The Advanced Synthesis of MOFs-Based Materials in Photocatalytic HER in Recent Three Years

Abstract

Since the advent of metal–organic frameworks (MOFs), researchers have paid extensive attention to MOFs due to their determined structural composition, controllable pore size, and diverse physical and chemical properties. Photocatalysis, as a significant application of MOFs catalysts, has developed rapidly in recent years and become a research hotspot continuously. Various methods and approaches to construct and modify MOFs and their derivatives can not only affect the structure and morphology, but also largely determine their properties. Herein, we summarize the advanced synthesis of MOFs-based materials in the field of the photocatalytic decomposition of water to produce hydrogen in the recent three years. The main contents include the overview of the novel synthesis strategies in four aspects: internal modification and structure optimization of MOFs materials, MOFs/semiconductor composites, MOFs/COFs-based hybrids, and MOFs-derived materials. In addition, the problems and challenges faced in this direction and the future development goals were also discussed. We hope this review will help deepen the reader’s understanding and promote continued high-quality development in this field.

1. Introduction

Since the 21st century, the world energy crisis has become increasingly urgent mainly due to the growing shortage of non-renewable energy reserves such as coal, oil, and natural gas [1,2,3]. It is crucial to find clean renewable energy, especially one that can be produced on an industrial scale [4,5]. Hydrogen (H₂) is regarded as one of the most ideal substitutes for carbon-based energy sources because of its high calorific value and lack of pollution from its combustion products [6,7]. At present, the electrocatalytic decomposition of water is mostly used in industrial hydrogen production, but this undoubtedly brings a large amount of waste of electrical energy [8,9]. As solar energy is inexhaustible, it would be a satisfactory advantage to fully utilize and convert solar energy to catalyze the hydrogen evolution reaction [10,11]. In the early days of this field, some complexes of precious metals were explored as photosensitizers. Although they were highly capable of capturing light, these complexes were too expensive to meet the economic principle [12,13,14], which promoted the researchers to develop some low-cost materials.

With the continuous in-depth research of the theory and practice of photoelectric chemistry, a multitude of semiconductor materials with superb light trapping abilities emerged, such as CdS [15,16,17,18,19], TiO₂ [20,21], C₃N₄ [22,23,24,25], etc. However, these classical semiconductor materials also had some inevitable shortcomings and deficiencies, despite that they were generally low toxicity, cheap, and easy to synthesize. In the case of CdS, serious photocorrosion would occur in the process of photocatalysis, which reduced the available components and made it difficult to maintain a high efficiency catalysis for a long-term reaction [26,27,28]. As far as TiO₂ was concerned, it mainly absorbed ultraviolet light and was not sensitive to visible light, which was obviously not the optimal way to utilize solar energy [29,30]. C₃N₄ did not have the above-mentioned disadvantages as a kind of novel organic semiconductor material, but its apparent quantum yields (AQYs) in photocatalysis were not satisfactory, usually no more than 10% [31,32]. Therefore, it is a prerequisite for researchers to make unremitting efforts to extend such semiconductor materials from design to practice.

In the past three decades, metal–organic frameworks (MOFs) have attracted the attention of many chemists due to their controllable pore size, redox ability of central ions, and changeful ligands [33,34,35,36,37,38,39,40,41,42,43,44]. Many literature reports in the fields of gas separation and adsorption [45,46,47,48], fluorescence recognition [49,50,51,52], electrocatalysis [53,54,55], and photocatalysis [56,57,58] have confirmed this hotspot. In the aspect of photocatalytic (hydrogen evolution reaction) HER, MOFs are particularly outstanding, and the orientations are mainly focused on the following three aspects. Firstly, the ligands and central ions of MOFs usually play various roles in the photocatalytic process, which makes reasonable design and adjustment of internal structures become one of the most common modification methods for MOFs. Secondly, the integration of MOFs and other types of materials is one of the frequent techniques, which can not only make full use of the stable skeleton of MOFs, but also make the supported substances evenly dispersed to improve the utilization of the cocatalyst. Thirdly, MOFs can be appointed as precursors to synthesize and prepare some substances with special morphology or composition. These corresponding measures give MOFs and their derivatives or composites better development potential and vitality in the field of photocatalytic HER for practical application.

The unique property of one substance is inseparable from its peculiar structure. Therefore, continuous progress in the synthesis and modification strategies for MOFs and their derived materials is the cornerstone of sustainable and efficient development. Most of the published reviews may only focus on one aspect of MOFs material modification and lack a systematic summary. Based on the concept of “from inside to outside”, we reviewed the synthetic progress of MOFs materials in the field of photocatalytic HER in the recent three years, from the internal modification of MOFs materials to the participation of MOFs materials as independent components in composite materials, and then to the derivatives of MOFs materials. We hope that this timely review will help readers gain an in-depth understanding about the relevant advances in this field and promote its rapid development with high quality.

2. Internal Modification and Structure Optimization of MOFs

Reasonable internal modification and structure optimization of MOFs may not only make the photosensitive units orderly, but also shorten the distance between the photosensitive units and the catalytic centers, so as to improve the efficiency of photogenerated electrons separation and accelerate the rate of hydrogen evolution [59,60].

