Metal-Organic Framework (MOF) Compounds: Photocatalysts for Redox Reactions and Solar Fuel Production.

Review: 148 refs.

Chapter 10 - Metal organic framework-based photocatalysts for hydrogen production

To relieve and solve the energy problem, effective methods to use solar energy must be built up. To be more specifically, we need figure out how to utilize sunlight for water splitting reaction, giving rise to hydrogen as clean energy, and photoreduction of carbon dioxide, leading to the formation of useful liquid products (e.g., HCOO−, HCHO and CH3OH) or gaseous products (e.g., CH4 and CO). As a class of distinguished and unique materials, metal-organic frameworks (MOFs) have drawn a lot of attention, considering that they display special physical and chemical properties such as exceedingly high surface areas, designable and controllable cavities, different mechanisms of photo-induced electrons transfer, and moreover photoactive parts can be easily introduced into MOFs by either encapsulating dye molecules into the cavities or constructing the frameworks with optically active bridging ligands or metal nodes. In this review, we have commented on the challenges in this field, summarized the unique advantages and inherent merits of MOFs as the emerging materials, and pointed out the opportunities and development strategies of MOFs for their applications in photocatalysis. Firstly, we have introduced MOFs′ concepts and features, distinguishing them from other porous materials, and their advantages in photocatalysis. MOFs are crystalline porous materials formed from ligands (including metalloligands) and transition-metal nodes. The structures of MOFs are of facile design, and can be further modified through post-synthetic methods. Some MOFs display high thermal and chemical stability, which can be stable up to 500°C and resist a variety of reaction media either organic solvents or aqueous, even in acidic and basic solutions. MOFs can be photoresponsive through light absorption by the organic linker, the metal oxide nodes or the photoactive species entrapped in the voids. Photoexcitation of the light absorbing units in MOFs generates the excited state, which might induce photocatalytic activity. Next, we have classified photocatalytic MOFs into three types, including (1) metal-oxo clusters as semiconductor dots, (2) ligands/metalloligands as photocatalysts, and (3) photocatalytic species (nano- particles, polyoxometalates, nano-composites, and etc.) encapsulated into the pore, and discussed their applications in photocatalysis in details. As for type I, metal-oxo clusters, especially Zr-O or Ti-O clusters, as the nodes have been assembled into MOFs. Upon the absorption of photons with the energy greater than the bandgap of the ligand, a ligand-to-metal charge-separation state was generated, resulting in photocatalytic activity. In type II, a few molecular photocatalysts based on metal-polypyridine complexes, usually being Ru and Ir complexes, metalloporphyrins and organic dyes have been incorporated into MOFs to afford photocatalysts under visible light. Considering type III, photoactive species, including polyoxometalates and metal nanoparticles (e.g., Pt, Pd, Au, and Ag NPs), have been doped into the cavities of MOFs. In addition, integration of an inorganic semiconductor with a MOF gives rise to a composite photocatalyst, which combines the advantages of both materials and then results in higher efficiency, selectivity and stability (especially low metal leaching and recyclability). Finally, we have provided our perspectives for the future of MOFs as photocatalysts. MOFs have displayed the potentials in photocatalysis, but there still exist large improvement spaces. The relatively low stability of MOFs compared to inorganic semiconductors limits their applications for practice. In most reports of MOF photocatalysts, sacrificial agents are required, which isn′t consistent with the sustainable development concept. MOFs with strong absorption of visible light, long lifetime of excited state, high product selectivity and stability are in pursuing.

Metal-Organic Frameworks for solar energy utilization

Imparting Superhydrophobicity to Porphyrinic Coordination Frameworks Using Organotin

Advances in Emerging Crystalline Porous Materials.

