Switching on the Photocatalysis of Metal–Organic Frameworks by Engineering Structural Defects

Accelerated Multi-step Sulfur Redox Reactions in Lithium-Sulfur Batteries Enabled by Dual Defects in Metal-Organic Framework-based Catalysts

The sluggish sulfur redox kinetics and shuttle effect of lithium polysulfides (LiPSs) are recognized as the main obstacles to the practical applications of the lithium-sulfur (Li-S) batteries. Accelerated conversion by catalysis can mitigate these issues, leading to enhanced Li-S performance. However, a catalyst with single active site cannot simultaneously accelerate multiple LiPSs conversion. Herein, we developed a novel dual-defect (missing linker and missing cluster defects) metal-organic framework (MOF) as a new type of catalyst to achieve synergistic catalysis for the multi-step conversion reaction of LiPSs. Electrochemical tests and first-principle density functional theory (DFT) calculations revealed that different defects can realize targeted acceleration of stepwise reaction kinetics for LiPSs. Specifically, the missing linker defects can selectively accelerate the conversion of S8 →Li2 S4 , while the missing cluster defects can catalyze the reaction of Li2 S4 →Li2 S, so as to effectively inhibit the shuttle effect. Hence, the Li-S battery with an electrolyte to sulfur (E/S) ratio of 8.9 mL g-1 delivers a capacity of 1087 mAh g-1 at 0.2 C after 100 cycles. Even at high sulfur loading of 12.9 mg cm-2 and E/S=3.9 mL g-1 , an areal capacity of 10.4 mAh cm-2 for 45 cycles can still be obtained.

Coordination Defect-Induced Frustrated Lewis Pairs in Polyoxo-metalate-Based Metal-Organic Frameworks for Efficient Catalytic Hydrogenation

Precise control of the structure and spatial distance of Lewis acid (LA) and Lewis base (LB) sites in a porous system to construct efficient solid frustrated Lewis pair (FLP) catalyst is vital for industrial application but remains challenging. Herein, we constructed FLP sites in a polyoxometalate (POM)-based metal-organic framework (MOF) by introducing coordination-defect metal nodes (LA) and surface-basic POM with abundant oxygen (LB). The well-defined and unique spatial conformation of the defective POM-based MOF ensure that the distance between LA and LB is at ~4.3 Å, a suitable distance to activate H2 . This FLP catalyst can heterolytically dissociate H2 into active Hδ- , thus exhibiting high activity in hydrogenation, which is 55 and 2.7 times as high as that of defect-free POM-based MOF and defective MOF without POM, respectively. This work provides a new avenue toward precise design multi-site catalyst to achieve specific activation of target substrate for synergistic catalysis.

Tailoring the Nanoporosity and Photoactivity of Metal-Organic Frameworks With Rigid Dye Modulators for Toluene Purification

Facile synthesis of hierarchically porous metal-organic frameworks (MOFs) with adjustable porosity and high crystallinity attracts great attention yet remains challenging. Herein, a micromolar amount of dye-based modulator (Rhodamine B (RhB)) is employed to easily and controllably tailor the pore size of a Ti-based metal-organic framework (MIL-125-NH2 ). The RhB used in this method is easily removed by washing or photodegradation, avoiding secondary posttreatment. It is demonstrated that the carboxyl functional group and the steric effects of RhB are indispensable for enlarging the pore size of the MIL-125-NH2 . The resulting hierarchically porous MIL-125-NH2 (RH-MIL-125-NH2 ) exhibits optimized adsorption and photocatalytic activity because the newly formed mesopore with defects concurrently facilitates mass transport of guest molecules (toluene) and photogenerated charge separation. This work offers a meaningful basis for the construction of hierarchically porous MOFs and demonstrates the superiority of the hierarchical pore structure for adsorption and heterogeneous catalysis.