Jiang et al. proposed a facile strategy for the construction of a deployable coordination microenvironment based on MOFs [61]. In this work, the earth-abundant metals (such as Ni²⁺, Co²⁺, and Cu²⁺) were fixed to the Zr₆-oxo cluster in the form of single-atom catalysts (SACs) via a rapid and facile microwave-assisted method (Figure 1). The adjacent -O/OH_x groups in the Zr₆-oxo cluster of MOFs provided the lone pair electrons and the charge balance to anchor the additional single metal atoms. The atomically dispersed metal site was in close proximity to the photosensitive unit (the linker), which greatly accelerated the transfer of photogenerated electrons and thus facilitated the redox reactions. Therefore, the optimized Ni₁-S/MOF had a unique Ni(I) microenvironment and exhibited excellent photocatalytic H₂ evolution performance, which was 270 times higher than that of the pure MOF and far exceeded the other Ni₁-X/MOF counterparts. This work unequivocally demonstrated the great advantage of MOFs in preparing high-content SACs with the proximity of variable microenvironments to photosensitive junctions, thereby promoting electrons transfer and facilitating photocatalysis.

In another example of structure optimization of MOFs, Jiang et al. discussed the role of linker engineering in MOFs for dark photocatalysis [62]. Figure 2 shows the dark photocatalysis based on MIL-125 and MIL-125 with different functional groups (MIL-125-X, X = NH₂, NO₂, Br). Notably, the introduction of different functional groups on the connector enabled the resulting MOFs to have markedly various activities. The dynamic and thermodynamic investigations manifested that the generation and the lifetime of Ti³⁺ intermediates were the most critical factors affecting their properties, due to the electron-donating/-withdrawing effect of the functional groups. The time-dependent density-functional theory (TD-DFT) calculation indicated that the introduction of an electron-donating group was beneficial, which helped to lengthen the distance between the photogenerated electrons and holes and improved the separation of these pairs. According to the investigation, this was the first study to systematically regulate the dark photocatalytic hydrogen production process of MOFs-based materials. The relevant works gave a new reference to the lifetime adjustment of electron relay for enhancing the dark photocatalysis.

The stripping of MOFs single crystals into 2D layered materials could increase the surface area and shorten the transfer path of electrons, which was regarded as an excellent medication method. Zhang and his co-authors [63] reported a water-stable nickel-based MOF single crystal (Ni-TBAPy-SC) and its exfoliated nanobelts (Ni-TBAPy-NB), which could bear a wide range of pH environments (Figure 3a). The optimized hydrogen production rate of Ni-TBAPy-NB could reach 98 μmol·h⁻¹ (5 mmol·h⁻¹·g⁻¹) with an apparent quantum efficiency (AQE) of 8.0% at 420 nm, which was 164 times higher than that of Ni-TBAPy-SC. Based on the DFT calculations, the transfer of photogenerated electrons from the H₄TBAPy to the [Ni₃O₁₆] cluster was thermodynamically permissible (Figure 3b). In addition, both the reduced [Ni₃O₁₆]⁻ and [Ni₃O₁₆] cluster had excellent properties of water absorption. In another report about 2D MOFs, Duan et al. [64] synthesized 2D indium-based porphyrin MOF cubic nanosheets (2D In-TCCP NS) via a surfactant-assisted method (Figure 4a). The corresponding characterization of 2D In-TCCP NS (powder XRD, SEM, TEM, and AFM) and 3D In-TCCP bulk (powder XRD and SEM) are shown in Figure 4b–f. The 2D In-TCCP NS showed great chemical stability in the range of 2-11 in acidic and basic solutions (Figure 4g). In the photocatalytic tests, the 2D In-TCCP NS exhibited a hydrogen evolution rate of 67.97 μmol·g⁻¹·h⁻¹, which was 11.5 times higher than that of 3D In-TCCP bulk (5.87 μmol·g⁻¹·h⁻¹). It was worth noting that there was not an obvious activity decrease after 40 h of photocatalysis, which reflected the practical commercial application.

3. MOFs/Semiconductor Composites

Some MOFs were not sensitive to light (especially in visible light), which led to their poor light absorption ability. The mutual combination with semiconductor materials could not only make full use of the stable skeleton of MOFs, but also improve the utilization rate of semiconductor materials [65,66,67,68]. Whereas, after they recombined or coexisted, some complicated issues such as the mechanism of interface electrons transfer between the photosensitizer and cocatalyst were not very unambiguous, and the uniform distribution of the cocatalyst on the surface or in the pores of MOFs needs to be solved urgently.

In 2020, Jiang et al. [69] incorporated the monodisperse, small-size and noble-metal-free transitional metal phosphides (TMPs; such as Ni₂P, Ni₁₂P₅) into a classical MOF (UiO-66-NH₂) for photocatalytic HER (Figure 5). The TEM images of Ni₂P@UiO-66-NH₂ (the inset is the size distribution of Ni₂P NPs) and Pt@UiO-66-NH₂ (the inset is the size distribution of Pt NPs) are shown in Figure 6a–d. Strikingly, the Ni₁₂P₅@UiO-66-NH₂ exhibited a hydrogen evolution rate of 293.2 μmol·g⁻¹·h⁻¹, which was about 80 times that of the parent MOF. Both thermodynamic and kinetic studies revealed that the TMPs had similar behaviors to Pt, which could greatly accelerate the photogenerated charge transfer from the linker to the cluster and promote its separation, thereby reducing the activation energy of H₂ generation. Alternatively, the relevant results manifested that Pt was thermodynamically favorable, yet Ni₂P was kinetically preferred for H₂ evolution. This work has laid a solid foundation for the development of novel non-precious metal composite photocatalysts.

