIL-100(Fe) and MIL-53(Fe), respectively. This large difference in the photodegradation performance may be due to higher visible-light harvesting attained by the Fe-MIL-101(Fe) MOF as shown in Fig. 14d. Moreover, the Fe-MIL-101(Fe) MOF had the maximum adsorption rate since it exhibited the highest pore volume and the largest pore size (0.86 cm³ g^–1 and 25.74 nm, respectively) compared to MIL-100(Fe) (0.34 cm³ g^–1 and 2.27 nm) and MIL-53(Fe) (0.04 cm³ g^–1 and 4.6 nm).

In 2021, Zhang and Guo129 reported the synthesis of a novel Fe-based MOF (Fe-BDC, BDC is benzenedicarb­oxylate) and a CdSe QDs nanocomposite ­(CdSe@Fe-­BDC) for photocatalytic degradation of rhodamine B (RhB) dye under visible light irradiation. Compared to the pristine Fe-BDC MOF, the ­CdSe@Fe-­BDC photocatalyst exhibited a degradation rate of 99.8% which is 5.8 times higher due to the interfacial contact and the formed heterostructure, thereby promoting the electron transfer from CdSe to MOF and significantly facilitating the separation of photogenerated electron-hole pairs.

In addition to the photodegradation of organic pollutants in aqueous phases, Fe-based MOFs were also applied for photocatalytic reduction of heavy metal ions such as Cr(VI), which is considered to be one of the toxic wastewater pollutants. One of the most effective strategies to remove this pollutant from wastewater is the reduction of Cr(VI) to Cr(III).130 Wu and co-authors131 investigated the use of the MIL-53(Fe) MOF for the photocatalytic degradation of Cr(VI) under visible light irradiation. The photocatalyst displayed an exceptional photocatalytic performance with a reduction rate of 100% within 40 min. Moreover, by using a Cr(VI)-dye solution, a photocatalytic Cr(VI) reduction of 60% and photocatalytic dye degradation of 80% were achieved, revealing that the MIL-53(Fe) MOF can act as a bifunctional oxidation/reduction photocatalyst for environmental applications. The recent advances of Fe-based MOFs and their composites for photocatalytic degradation of different pollutants are summarized in Table 6.

Table 6

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Recent developments of Fe-MOFs and Fe-MOF composites for photocatalytic degradation of pollutants

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Recently, the photocatalysis process has gained great attention as one of the best strategies to solve the problem of energy shortage and environmental crisis using four main photocatalytic applications such as H₂ production, O₂ production, CO₂ reduction and photodegradation of pollutants. Currently, metal-organic framework materials, in particular, Fe-based MOFs, are presented as successfully used photocatalysts in these fields. In this review, six main synthetic approaches to Fe-based MOFs are considered and explained in detail. Among these approaches, the solvothermal synthesis was primarily used; however, the microwave and sonochemical techniques are gaining more priority as being fast and energy-efficient. Some unconventional techniques such as electrochemical and mechanochemical methods can be used for industrial scale-up production. Minimum waste disposal, continuous production with controlled shape and structure of the resulting Fe-based MOFs can be achieved using a dry-gel method.

This review also summarizes the recent advances in the use of Fe-based MOFs for the aforementioned photocatalytic applications. Iron-based MOFs, Fe-MOF composites and nanostructured materials derived there from showed excellent performance. It can be concluded that the yields of photocatalytic production are greatly affected by many factors including operating parameters, morphological properties, Fe-based MOF functionalization, surface area and porosity. However, Fe-based MOF photocatalysts still have some limitations and issues, for which the following recommendations should be considered in the future:

- More research is needed to gain insight into the possibility of combining various conventional and non-conventional methods for the synthesis of Fe-based MOF to improve physical, chemical and electrical properties.

- Pure Fe-based MOF photocatalysts showed poor results compared to their composites; therefore, greater emphasis should be given to the preparation of Fe-based MOFs with other materials such as perovskite, LDH and metal oxide nanocomposites.

- Some of the reported Fe-based MOFs showed relatively poor stability in reactions involving water; hence, the development of more reliable MOF photocatalysts in water medium is highly essential.

- More attention needs to be paid to understanding the photocatalytic reaction mechanisms and the resulting heterojunctions such as Z- and S-schemes of Fe-based MOF composites.

- Research into photocatalytic applications involving biologically active and radioactive pollutants is very rare; hence, the use of Fe-based MOF photocatalysts for these purposes should be considered.

This study was funded by the Arabian Gulf Oil Company. I am also immensely grateful to my mother for the unwavering support in my difficult times.

BDC — benzenedicarboxylate (ligand),

BET — Brunauer--Emmett--Teller (theory),

bpy — 2,2’-bipyridine,

BTC — benzenetricarboxylic (MOF),

CB — conduction band,

CIP — ciprofloxacin (dye),

CNT — carbon nanotube,

DGC — dry-gel conversion,

DEF — diethylformamide,

DMA — dimethylacetamide,

DMF — dimethylformamide,

DRS — diffuse reflectance spectroscopy,

EY — Eosin Y (dye),

g-C₃N₄ — graphitic carbon nitride,

GHG — greenhouse gas,

GO — graphene oxide,

h⁺ — hole,

HOMO — highest occupied molecular orbital,

IBP — ibuprofen (dye),

LAG — liquid-assisted grinding,

LDH — layered double hydroxides,

LUMO — lowest unoccupied molecular orbital,

MAPbI₃ — MeNH₃PbI₃,

MIL — Materials Institute Lavoisier (MOF),

MNS — MOF nanosheet,

MOF — metal-organic framework,

MTBS — tributylmethylammonium methyl sulfate,

NC — nanocomposite,

NG — neat grinding,

NNU — Northeast Normal University,

NP — nanoparticle,

NU — Northwestern University (MOF),

OER — oxygen evolution reactions,

PC — photocatalyst,

PCN — porous coordination network (MOF),

PL — photoluminescence (spectrum),

POM — polyoxometalate (anions),

QD — quantum dots,

RhB — rhodamine B (dye),

SAG — steam-assisted conversion,

SEM — scanning electron microscopy,

TC — tetracycline (dye),

TEOA — triethanolamine,

TMAOH — tetramethylammonium hydroxide,

UiO — Universiteteti Oslo (MOF),

UV — ultraviolet,

VB — valence band,

ZIF — zeolitic imidazolate frameworks (MOFs).