With the fast development of the social economy and the improvement of people's living standards, energy and environmental issues are attracting more and more attention. In the future, the great challenge for mankind is to shift energy supply from fossil energy to renewable energy. Solar energy is the most important renewable energy on Earth. However, low energy density and intermittency limit its practical application. Photocatalysis has broad application prospects in solar energy utilization. Photocatalysis can utilize solar energy to decompose water to produce hydrogen, reduce carbon dioxide to synthesize solar fuel, and degrade pollutants to purify the environment. However, the low photocatalytic efficiency limits its practical application. Thus, from the viewpoint of practical utilization, the improvement in methods and new photocatalysts are highly required. A total of 16 papers have been published in this issue, covering H2 production, CO2 reduction, H2O2 synthesis, and pollutant degradation. Among them, there are 10 papers about hydrogen production and 6 papers related to S-scheme heterojunction photocatalysts. We would like to express our sincere thanks to all the authors who submitted their interesting works to this special issue. A summary of all 18 accepted papers is provided as follows. Firstly, in article number 2200394, the authors present different functional ligands or metals incorporated into the parent metal-organic frameworks (MOFs) to enhance the photocatalytic performance of multivariate MOFs. The synthesis methods and unique advantages of multivariate MOFs-based photocatalysts are discussed. The recent advance in three multivariate MOFs for solar-to-chemical energy conversion are summarized according to mixed-metal MOFs, mixed-metal and mixed-ligand MOFs, and mixed-ligand MOFs. Finally, future perspectives and challenges in CO2 conversion and H2 evolution over Multivariate MOFs-based photocatalysts are discussed. In article number 2200364, Liu and colleagues reported enhanced CO2 photoreduction over Ni(OH)2-x/WO3 nanofibers, which were prepared by in situ growth of freestanding oxygen-vacancy Ni(OH)2-x nanosheets on WO3 nanofibers. The Ni(OH)2-x/WO3 nanofibers exhibit an enhanced CO production rate with respect to WO3 (54.4 vs 8.1 µmol g−1 h−1). The 13CO2 isotope tracing experiment confirmed that the CO product originated from the input CO2. The article with number 2200189 presents n-type CoP2 semiconductors as one of the main active components for efficient hydrogen evolution obtained from bulk P-CoV-LDH (layered double hydroxide). To achieve oriented control of carrier migration, the ZnxCd1−xS solid solution is effectively combined with P-CoV-LDH to synthesize a highly efficient and stable S-scheme heterojunction photocatalyst. The best P-CoV-LDH/ZnxCd1−xS 30% composite has a hydrogen evolution rate of 1244.3 µmol without noble metal additives, which is 6.4 times more than ZnxCd1−xS. Li and co-workers, in article number 2200143, reported g-C3N4 with edge grafting of 4-(1H-imidazol-2-yl) benzoic acid and NiS cocatalysts fabricated via a one-pot chemical condensation of monomers with urea and subsequent photodeposition. The obtained composites exhibit greatly enhanced visible-light photocatalytic performance for H2 evolution, in comparison with the undoped g-C3N4. The synergistic effect of bimetallic sulfide is discussed in article number 2200139, which reports the composite bimetallic sulfide ZnCo2S4 and CdS with excellent photocatalytic hydrogen evolution capability. The synergistic effect of zinc ions and cobalt ions enriches the redox-active sites, which provides favorable conditions for the photocatalytic hydrogen evolution reaction. The synergistic effect of bimetallic ions as the main driving force for the accelerated hydrogen precipitation reaction is analyzed by fluorescence and electrochemical characterization. The results of hydrogen production experiments show that the hydrogen evolution amount of ZnCo2S4/CdS is about 10 times that of single CdS. Article number 2200134 describes carbon nanotubes in situ grown onto g-C3N4 nanosheets via a chemical vapor deposition process, catalyzed by Au nanoparticles pre-deposited on g-C3N4 surface via deposition-precipitation. Systematic characterizations, in particular femtosecond transient absorption spectroscopy and time-resolved photoluminescence, prove that carbon nanotubes can efficiently extract the localized electrons in the tri-s-triazine units of g-C3N4, thereby enhancing charge carrier diffusion and separation. In article number 2200130, a donor–acceptor modified g-C3N4 conjugated copolymer is fabricated via facile thermal copolymerization of 2-aminobenzimidazole (abIM) and urea. The experimental results demonstrate that the abIM units are successfully incorporated into the framework of g-C3N4 and the main chemical structure of g-C3N4 is still preserved. These abIM units can serve as electron acceptors, extending the π-conjugated system and inducing the intramolecular charge transfer via an internal electric field. As a result, the construction of D–A structure not only improves the optical utilization efficiency but also facilitates the intramolecular migration of electrons and holes, leading to enhanced photocatalytic hydrogen evolution (2566 µmol g−1 h−1) as compared to pristine g-C3N4. Article number 2200113 presents a novel S-scheme heterojunction photocatalyst g-C3N4/PDA comprised of ultrathin g-C3N4 and polydopamine (PDA) constructed by in situ self-polymerization. The optimal photocatalyst presents an excellent H2O2 production rate of 3801.25 µmol g−1 h−1 under light irradiation, which is about 2 and 11 times higher than that of pure g-C3N4 and PDA, respectively, and exceeds most of the reported C3N4-based photocatalysts. The improvement of photocatalytic activity is ascribed