The surfactant coating on the metal cocatalyst had a great regulating effect on the microenvironment of the catalytic site, which had been largely ignored. In 2021, Jiang and his co-authors [70] reported a series of Pt cocatalysts with adjustable microenvironments, including polyvinylpyrrolidone (PVP)-capped Pt nanoparticles (Pt_PVP), Pt with partially removed PVP (Pt_rPVP), and clean Pt without PVP (Pt) were encapsulated into UiO-66-NH₂ to obtain Pt_PVP@ UiO-66-NH₂, Pt_rPVP@UiO-66-NH₂, and Pt@UiO-66-NH₂ (Figure 7a,b). The experimental results indicated that the PVP intervention had a negative effect on the transfer of photogenerated electrons, and this negative effect was ameliorated after the removal of a portion of PVP, thus increasing the rate of hydrogen precipitation. Inspired by this rule, the Pt-Fc@UiO-66-NH₂ with the best activity was synthesized via introducing an electron mediator of Fc into Pt@UiO-66-NH₂. In this research, the effects of interfacial surfactants between the photosensitizer and cocatalyst on electron transfer kinetics and related activities were thoroughly investigated, which also provided vital implications for the microenvironment regulation of photocatalytic catalytic centers. In contrast, PVP played a positive role in an example reported by Li and his co-authors [71] for constructing 2D-on-3D MOFs materials. They achieved the “anti-epitaxial growth” pattern of foreign MOF nucleus on the (111) facets of the UiO-66-NH₂ octahedron seed with the assistance of PVP and successfully extended this method to Zn, Cd, Co, and Ni elements for building this type of 2D-on-3D heterojunction (Figure 8a). The influence of the presence or absence of PVP on the control of product morphology was also discussed in detail (Figure 8b). The photocatalytic performance of Cu 2D-on-3D exhibited over one order of magnitude times that of the “dimensionality-identical” MOF. Especially, the apparent quantum efficiency (AQE) of 2D-on-3D Cu heterostructures could reach 12.04% with an excitation wavelength of 380 nm. Thanks to the ultrathin cover layers, the faster electron transfer and more efficient electron–hole pairs separation promoted the photocatalysis. The emergence of these two diametrically opposed situations fully illustrated and verified the vital role and the influence of the surfactants, such as PVP, in the construction of the composite materials.

Encapsulation of semiconductor materials in the internal holes of MOFs had higher electron transport and separation efficiency than encapsulation on the surface, which will be more conducive to hydrogen production. In Maji’s article [72], a MOF-based composite where CdS NPs were confined inside the nanosized pores of Zr-based MOF-808 was synthesized and named CdS@MOF-808. The authors anchored L-cysteine in the nanospace of MOF-808 by post-synthetic ligand change, where it could capture Cd²⁺ ions from the aqueous solution, which further facilitated the in situ growth of CdS NPs in MOFs pores (Figure 9). The CdS₄@MOF-808 achieved an excellent visible-light-driven H₂ evolution rate of 10.41 mmol·h⁻¹·g⁻¹, which was about 60-fold higher than that of similar MOFs composites (CdS NPs simply supported on the surface of MOFs). Powerful studies had shown that the key factor leading to the large difference in the photocatalytic performance between these two types MOFs composites was the rapid and effective transfer from CdS to Zr⁴⁺ clusters in nanospace, which inhibited the electron–hole pairs recombination. Thus, this research revealed the critical role of the NPs photosensitizer stabilized near the catalytic site for the development of efficient H₂ evolution photocatalysts in MOFs-based systems.

In the field of the photocatalytic decomposition of water, the construction of semiconductor heterojunctions usually requires energy matching and band bending, so that the formed heterojunctions are thermodynamically permissible for hydrogen generation. However, the band bending had not been demonstrated in MOF photocatalysts. Jiang’s research group [73] expounded and demonstrated this effect in MOFs. In this work, a representative MOF (MIL-125-NH₂) was integrated with metal oxides (MoO₃ and V₂O₅) with appropriate work functions and energy levels to provide the corresponding MOF composite (Figure 10a,b). The consequence of the surface photovoltage displayed that the band bending of the MOF composite led to the generation of the built-in electric field of MIL-125-NH₂, which promoted the charge separation. In the end, the MOF composites exhibited 56 and 42 times that of the pristine MOF in photocatalytic HER. This literature had a novel perspective, elucidating the band bending of MOF for the first time and supporting its semiconductor-like properties, which would greatly promote the practical applications of MOF in photocatalysis.