Designing Heteroatom-Codoped Iron Metal-Organic Framework for Promotional Photoreduction of Carbon Dioxide to Ethylene

Rational engineering active sites and vantage defects of catalysts are promising but grand challenging task to enhance photoreduction CO₂ to high value-added C2 products. In this study, we designed an N,S-codoped Fe-based MIL-88B catalyst with well-defined bipyramidal hexagonal prism morphology via a facile and effective process, which was synthesized by addition of appropriate 1,2-benzisothiazolin-3-one (BIT) and acetic acid to the reaction solution. Under simulated solar irradiation, the designed catalyst exhibits high C₂ H₄ evolution yield of 17.7 μmol g^-1 ⋅h, which has been rarely achieved in photocatalytic CO₂ reduction process. The synergistic effect of Fe-N coordinated sites and reasonable defects in the N,S-codoped photocatalyst can accelerate the migration of photogenerated carriers, resulting in high electron density, and this in turn helps to facilitate the formation and dimerization of C-C coupling intermediates for C₂ H₄ effectively.

Design of Imide Oligomer-Mediated MOF Clusters for Solar Cell Encapsulation Systems by Interface Fusion Strategy

Dielectric encapsulation materials are promising for solar cell areas, but the unsatisfactory light-management capability and relatively poor dielectric properties restrict their further applications in photovoltaic and microelectronic devices. Herein, an interface fusion strategy to engineer the interface of MOF (UiO-66-NH₂ ) with anhydride terminated imide oligomer (6FDA-TFMB) is designed and a novel MOF cluster (UFT) with enhanced forward scattering and robust porosity is prepared. UFT is applied as an optical and dielectric modifier for bisphenol A epoxy resin (DGEBA), and UFT epoxy composites with high transmittance (>80%), tunable haze (45-58%) and excellent dielectric properties can be prepared at low UFT contents (0.5-1 wt%), which delivers an optimal design for dielectric encapsulation systems with efficient light management in solar cells. Additionally, UFT epoxy composites also show excellent UV blocking, and hydrophobic, thermal and mechanical properties. This work provides a template for the synthesis of covalent bond-mediated nanofillers and for the modulation of haze and dielectric properties of dielectric encapsulation materials for energy systems, semiconductors, microelectronics, and more.

MOF-Integrated Hierarchical Composite Fiber for Efficient Daytime Radiative Cooling and Antibacterial Protective Textiles

Incorporating passive radiative cooling into textiles is an effective way to improve individual personalized thermophysiological comfort for the human body. Based on radiative cooling textile design, rational functionalization further facilitates practical applications, especially for medical protective products with customized requirements. Herein, we present a hierarchical polyurethane/metal-organic framework (MOF) composite nanofiber membrane with an integrated radiative cooling effect and photocatalytic antibacterial property. Fabricated by a scalable electrospinning method, the hierarchical nanofiber membrane shows high solar reflectance of 97% and improved thermal emissivity of 93% attributed to the abundant chemical bonds in ZIF-8 nanoparticles, rendering a temperature drop of ∼7.2 °C under direct sunlight and ∼5.5 °C at night. In addition, the photocatalytic activity of ZIF-8 ensures a 96% bacterial mortality rate for preventing bacterial infection. In practical application, our composite fabric can prevent superheating by 4.4 °C compared with the traditional protective suit under direct sunlight. Along with its anti-infection ability, the composite fabric is desirable for medical protective textiles. The innovative integration of passive radiative cooling design and functional MOFs breaks through the traditional cooling mode and provides huge substantial advantages for smart textiles and personal cooling applications.

Covalent Grafting of a Nickel Thiolate Catalyst onto Covalent Organic Frameworks for Increased Photocatalytic Activity

Covalent organic frameworks (COFs) have recently emerged as prospective photoactive materials with noble Pt as a cocatalyst for photocatalytic hydrogen evolution. In this work, a series of SH-group-functionalized covalent organic frameworks, TpPa-1-SH-X, is prepared by reaction of p-phenylenediamine (Pa) and 1,3,5-triformylphloroglucinol (Tp) with p-NH₂ C₆ H₄ SH as a modulating agent. The reaction of TpPa-1-SH-X with Ni^II acetylacetonate Ni(acac)₂ gave nickel thiolate-immobilized TpPa-1 (TpPa-1-SNi-X). The highest hydrogen evolution rate was 10.87 mmol h^-1  g^-1 , which was an enhancement of 16.47, 3.83, and 1.84 times than that of the parent TpPa-1, covalent-bond-free [(p-NH₂ C₆ H₄ S)₂ Ni]_n /TpPa-1-SH-10, and 3 wt % Pt-deposited TpPa-1, respectively. This enhanced photocatalytic hydrogen evolution is ascribed to enhanced crystallinity, the use of Ni^II thiolate as a cocatalyst and covalent bonding between the cocatalyst and TpPa-1.