to the synergistic effect of improved light absorption and promoted charge separation and transfer induced by the S-scheme heterojunction. In article number 2200030, a novel quaternary CdIn2S4-xSex solid-solution nanocrystal photocatalyst was prepared by one-step hydrothermal synthesis. The bandgap structure of CdIn2S4-xSex nanocrystals can be adjusted from 2.42 to 1.87 eV by varying the molar ratio of Se/S. Compared with pure CdIn2S4, the CdIn2S4-xSex solid-solution photocatalyst clearly represents excellent photocatalytic hydrogen production performance, while the CdIn2S4-xSex (x = 0.4) solid-solution nanocrystal exhibits the optimal hydrogen-production efficiency of 314.24 µmol h−1, which is 3.3 times superior to that of CdIn2S4 (94.83 µmol h−1). In article number 2200027, ZnS/TiO2 S-scheme heterojunction photocatalysts were constructed by in situ depositing ZnS nanoparticles on TiO2 nanofibers via hydrothermal method. A highly improved photocatalytic H2 evolution rate is achieved for the ZnS/TiO2 heterojunction as compared to the mono-component ZnS and TiO2. Remarkably, the TiO2/ZnS-5 sample possesses the highest H2 evolution rate of 5503.8 µmol g–1 h–1, which is 4.8 times of ZnS and 38.8 times of TiO2, respectively. In article number 2200009, highly dispersed Ni sites are planted on C3N5, an N-rich carbon nitride, by a facial two-step annealing method to construct a Ni-C3N5 material. The incorporation of Ni sites can significantly enhance the e–/h+ separation efficiency of C3N5 under light irradiation and promote the activation of O2 to produce reactive oxygen species. Compared with pristine C3N5 (with NO removal ratio of ≈35%), the as-prepared 0.1- or 0.25-Ni-C3N5 material can remove ≈54% continuous-flowing NO (initial concentration: 600 ppb) quickly in less than 25 min under white LED light irradiation. A novel sandwich-like hierarchical heterostructure of Ti3C2 MXene/WO3 is created by in situ growth of ultrathin WO3 nanosheets onto the surface of few-layer Ti3C2 nanosheets via a one-pot solvothermal synthesis strategy (article number 2100507). The resultant Ti3C2/WO3 heterostructure holds a large interface contact area, an intimate electronic interaction, and a short carrier migration distance, which is beneficial for bulk-to-surface and interfacial charge transfer. As expected, the as-prepared Ti3C2/WO3 nanohybrids exhibit superior visible-light-driven photoactivity and stability toward tetracycline hydrochloride decomposition. Article number 2100498 presents an S-scheme of Mn0.2Cd0.8S-diethylenetriamine/porous g-C3N4 heterojunction designed, which accelerates the charge transfer at the interface of Mn0.2Cd0.8S-diethylenetriamine and porous g-C3N4, and provides electrons for photocatalytic hydrogen production. Under the same light conditions, the hydrogen production efficiency of the composite is 11.42 mmol h–1 g–1, which is 30 times higher than that of porous g-C3N4. The paper "Porous Zn conformal coating on dendritic-like Ag with enhanced selectivity and stability for CO2 electroreduction to CO" (article number 2200374) presents uniform porous Zn conformal coating on high-curvature dendritic Ag nanoneedles by vacuum thermal evaporation. As the surface sacrificial shell, the dissolution and reconstruction of Zn protect the inner Ag core, thus enhancing the CO2 reduction stability of the composited samples. In article number 2200402, a step-scheme heterojunction consisting of thin TiO2 nanosheets and few-layered MoO3 structures is reported. With a decoration of a low dose of MoO3 layer by ball milling method, TiO2 shows a 3-fold increase in the hydrogen evolution rate. The presence of MoO3 promotes the electron-hole pair separation via the Step-scheme mechanism. Finally, in article number 2200381, a novel MOFs-derived In2O3/ZnO tubular S-scheme heterojunction photocatalyst for CO2 photoreduction is reported. Because of Fermi level difference and electron transfer, an internal electric field is produced at In2O3/ZnO heterojunction interfaces, which results in the formation of S-scheme heterojunctions. The CO2 photoreduction follows a *COOH-intermediate mechanism and the CO production rate (12.6 µmol g−1) with nearly 100% selectivity is obtained over In2O3/ZnO S-scheme photocatalyst. The authors declare no conflict of interest. Jiaguo Yu is a professor in the Faculty of Materials Science and Chemistry at the China University of Geosciences. He received his BS and MS degrees in chemistry from Central China Normal University and Xi'an Jiaotong University, respectively, and his PhD degree in materials science in 2000 from Wuhan University of Technology. In 2000, he became a Professor at Wuhan University of Technology. In 2021, he moved to the China University of Geosciences (Wuhan). His research interests include photocatalysis, adsorption, electrocatalysis and so on. He is a Foreign Member of Academia Europaea (2020), a Foreign Fellow of the European Academy of Sciences (2020), and a KIA Laureate of the 35th Khwarizmi International Award (2022). Kai Dai is a professor at the College of Physics and Electronic Information, Huaibei Normal University, Huaibei, China. He received Ph.D. degree from Shanghai University in 2007 and then worked as an assistant professor in Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences. He joined Huaibei Normal University in 2010 and his research interests mainly focus on semiconductor photocatalysis. Chuanbiao Bie obtained his Ph.D. degree in Materials Science and Engineering from Wuhan University of Technology (2021). He is now a postdoctoral researcher working in the Laboratory of Solar Fuel, Faculty of Materials Science and Chemistry, China University of Geosciences (Wuhan). His research interests are focused on semiconductor photocatalysis, including H2 evolution, CO2 reduction, H2O2 production, and organic synthesis.