Inspired by the electron transfer channel in natural photosynthesis, a novel strategy of multi-stepwise charge transfer was proposed by Jiang and his co-authors [74] for the ingenious design of photocatalysts. The NM@OM/TiO₂ dual heterojunction was constructed via depositing TiO₂ nanoparticles on the surface of a core-shell NM@OM hetero-skeleton and the preparation procedure of the NM@OM/TiO₂ is shown in Figure 11a. The obtained NM@OM/TiO₂ photocatalyst could reach the H₂ evolution rate of 7.108 mmol·g⁻¹·h⁻¹, which was much superior to that of NM/TiO₂, NM@OM, TiO₂, OM, and NM. In the NM@OM/TiO₂ dual heterojunction, the charge transfer path from NM (−1.27 eV), OM (−1.10 eV) to TiO₂ (−0.99 eV) and the hole transfer path from TiO₂ (2.35 eV), OM (2.23 eV) to NM (1.41 eV) were established (Figure 11b). This three-step electron transfer process, which imitated natural fabrication, had thermodynamic advantages over most heterojunctions composed of two materials. This work might give guidance for the rational design of high performance photocatalysts and stimulate interest in charge transfer enhancement.

4. MOFs/COFs-Based Hybrids

The coupling of MOFs and COFs could not only make full use of the 2D layered plane structure of COFs as a solid substrate, but also maximized the advantages of the high porosity and easy mass transfer of MOFs [75,76,77,78]. Achieving efficient and stable hydrogen evolution of MOFs/COFs-based hybrids without the assistance of precious metals as cocatalysts was still an arduous and meaning task.

Wang et al. [79] reported the synthesis of a new series of Cu₃(HHTP)₂-MOF/Tp-Pa-1-COF hybrids with differences in MOF content (Figure 12). The as-synthesized MOF/COF hybrids exhibited close interactions through the coordination of Cu ions with the carbonyl oxygen and enamine nitrogen groups in Tp-Pa-1. In the optimal conditions, the hydrogen generation rate of the photocatalysts with a Cu₂(HHTP)₂:Tp-Pa-1 ratio of 1:15 could achieve 1.76 mmol·h⁻¹·g⁻¹ under visible-light illumination, which was about 93-fold higher than that of the pure Tp-Pa-1. It was worth noting that such a superior hydrogen production rate was even higher than the performance of 3% Pt/Tp-Pa-1. This relevant research provided some inspiration for the design and construction of porous heterostructure photocatalyst materials for efficient visible-light-driven HER in the absence of precious metals.

As an attractive artificial photocatalyst, titanium metal–organic frameworks (Ti-MOFs) exhibited great potentials in the field of solar energy conversion due to their good photo-redox activities (similar to TiO₂), which could be appointed as the antenna to absorb visible light. Although many efforts had been made to develop Ti-MOFs with a high photocatalytic performance, their conversion rates of solar energy were still unsatisfactory. In Ma’s literature [80], they implemented a strategy of covalent integration to synthesize a series of multivariate Ti-MOF/COF hybrid materials PdTCCP@PCN-415(NH₂)/TpPa (composites 1, 2, and 3) with excellent visible-light utilization (Figure 13). The experimental results manifested that material 2 presented a H₂ evolution rate of 13.98 mmol·h⁻¹·g⁻¹ with a TON value of 24,508 after irradiation of 120 h (TOF = 204·h⁻¹). Importantly, this reaction rate was the highest H₂ production performance of MOFs-/COFs-based photocatalysts reported thus far. This paper contributed a novel covalent integration strategy for the construction of MTV-Ti-MOF/COF hybrid materials, which might promote the design and application of other types of MTV-MOF/COF in photocatalysis.

In Pan’s work [81], aldehyde-modified MOFs (NH₂-MIL-101(Fe)) were covalently bonded with covalent organic frameworks (Schiff-base network-1, SNW-1) to form the NH₂-MIL-101(Fe)@SNW-1 (MS-x) heterojunction via a post-synthetic covalent modification method (Figure 14a). Under the simulated sunlight, the MS-0.8 exhibited great photodegradation pollutant ability (93% in 90 min for TC and 97.2% in 90 min for RMB) and photocatalytic H₂ production performance (1949.56 μmol·g⁻¹·h⁻¹). The formation of -C=N- bonds between the NH₂-MIL-101(Fe) and SNW-1, accelerated the separation of photogenerated electrons and ameliorated the recombination of electron–hole pairs. The corresponding tests indicated that the matched band structure of NH₂-MIL-101(Fe) and SNW-1 could generate a type-II heterojunction (Figure 14b), which explained why the composite system had such superior photocatalytic performance.

Similarly, using the Schiff-base condensation reaction, Jiang et al. [82] in situ constructed a core-shell MOF@COF hetero-framework materials (NH₂-UiO-66@TFPT-DETH) via a two-step method (Figure 15a). The SEM image of NH₂-UiO-66 and the SEM and TEM images of U@TDE4 (NH₂-UiO-66@TFPT-DETH₄) are displayed in Figure 15b,d, where both the large differences in morphology between the two substances and the core-shell structure of U@TDE4 could be clearly observed. The as-synthesized photocatalyst exhibited an excellent H₂ production rate of 7178 μmol·g⁻¹·h⁻¹, which was quite competitive, especially compared to the pristine COF materials reported. In addition, there were no obvious decreases in long-term photocatalytic test for 12 h, which indicated that the U@TDE4 had outstanding reaction stability. The MOF@COF hetero-skeleton had a high surface area with the coexistence of micropores and mesopores, which extended the visible-light absorption and facilitated the efficient exciton dissolution and transfer. This work might give the inspiration for the design of hetero-framework photocatalysts and their application in solar energy conversion.