Recent Advances in g-C3N4-Based Photocatalysts for NOx Removal

Nitrogen oxides (NOx) pollutants can cause a series of environmental issues, such as acid rain, ground-level ozone pollution, photochemical smog and global warming. Photocatalysis is supposed to be a promising technology to solve NOx pollution. Graphitic carbon nitride (g-C3N4) as a metal-free photocatalyst has attracted much attention since 2009. However, the pristine g-C3N4 suffers from poor response to visible light, rapid charge carrier recombination, small specific surface areas and few active sites, which results in deficient solar light efficiency and unsatisfactory photocatalytic performance. In this review, we summarize and highlight the recent advances in g-C3N4-based photocatalysts for photocatalytic NOx removal. Firstly, we attempt to elucidate the mechanism of the photocatalytic NOx removal process and introduce the metal-free g-C3N4 photocatalyst. Then, different kinds of modification strategies to enhance the photocatalytic NOx removal performance of g-C3N4-based photocatalysts are summarized and discussed in detail. Finally, we propose the significant challenges and future research topics on g-C3N4-based photocatalysts for photocatalytic NOx removal, which should be further investigated and resolved in this interesting research field.

Defective Homojunction Porphyrin-Based Metal-Organic Frameworks for Highly Efficient Sonodynamic Therapy

Sonodynamic therapy (SDT) with non-invasiveness and high tissue-penetrating ability has attracted widespread interest in treating deep-seated tumors or infections. To enhance the treatment efficacy of SDT, the development of high-efficiency and stable sonosensitizers are still needed. Herein, a defective homojunction porphyrin-based metal-organic framework (MOF) with greatly enhanced sonocatalytic ability is easily prepared and used for SDT of osteomyelitis infected by methicillin-resistant Staphylococcus aureus (MRSA). Acetic acid and benzoic acid are chosen as modulators during the hydrothermal synthesis of porphyrin-based MOF. It is found that the crystal structure of MOF shifts from PCN-222 to PCN-224 as the amount of acetic acid increases. Interestingly, the defective PCN (D-PCN) contains a two-phase homojunction structure of PCN-222/PCN-224. The sonocatalytic reactive oxygen species production presents a volcano-type trend with increased acetic acid, among which D-PCN-2 with more content of PCN-224 has the best sonocatalytic antibacterial ability. The reduced band gap introduced a defect, and type-II homojunction structures of D-PCN-2 improve the separation of the ultrasound-triggered electron hole, which significantly enhances the SDT effect. Through a mixed linker approach, this work develops a new defect-induced homojunction MOF with great performance for SDT of MRSA-infected osteomyelitis.

Bioinspired Metalation of the Metal-Organic Framework MIL-125-NH2 for Photocatalytic NADH Regeneration and Gas-Liquid-Solid Three-Phase Enzymatic CO2 Reduction

Coenzyme NADH regeneration is crucial for sustained photoenzymatic catalysis of CO₂ reduction. However, light-driven NADH regeneration still suffers from the low regeneration efficiency and requires the use of a homogeneous Rh complex. Herein, a Rh complex-based electron transfer unit was chemically attached onto the linker of the MIL-125-NH₂ . The coupling between the light-harvesting iminopyridine unit and electron-transferring Rh-complex facilitated the photo-induced electron transfer for the NADH regeneration with the yield of 66.4 % in 60 mins for 5 cycles. The formate dehydrogenase was further deposited onto the hydrophobic layer of the membrane by a reverse filtering technique, which forms the gas-liquid-solid reaction interface around the enzyme site. It gave an enhanced formic acid yield of 9.5 mM in 24 hours coupled with the in situ regenerated NADH. The work could shed light on the construction of integrated inorganic-enzyme hybrid systems for artificial photosynthesis.