Optical Properties and Applications of Crystalline Materials

Metal organic frameworks (MOFs) are a class of porous crystalline materials that feature a series of unique properties, such as large surface area and porosity, high content of transition metals, and possibility to be designed and modified after synthesis, that make these solids especially suitable as heterogeneous catalysts. The active sites can be coordinatively unsaturated metal ions, substituents at the organic linkers or guest species located inside the pores. The defects on the structure also create these open sites. The present review summarizes the current state of the art in the use of MOFs as solid catalysts according to the type of site, making special emphasis on the more recent strategies to increase the population of these active sites and tuning their activity, either by adapting the synthesis conditions or by post-synthetic modification. This review highlights those reports illustrating the synergy derived from the presence of more than one of these types of sites, leading to activation of a substrate by more than one site or to the simultaneous activation of different substrates by complementary sites. This synergy is frequently the main reason for the higher catalytic activity of MOFs compared to homogeneous catalysts or other alternative solid materials. Besides dark reactions, this review also summarizes the use of MOFs as photocatalysts emphasizing the uniqueness of these materials regarding adaptation of the linkers as light absorbers and metal exchange at the nodes to enhance photoinduced electron transfer, in comparison with conventional inorganic photocatalysts. This versatility and flexibility that is offered by MOFs to optimize their visible light photocatalytic activity explains the current interest in exploiting these materials for novel photocatalytic reactions, including hydrogen evolution and photocatalytic CO2 reduction.