5. MOF-Derived Materials

MOFs are composed of regularly arranged structures that extend indefinitely in 3D space, which gives reprocessed MOF-derived materials an additional high regularity. The similar composition of retained components, orderly morphology change, brief and efficient post-processing, and other characteristics make MOFs become a kind of crucial precursor for the preparation of those materials with special needs [83,84]. After pyrolysis, MOFs can provide a variety of derivatives, including metal oxides, metal composites, metal carbides, and metal nitrides. In addition, the photo-response of these derivatives can be enhanced via doping some other atoms. MOF derivatives significantly expand the range of catalysts available and avoid some of the drawbacks associated with the direct utilization of MOFs as the catalysts.

Because of the composition diversity of MOFs, MOF-derived polymetallic compounds are easy to prepare. Nevertheless, there are few reports on the synthesis and preparation of MOF-derived porous metal materials at lower temperatures due to the collapse of the pore structure (prolonged treatment at high temperatures) during the synthesis process. Porous ternary metal nitrides (Ni₃FeN) derived from Prussian blue analogue (PBA) were synthesized via a simple two-step oxidation rapid nitride method reported by Yang and his co-authors (Figure 16a) and used in photocatalytic hydrogen evolution [85]. When the Eosin-Y was appointed as a dye photosensitizer, the porous ternary Ni₃FeN photocatalyst exhibited the excellent H₂ production rate of 16.96 mmol·g⁻¹·h⁻¹ with an AQE of ~3.03% at 520 nm, which was more than 3.45, 3.83, and 1.31 times that of the PBA, NiFeO_x, and Prussian-blue-derived Fe₃N samples. On the one hand, the reaction kinetics of the photocatalytic HER was ameliorated through the well-proportioned adsorption of the Eosin-Y on the porous Ni₃FeN nanocube. On the other hand, the superior conductivity of ternary Ni₃FeN boosted the separation and transfer of photogenerated electrons from the Eosin-Y and enhanced the HER performance of the Ni₃FeN/Eosin-Y photocatalytic system (Figure 16b). This report was expected to give a novel guideline for MOF-derived ternary metal nitrides to realize the efficient conversion from solar energy to chemical energy.

In Qin’s paper [86], they described the competitive evolution, the morphological and structural changes from Zn-based crystals to amorphous particles. The controlled contribution of organic linkers selectively derived six Zn-CPPs with multivariate characteristics (Figure 17). The FESEM images and the powder XRD patterns of CPPS-derived hollow ZnO microtubes and microspheres are displayed in Figure 18a–c. Based on the diversity of these substructures, hollow ZnO particles were generated via self-pyrolysis and effectively modified by ultrathin doped nanosheets (Figure 18d). The as-synthesized double-sided heterojunctions offered fully covered active sites and brought the efficient photo-excited electron transfer nanochannels (Figure 18e), which gave the superior H₂ evolution rate of 4512.5 μmol·h⁻¹·g⁻¹ with stable cyclability. In addition, the basic steps of HER on the O-ZnIn₂S₄/ZnO heterostructure were determined by DFT calculation (see the original text), which could also be used with activity descriptors to further understand the HER performance of the present photocatalytic system. Herein, this current work opened a new paradigm for the synthesis of custom metal oxides and refined composites for efficient and functional photocatalytic applications.

In other research on the regulation of electron transfer pathways reported by Jin et al. [87], the CdS-MoS₂ dumbbell structure was primarily synthesized by a solvothermal method to make the photo-excited electrons flow along the one-dimensional axis. The confinement effect of MOF nanoparticles produced by the silica hollow hexagonal prism greatly expanded the spectral absorption range of the composite photocatalyst. After that, the CdS-In₂O₃ S-scheme was successfully constructed through a simple electrostatically driven self-assembly method (Figure 19a). The authors attributed the superior photocatalytic activity (198.58 mmol·h⁻¹·g⁻¹) and stability (without obvious decrease for 20 h in four cycles) of this composite system to two aspects. Firstly, the formation of an internal electric field drove the photogenerated electrons in In₂O₃ to move to CdS, which accelerated the separation of electron–hole pairs. Secondly, the combination of In₂O₃ with the side wall of CdS-MoS₂ dumbbells weakened the surface oxidation kinetics, thus inhibiting the photocorrosion reaction and giving such a good performance (Figure 19b).