Charge Separation by Creating Band Bending in Metal-Organic Frameworks for Improved Photocatalytic Hydrogen Evolution

Metal-organic frameworks (MOFs) have been intensively studied as a class of semiconductor-like materials in photocatalysis. However, band bending, which plays a crucial role in semiconductor photocatalysis, has not yet been demonstrated in MOF photocatalysts. Herein, a representative MOF, MIL-125-NH₂ , is integrated with the metal oxides (MoO₃ and V₂ O₅ ) that feature appropriate work functions and energy levels to afford the corresponding MOF composites. Surface photovoltage results demonstrate band bending in the MOF composites, which gives rise to the built-in electric field of MIL-125-NH₂ , boosting the charge separation. As a result, the MOF composites present 56 and 42 times higher activities, respectively, compared to the pristine MOF for photocatalytic H₂ production. Upon depositing Pt onto the MOF, ∼6 times higher activity is achieved. This work illustrates band bending of MOFs for the first time, supporting their semiconductor-like nature, which would greatly promote MOF photocatalysis.

Switching on photocatalytic NO oxidation and proton reduction of NH2-MIL-125(Ti) by convenient linker defect engineering

Photocatalysis technology has been widely adopted to abate typical air pollutants. Nevertheless, developing photocatalysts aimed at improving photocatalytic efficiency is a challenge. Herein, the linker-defect NH₂-MIL-125(Ti) photocatalyst was synthesized through a convenient one-step heating-stirring method (just adjusting multiple temperatures) to firstly realize efficient photocatalytic performances of NO removal and hydrogen evolution. The optimal sample (named 65-NMIL) with a linker-defect content of 32.08% exhibited a NO removal ratio of 65.49%, which was 37.57% higher than that of pristine NH₂-MIL-125(Ti), and displayed better H₂-production activity. Through ESR, it was confirmed that 65-NMIL can generate more •O₂^- and •OH under visible light, and the radical trapping experiment further proved that •O₂^- played a more important role in photocatalytic activity. Moreover, the photocatalytic NO oxidation process was also monitored by in situ DRIFTS, it was found that the defective samples could promote the oxidation of NO and intermediates to the final product (NO₃^-). On the basis of the above-mentioned photocatalytic experimental results and characterization, a possible mechanism or pathway was proposed and illustrated. This work can provide a new strategy for the subsequent defect engineering for photocatalytic MOFs materials to further solve environmental and energy crises.

State of the art developments and prospects of metal-organic frameworks for energy applications

The progress on technologies for the cleaner and ecological transformation and storage of energy to combat effluence or pollution and the impending energy dilemma has recently attracted interest from energy research groups, particularly in the field of coordination chemistry, among inorganic chemists. Carriers for storing energy or facilitating mass and e^- transport are considered significant for energy conversion. Accordingly, considering their properties such as large surface area, low cost, customizable pore diameter, tunable topologies, low densities, and variable frameworks, MOFs (metal-organic frameworks) and their derivatives are well-suited for this purpose. MOFs are an innovative category of porous and crystalline materials, which have gained significant interest in recent years. Thus, herein, we highlight the state of the art progress on MOFs for energy-based applications, as perfect compounds and elements in compound assemblies for converting solar energy, lithium-ion arrays, fuel devices, hydrogen production, photocatalytic CO₂ reduction, proton conduction, etc. In addition, the substantial progress achieved in the production of various composites and derivatives containing MOFs with particular focus on supercapacitors and gas adsorption and storage is summarized, concentrating on the correlation between their coordination structural frameworks and applications in the field of energy. The current improved strategies, challenges, and future prospects are also presented in view of the coordination chemistry governing the structural modification of MOFs for energy applications.

Recent Advances and Challenges in Photoreforming of Biomass-Derived Feedstocks into Hydrogen, Biofuels, or Chemicals by Using Functional Carbon Nitride Photocatalysts

Photoreforming of biomass into hydrogen, biofuels, and chemicals is highly desired, yet this field of research is still in its infancy. Developing an efficient, novel, and environmentally friendly photocatalyst is key to achieving these goals. To date, the nonmetallic and eco-friendly material carbon nitride has found many uses in reactions such as water splitting, CO₂ reduction, N₂ fixation, and biorefinery, owing to its outstanding photocatalytic activity. However, a narrow light absorption range and fast charge recombination are often encountered in the pristine carbon nitride photocatalytic system, which resulted in unsatisfying photocatalytic activity. To improve the photocatalytic performance of pure carbon nitride in biomass reforming, modification is needed. In this Review, the design and preparation of functional carbon nitride, as well as its photocatalytic properties for the synthesis of hydrogen, biofuels, and chemicals through biomass reforming, are discussed alongside potential avenues for its future development.