Post-synthetic modification of metal-organic framework-based membranes for enhanced molecular separations

Ti-based metal-organic frameworks (MOFs) are demonstrated as promising photosensitizers for photoelectrochemical (PEC) water splitting. Photocurrents of TiO2 nano wire photoelectrodes can be improved under visible light through sensitization with aminated Ti-based MOFs. As a host, other sensitizers or catalysts such as Au nanoparticles can be incorporated into the MOF layer thus further improving the PEC water splitting efficiency.

Rapid growth in economy and society currently relies on fossil fuels heavily, leading to serious concern on energy sustainability and environmental pollution. This has aroused increasing interest in solar energy, which is the most abundant and green energy source. However, it remains a significant challenge in storing and harnessing solar energy due to its diurnal and seasonal fluctuation along with uneven distribution. Moreover, its energy density is relatively low; and the energy supply from traditional solar cells is time-varying. Consequently, it is difficult to synchronize the generation and usage of electricity from solar cells. In contrast, photocatalytic water splitting can convert solar energy to chemical energy, which can be stored in the form of hydrogen, an ideal energy carrier that is characteristic of cleanness and operational convenience. Also, photocatalytic CO2 reduction can transform greenhouse gas to valuable fuels. Furthermore, photocatalytic degradation of organic pollutants can decontaminate air, water, and soil. Additionally, photocatalytic disinfection and sterilization can reduce human exposure to pathogens and toxins. Therefore, photocatalysis has received extensive attention globally, becoming the hotspot and frontier across several fields, including chemistry, materials science, energy and environment engineering. In this special issue (Part 1), 2 progress reports, 12 review articles and 15 research articles have been published. Classified based on applications, 8 papers concern with hydrogen evolution, 7 papers involve CO2 reduction, 2 articles deal with pollutant decomposition, and 11 papers are related to nitrate reduction, nitrogen fixation, organic synthesis, syngas synthesis, NADH (nicotinamide adenine dinucleotide) regeneration and so on. From the materials perspective, 8 papers discuss C3N4, 4 articles involve CdS, 3 paper concern with conjugated polymers, other papers cover TiO2, CdSe, CoMn alloy, PbI2/CuI, silicon, metal-organic frameworks (MOFs), perovskite oxides, Bi2MoO6 and so on. Photocatalytic H2 production is of great interest from both theoretical and practical viewpoints because of its potential application in converting solar energy into storable chemical energy. Herein, Zhen and Xue (solr.202000440) have reviewed surface functionalization of polymeric carbon nitride at atomic and molecular levels for photocatalytic H2 production and CO2 reduction applications. Then, Yu et al. have (solr.202000372) reported enhanced photocatalytic H2 production activity of g-C3N4, which are prepared by one-step crystallization and cyano-group modification. Yang and co-workers (solr.202000414) have presented the fabrication of 2D/2D CdS/MXene Schottky heterojunctions by electrostatic self-assembly and solvothermal method and their application in high-efficiency photocatalytic hydrogen production. Tang and colleagues (solr.202000281) have reported the molecular cobalt catalysts grafted on a conjugated microporous polymer for high-efficiency H2 production. Zhao et al. (solr.202000415) have prepared a CdS/MoS2 nanooctahedron heterostructure with a tight interface for enhanced photocatalytic H2 evolution and biomass upgrading. Wu's group (solr.202000474) have reported a per-6-thiol-cyclodextrin engineered [FeFe]-H2ase mimic/CdSe quantum dot assembly for effective photocatalytic H2 evolution. Xu et al. (solr.202000486) have synthesized a lignin-modified g-C3N4 nanoarchitecture with an ultrathin layered topography for efficient photocatalytic H2 production. Finally, Kwon et al. (solr.202000411) have reported a self-assembly between the CdS quantum dots and the RuO2/reduced graphene oxide nanosheets, showing enhanced photocatalytic H2 production activity. Nowadays, there is increasing interest in solutions to the increasing CO2 level in the atmosphere. Photocatalytic reduction of CO2 into storable solar fuels is an appealing strategy to simultaneously overcome both environmental problems and energy crisis. In this special issue, Wang's group (solr.202000443) has firstly discussed the active sites of catalysts for CO2 activation and conversion. Then, Liang et al. (solr.202000478) have summarized recent research progress in g-C3N4 and its composite photocatalysts for CO2 reduction. Huang's group (solr.202000430) has reviewed junction engineering for photocatalytic and photoelectrocatalytic CO2 reduction. Zhang et al. (solr.202000387) have demonstrated an all-earth-abundant