In contrast to the method mentioned above in which the electrons were separated in multiple dimensions, Dai et al. [88] inhibited the photogenerated carrier recombination via introducing C with high electronic conduction between the heterojunctions. A hierarchical C/HT-In₂O₃/ZnIn₂S₄ heterostructure was fabricated via growing thin-layered ZnIn₂S₄ on the surface of carbon-coated hollow tubular In₂O₃ (C/HT-In₂O₃) derived from In-MOF (Figure 20a). In the optimal conditions, the hybrid system exhibited a hydrogen evolution rate of 920.5 μmol/m², which was about 13.2 and 6.6 times higher than that of pristine C/HT-In₂O₃ and ZnIn₂S₄, respectively. The comparison of interfacial electron transfer pathways for straddling type (In₂O₃/ZnInS₄) and staggered model (C/HT-In₂O₃/ZnInS₄) is displayed in Figure 20b. The EXAFS analysis and density functional theory (DFT) calculation revealed that the faster charge transfer and significantly enhanced photocatalytic HER performance could be attributed to the narrowed band gap of C/HT-In₂O₃ and the formation of the staggered heterostructure between C/HT-In₂O₃ and ZnIn₂S₄. This work manifested that the feasibility of establishing coordinated In-N-In sites at the carbon-coated HT-In₂O₃/ZnIn₂S₄ heterostructure interface to facilitate charge transfer and introduced an ideal photo-activated HER catalyst.

If bimetallic MOFs were appointed as precursors, proper regulation of the ratio of the two metals in the MOFs and subsequent correct reprocessing were both vital. In Jin’s work [89], the bimetallic CeCo-MOF was synthesized via a hydrothermal method, and various composite catalysts were obtained from different treatment methods including phosphating, oxidation, and sulfurization (Figure 21). After reasonable adjustment of the Ce:Co ratio, it was finally determined that when the ratio of Ce:Co was 1:2, the composite phosphating catalyst had the best performance, and the photocatalytic HER rate could reach 8049 μmol·g⁻¹·h⁻¹. They attributed the superior reactivity of the as-synthesized CeCo1:2/P catalyst to three factors. Firstly, the Ce and Co sources were uniformly dispersed using bimetallic CeCo-MOF synthesized by a one-pot solvothermal method. Secondly, phosphating ameliorated the photo-absorption performance of the catalyst, which provided the prerequisite conditions for the generation of photogenerated electron–hole pairs. Thirdly, the close contact between CeO_1.66 and CoP ensured the smooth transfer of photogenerated electrons between the catalysts. This report might provide novel insights for post-processing bimetallic MOF materials and their further applications in photocatalytic HER.

6. Conclusions and Prospects

In this review, we summarized the research advances of MOFs catalysts in the field of photocatalytic HER in the past three years. The contents mainly focused on the preparation and synthesis of these catalysts, which included four separate aspects: internal modification and structure optimization of MOFs materials, MOFs/semiconductor composites, MOFs/COFs-based hybrids, and MOFs-derived materials. The MOFs-based reaction system and corresponding HER rates mentioned in this review are listed in Table 1, and the unit of H₂ evolution rate was unified as μmol∙g⁻¹∙h⁻¹. Compared with semiconductor materials and molecular catalysts, although the corresponding research about MOF materials started relatively late, their excellent potential in this field was quite stunning. Nevertheless, some unavoidable shortcomings remained, and the challenges were complicated, meaning that necessary efforts would contribute to the long-term and sustained development of this field.

(1) In most of the examples that have been reported in the field of photocatalysis, the MOFs that were mainly focused on were In [90,91,92], Ti [93,94], and Zr [95,96] MOFs, while there were few reports based on traditional transition metals such as Fe, Ni, and Cu, etc. This reduced the universality of synthetic methods and limited the large-scale utilization of similar materials. Some universal MOFs such as UiO-66, ZIF-8, and MOF-67, with temperature tolerance and acid-base environment tolerance need to be expanded and developed, which would lay a satisfactory foundation for building a broader MOF platform. Furthermore, some MOFs with high crystallinity can be regarded as a kind of special semiconductor material with band gap, so some methods of constructing heterojunctions can be referred to load semiconductor materials on MOFs [97,98].

(2) In terms of light sources, most laboratory studies use 300 W Xe lamps to simulate visible light rather than direct sunlight. In general, the amount of hydrogen produced by direct sunlight irradiation was usually smaller than produced by simulated visible light. Therefore, it was necessary to test the catalytic performance of the as-synthesized MOF-based materials under outdoor or direct light conditions. Alternatively, the single input amount of catalyst did not reach the gram level. The catalyst used in the experiments was usually between 5–50 mg, which greatly hindered the expansion for the practical application of this kind of catalyst. The reason for this phenomenon might be due to the inability of larger mass catalysts to disperse uniformly during photocatalytic reactions in mixed solutions, leading to agglomeration, which means that the catalyst per unit mass cannot be fully utilized. Hence, how to increase the amount of catalyst to a gram level reaction and how to carry out the hydrogen evolution reaction in real sunlight were two urgent problems to be solved in the process of the industrialization of this field.

(3) Although the high reactivity of photocatalytic HER was the most pursued goal of researchers, the long-term reaction stability and cyclic stability were also of great significance. There was no systematic evaluation of cyclic stability in some articles, and the characterization and further analysis of the cycled catalysts were also ambiguous. For example, the morphology of some catalysts changed greatly after the reaction, which might be the main reason for the decrease in the catalytic cycle reaction efficiency, but some authors did not discuss this part in detail. The superior cycle stability of these photocatalysts would lay a solid foundation for their subsequent industrial development.