photothermal silicon platform for CO2 catalysis with nearly 100% sunlight absorption ability. Jia's group (solr.202000313) has reported anchoring single-atom Ru on CdS, showing enhanced CO2 capture and charge accumulation for highly selective photothermocatalytic CO2 reduction to solar fuels. Xiang et al. (solr.202000351) have prepared an ultrathin S-scheme heterojunction based on few-layer g-C3N4 and monolayer Ti3C2Tx MXene for photocatalytic CO2 reduction. Finally, Zhang's group (solr.202000326) has reported an ultraviolet-visible-near-infrared responsive Cu2-xS/g-C3N4 composite photocatalyst and its photocatalytic CO2 reduction performance. It worth noting that significant efforts have been made to prepare high-performance photocatalysts for environment remediation including air purification, hazardous waste removal, water purification, and etc. Herein Xu et al. (solr.202000416) have fabricated a g-C3N4/NH2-UIO-66 composite photocatalyst with enhanced photocatalytic removal efficiency for hexavalent chromium. Zhu's group (solr.202000453) has reported the enhanced photocatalytic phenol degradation activity in the presence of g-C3N4/PDI (perylenetetracarboxylic diimide). Janáky and co-workers (solr.202000418) have reported the preparation of PbI2/CuI nanocomposite electrode and its solar photoelectroreduction of nitrate ions. Peng's group (solr.202000487) has summarized updated research progresses in photocatalytic nitrogen-fixation reaction over semiconductors. Su and colleagues (solr.202000444) have surveyed recent advance in the rational harnessing of photoexcited hole-electron pairs in semiconductor photocatalysts, and the application in oxidative and reductive synthetic transformations for chemical and pharmaceutical production. Ouyang's group (solr.202000488) has reported the fabrication of CoMn alloy using a metal-segregation method and its enhanced photothermal conversion of syngas to light olefins. Qian and Zhang (solr.202000489) have commented the recent advance in the conjugated microporous polymers in visible light promoted chemical transformations such as water splitting, CO2 reduction, organic photoredox reactions, and etc. Chen and colleague (solr.202000442) have reviewed the current research status of Bi2MoO6-based photocatalysts and their surface/interface modification strategies and applications. Dong and co-workers (solr.202000419) have reviewed the synthesis strategy, interfacial effect and photocatalytic application of perovskite nanocrystals-based heterostructure photocatalysts. Hao and Li (solr.202000454) have reviewed visible-light initiated synergistic/cascade reactions over metal-organic frameworks. Ma's group (solr.202000397) has highlighted the 2D/2D Z-Scheme heterojunctions for photocatalytic application. Wang et al. (solr.202000392) have reviewed two-dimensional silicon (2D Si) for catalysis and photocatalysis applications. Liu's group (solr.202000339) has summarized the key developments of conjugated photocatalytic systems for NADH (nicotinamide adenine dinucleotide) regeneration. As the guest editors, we thank all the authors for their prompt response to the paper call and their valuable contribution to this special issue. All the manuscripts were refereed through rigorous peer-review processes. We greatly appreciate the timely and conscientious evaluation of manuscripts by the reviewers. Last but not least, we are grateful to Dr. Lulu Ma, Editor of Solar RRL, for her tremendous support and dedication. Jiaguo Yu received his B.S. and M.S. in chemistry from Central China Normal University and Xi'an Jiaotong University, respectively; his Ph.D. in Materials Science from Wuhan University of Technology (WUT). In 2000, he became a Professor at WUT. His research interests include photocatalysis, adsorption, supercapacitor, electrocatalysis, formaldehyde removal and so on. He is Foreign Member of Academia Europaea (The Academy of Europe) (2020), Foreign Fellow of the European Academy of Sciences (2020) and Fellow of the Royal Society of Chemistry (2015). Tierui Zhang is currently Professor at the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences. He obtained his Ph.D. degree in Chemistry from Jilin University, China in 2003. He worked as a postdoctoral fellow in the labs of Prof. Markus Antonietti, Prof. Charl F.J. Faul, Prof. Hicham Fenniri, Prof. Z. RyanTian, Prof. Yadong Yin, and Prof. Yushan Yan. His current scientific interests focus on catalyst nanomaterials for energy conversion. Nianqiang Wu is currently Armstrong-Siadat Endowed Professor in Materials Science at University of Massachusetts Amherst, USA. He has received his Ph.D. degree in Materials Science and Engineering from Zhejiang University, China. Dr. Wu is Fellow of the Electrochemical Society (FECS) and Royal Society of Chemistry (FRSC). His research interest lies in: 1) photocatalysts and photoelectrochemical cells, 2) electrochemical energy storage, and 3) biosensing, microfluidics and photodynamic therapy.

The field of coordination polymers and metal–organic frameworks (MOFs) has evolved over a period of approximately 25 years to a stage where it is one of the most widely investigated areas of materials chemistry. The field has impacted many areas of science including commercial applications from

Metal-organic frameworks (MOFs) are regarded as promising materials for energy applications, particularly in photocatalytic hydrogen (H2) production. This is due to their structural architectures that facilitate charge transfer, and tunable porous and light absorption properties. However, the many characteristics of MOFs including crystal morphology and sizes, surface facets, porosity, light absorption properties, and optical band gaps, can significantly influence their photocatalytic activity, presenting challenges in achieving reproducibility. In this study, we describe the synthesis of five distinct batches of the photoactive MOF, MIL-125-NH2, utilizing different synthetic conditions. Solid-state characterization confirmed the purity, porosity, and light absorption properties of each MOF batch. Each material was then combined with nano sized Ni2P as a cocatalyst, and their photocatalytic activity for H2 evolution was evaluated. We observed variations in their photocatalytic H2 evolution rates, which depended on the batch of MIL-125-NH2 utilized, ranging from the lowest rate of 2980 μmol·h-1·g-1 to the highest of 4327 μmol·h-1·g-1. Notably, different H2 evolution rates were also observed even when MIL-125-NH2 was synthesized under identical synthetic conditions but by different students. Our research highlights the critical relationship between MOF synthesis parameters─such as reaction time, temperature, and precursor concentration─and resulting properties, including particle size, morphology, surface facets, and light absorption characteristics. These factors significantly influence their photocatalytic activity, as evidenced by varying H2 evolution rates. This underscores the importance of optimizing materials synthesis conditions to improve reproducibility and efficiency in photocatalytic applications.

Metal–organic frameworks (MOFs) are a class of porous materials with immense chemical tunability derived from their organic and inorganic building blocks. Presynthetic approaches have been used to construct tailor-made MOFs, but with a rather restricted functional group scope limited by the typical MOF solvothermal synthesis conditions. Postsynthetic modification (PSM) of MOFs has matured into an alternative strategy to broaden the functional group scope of MOFs. PSM has many incarnations, but two main avenues include (1) covalent PSM, in which the organic linkers of the MOF are modified with a reagent resulting in new functional groups, and (2) coordinative PSM, where organic molecules containing metal ligating groups are introduced onto the inorganic secondary building units (SBUs) of the MOF. These methods have evolved from simple efforts to modifying MOFs to demonstrate proof-of-concept, to becoming key synthetic tools for advancing MOFs for a range of emerging applications, including selective gas sorption, catalysis, and drug delivery. Moreover, both covalent and coordinative PSM have been used to create hierarchal MOFs, MOF-based porous liquids, and other unusual MOF materials. This Outlook highlights recent reports that have extended the scope of PSM in MOFs, some seminal reports that have contributed to the advancement of PSM in MOFs, and our view on future directions of the field.