(4) We should acknowledge that there were still many unknown processes or speculative results when it came to mechanism verification. This was related to the complexity of the composite system, which included the interaction between catalyst and reaction solution, the structure-activity relationship of catalysts themselves, whether photogenerated electron transfer between photosensitizer and cocatalyst was thermodynamically allowed, etc. More detailed and clear explanations of the mechanism might require advanced characterization techniques, more accurate theoretical calculations [99], and unremitting efforts of relevant researchers.

(5) For MOF derivatives and their composite systems, researchers should focus on the original driving forces for their formation or co-existence, including coordination bonds, covalent bonds, intermolecular or electrostatic forces, etc., which will provide guidance for the development and utilization of novel MOFs-based materials in a broader sense. In addition, when some newly synthesized MOFs are appointed as precursors, it will be more economical if the cost can be reduced and the preparation process can be simplified. This will make the advantages of MOFs more obvious and prominent when compared with the pure inorganic salts as precursors.

(6) Last but not the least, the solvents of many reaction systems are not pure water. The addition of organic solvents not only increases the cost, but also introduces the instability of the reaction. The addition of organic solvents with strong solubility may change the reaction system from heterogeneous to homogeneous. The comparison of activity and stability of homogeneous or heterogeneous systems is also vital for MOFs composite systems. At the same time, the recyclability of MOFs is very important and should be taken seriously. In addition, for some photocatalytic systems involving electron sacrificial agents, such as ascorbic acid, that can dissociate hydrogen ions, the hydrogen atoms in the precipitated hydrogen may be ionized by ascorbic acid, so some necessary verification experiments are needed.

Author Contributions

Writing—original draft preparation, H.Z.; validation, C.L.; writing—review and editing, Y.L.; resources, J.P.; project administration, funding acquisition, X.B. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Table

Figure 1. The illustration of the synthetic strategy for UiO-66-NH2 with Ni2+ encapsulated via an efficient microwave-assisted method and the follow-up coordination environment modulation of single Ni atom to obtain Ni1-X/MOF, in contrast to NiNP/MOF and the NiSNP/MOF via one-pot synthesized. Reproduced with permission from Ref. [61].

Figure 2. The illustration of the dark photocatalysis based on the MIL-125 and MIL-125-X (X = NH2, NO2 and Br). Reproduced with permission from Ref. [62].

Figure 3. (a) The 2D structure of Ni-TBAPy-SC and the coordinated environment of middle [Ni3O16] SBU. Color: red, O; yellow, C; green, Ni; cyan, H. (b) The schematic diagram of the photocatalytic hydrogen evolution reaction for Ni-TBAPy-NB. Reproduced with permission from Ref. [63].

Figure 4. (a) Schematic diagram of synthesis for different morphologies of In-TCPP MOFs. (b) The powder XRD patterns for 2D In-TCPP NS and 3D In-TCPP bulk. (c) SEM, (d) TEM, and (e) AFM images for 2D In-TCPP NS. (f) The SEM image of 3D In-TCPP bulk. (g) The powder XRD patterns after soaking and stirring in acidic and basic solutions for 12 h. Reproduced with permission from Ref. [64].

Figure 5. The schematic diagram of the synthesis steps for TMPs and TMPs@UiO-66-NH2. Reproduced with permission from Ref. [69].

Figure 6. The TEM images of (a,b) Ni2P@UiO-66-NH2 (the inset is the size distribution of Ni2P NPs) and (c,d) Pt@UiO-66-NH2 (the inset is the size distribution of Pt NPs). Reproduced with permission from Ref. [69].

Figure 7. The schematic diagram of synthesis steps for various photocatalysts. (a) PtPVP@UiO-66-NH2 and PtrPVP@UiO-66-NH2; (b) Pt@UiO-66-NH2 and Pt-Fc@UiO-66-NH2. Reproduced with permission from Ref. [70].

Figure 8. (a) The strategy exploration to form well-defined 2D-on-3D structures and to achieve the “anti-epitaxial growth” pattern of MOF nuclei. (b) The SEM image of the core satellite structure without the assistance of PVP. (c–h) display the SEM and TEM images when the system is in the presence of PVP and the relevant kinetic control results. (c,d) Vdeposition >> Vgrowth, (e,f) Vdeposition ≈ Vgrowth, (g,h) Vdeposition << Vgrowth. Reproduced with permission from Ref. [71].

Figure 9. Illustration of the preparation of CdS@MOF-808 composites and the comparison of hydrogen production rates between CdS4@MOF-808 and CdS/MOF-808-cys. Reproduced with permission from Ref. [72].

Figure 10. The schematic diagram of the photocatalytic hydrogen evolution for MIL-125-NH2 (a) before and (b) after integration with the metal oxides. Reproduced with permission from Ref. [73].

Figure 11. (a) The preparation procedure of the NM@OM/TiO2 dual heterojunction. (b) The possible mechanism for photocatalytic hydrogen evolution on NM@OM/TiO2 under full-spectrum illumination. Reproduced with permission from Ref. [74].

Figure 12. The illustration of synthesis steps of Cu3(HHTP)2/Tp-Pa-1 hybrid photocatalysts. Reproduced with permission from Ref. [79].

Figure 13. The schematic diagram of the synthesis steps for MTV-Ti-MOF/COF. Reproduced with permission from Ref. [80].

Figure 14. (a) The schematic illustration of the synthesis steps of the NH2-MIL-(101)Fe@SNW-1. (b) The mechanism diagram of photodegradation pollutant and photocatalytic H2 evolution for MS-0.8 under the visible-light irradiation. Reproduced with permission from Ref. [81].

Figure 15. (a) The schematic illustration of the synthesis steps of U@TDEn (NH2-UiO-66@TFPT-DETH) core-shell hetero-frameworks. (b) The SEM image of NH2-UiO-66. (c,d) The SEM and TEM images of U@TDE4. Reproduced with permission from Ref. [82].

Figure 16. (a) The synthesis schematic illustration of the Ni3FeN. (b) A possible mechanism of photocatalytic Eosin-Y-sensitized hydrogen evolution based on the Ni3FeN catalyst. Reproduced with permission from Ref. [85].

Figure 17. Morphology-selective formation of Zn-based coordination polymer particles with Zn2+, H2bdc, and/or salen linkers. [86].

Figure 18. (a,b) The FESEM images and (c) the powder XRD patterns of CPPs-derived hollow ZnO microtubes and microspheres. (d) The schematic illustration of the formation for the hollow O-doped ZnIn2S4/ZnO microtubes. (e) The illustration of the hydrogen evolution reaction on O-ZnIn2S4/ZnO-T heterosurfaces. Reproduced with permission from Ref. [86].

Figure 19. (a) The schematic illustration of the synthesis of CdS-MoS2-In2O3. (b) A possible hydrogen evolution mechanism of the CdS-MoS2-In2O3 ternary nanocomposite photocatalyst. Reproduced with permission from Ref. [87].

Figure 20. (a) The schematic illustration of the synthesis process for the C/HT-In2O3/ZnInS4. (b) The comparison of interfacial electron transfer pathways for straddling type (In2O3/ZnInS4) and staggered model (C/HT-In2O3/ZnInS4). Reproduced with permission from Ref. [88].

Figure 21. The preparation diagram of the CeO1.66/CoP and CeO2/Co3O4 catalysts. Reproduced with permission from Ref. [89].

Table 1

The comparison of the H₂ evolution rates in the articles mentioned in this review.

MOFs	Cocatalysts/Derivatives	Light Source	Sacrificial Agents	H2 Evolution Rate (μmol∙g−1∙h−1)	Ref.
UiO-66-NH2 (5 mg)	Ni SACs	300 W Xe lamp(λ > 380 nm)	TEA	1360	[61]
MIL-125-NH2 (10 mg)	Pt	300 W Xe lamp(800 nm > λ > 200 nm)	TEOA	~140	[62]
Ni-TBAPy-NB (20 mg)	―	300 W Xe lamp(λ > 420 nm)	Ascorbic acid	5000	[63]
In-TCPP NS (5 mg)	Pt	300 W Xe lamp	TEA	539.07	[64]
UiO-66-NH2 (5 mg)	Ni2P	300 W Xe lamp(λ > 380 nm)	TEA	409	[69]
UiO-66-NH2 (5 mg)	Pt-Fc	300 W Xe lamp(λ > 380 nm)	TEOA	102	[70]
UiO-66-NH2 (5 mg)	Cu-TCPP MOF	300 W Xe lamp	TEOA	25	[71]
MOF-808 (5 mg)	CdS4 NPs	300 W Xe lamp(800 nm > λ > 400 nm)	TEA	10,410	[72]
MIL-125-NH2 (10 mg)	MoO3	300 W Xe lamp(λ > 380 nm)	TEA	399	[73]
NH2-MIL-125@OH-MIL-125 (8 mg)	TiO2	300 W Xe lamp	TEOA	7108	[74]
Cu3(HHTP)2 (10 mg)	Tp-Pa-1-COF	300 W Xe lamp(λ > 400 nm)	sodium ascorbate	1760	[79]
PdTCPP@PCN-415(NH2) (10 mg)	TpPa-COF	300 W Xe lamp(λ > 400 nm)	Ascorbic acid	13,980	[80]
NH2-MIL-125(Fe) (50 mg)	SNW-1@Pt	300 W Xe lamp	TEOA	1949.56	[81]
NH2-UiO-66 (8 mg)	TFPT-DETH-COF	300 W Xe lamp(λ > 420 nm)	Ascorbic acid	7178	[82]
Prussian blue analogues	Ni3FeN (derivative)	300 W Xe lamp(λ > 400 nm)	TEOA	16,960	[85]
Zn-based CPP	O-ZnIn2S4/ZnO (derivative)	300 W Xe lamp	Na2S and Na2SO3	4512.5	[86]
In-CPP	CdS-In2O3(derivative)	300 W Xe lamp	LA	198,580	[87]
MIL-68	C/HT-In2O3 /Zn2InS4	300 W Xe lamp	Na2S and Na2SO3	230.1 μmol/m2/h	[88]
Ce/Co-MOF	CeO1.66/CoP(derivative)	5 W LED(λ≥400 nm)	TEOA	8040.9	[89]

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