Hierarchically Porous Metal-Organic Framework/MoS2 Interface for Selective Photocatalytic Conversion of CO2 with H2 O into CH3 COOH

Metal-organic frameworks (MOFs) provide a platform to design new heterogeneous catalysts for catalytic CO₂ reduction, but selective formation of C2 valuable liquid fuel products remains a challenge. Herein, we propose a strategy to synthesize composites by integrating MoS₂ nanosheets into hierarchically porous defective UiO-66 (d-UiO-66) to form Mo-O-Zr bimetallic sites on the interfaces between UiO-66 and MoS₂ . The active interfaces are favorable for the efficient transfer of photo-generated charge carriers and for promoting the activity, whereas, the synergy of the components at the interfaces achieves selectivity for C2 production. The d-UiO-66/MoS₂ composite facilitates the photo-catalytic conversion of gas phase CO₂ and H₂ O to CH₃ COOH under visible light irradiation without any other adducts. The evolution rate and selectivity of CH₃ COOH reached 39.0 μmol g^-1  h^-1 and 94 %, respectively, without any C1 products, suggesting a new approach for the design of highly efficient photocatalysts of CO₂ for C2 production. Theoretical calculations demonstrate the charge-polarized Zr-O-Mo aided the C-C coupling process with the largely reduced energy barrier.

Defect-Assisted Loading and Docking Conformations of Pharmaceuticals in Metal-Organic Frameworks

Understanding of drug-carrier interactions is essential for the design and application of metal-organic framework (MOF)-based drug-delivery systems, and such drug-carrier interactions can be fundamentally different for MOFs with or without defects. Herein, we reveal that the defects in MOFs play a key role in the loading of many pharmaceuticals with phosphate or phosphonate groups. The host-guest interaction is dominated by the Coulombic attraction between phosphate/phosphonate groups and defect sites, and it strongly enhances the loading capacity. For similar molecules without a phosphate/phosphonate group or for MOFs without defects, the loading capacity is greatly reduced. We employed solid-state NMR spectroscopy and molecular simulations to elucidate the drug-carrier interaction mechanisms. Through a synergistic combination of experimental and theoretical analyses, the docking conformations of pharmaceuticals at the defects were revealed.

Directional charge transportation and Rayleigh scattering for the optimal in-band quantum yield of a composite semiconductor nano-photocatalyst

The work propose a novel technique based on wavelength dispersive in situ photoluminescence spectroscopy for diagnosing the wavelength dependent directional charge transportation and Rayleigh scattering enhanced in-band quantum yield.

Co3O4@NiCo2O4 double-shelled nanocages with hierarchical hollow structure and oxygen vacancies as efficient bifunctional electrocatalysts for rechargeable Zn-air batteries

Highly efficient bifunctional oxygen electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucially important for the rechargeable Zn-air battery, a potential power source for applications in electric vehicles and grid-scale stationary storage systems. Herein, Co3O4@NiCo2O4 double-shelled nanocages (Co3O4@NiCo2O4 DSNCs) with hierarchical hollow structure and oxygen vacancies were designed and synthesized via annealing metal-organic frameworks. Co3O4@NiCo2O4 DSNCs with large specific surface area and three-dimensional interconnected mesopores and cavity not only provide more reaction sites, but also offer an efficient transport environment for reactants. Moreover, oxygen vacancies on the surfaces improve the capture of oxygen species to enhance the reactivity of the catalyst. Consequently, Co3O4@NiCo2O4 DSNCs displayed excellent bifunctional electrocatalytic performance, with a positive half-wave potential of 0.81 V (vs. reversible hydrogen electrode, RHE) for ORR (approaching the potential of commercial Pt/C catalyst) and a low potential of 1.65 V at 10 mA cm-2 for OER (exceeding Pt/C). In a practical demonstration, the Zn-air battery using Co3O4@NiCo2O4 DSNCs as the cathode delivered a satisfactory power density of 102.1 mW cm-2, comparable to the Zn-air battery with a Pt/C cathode, and exhibited much longer cycling stability.

Plasma-Induced Defects Enhance the Visible-Light Photocatalytic Activity of MIL-125(Ti)-NH2 for Overall Water Splitting

Defect engineering in metal-organic frameworks is commonly performed by using thermal or chemical treatments. Herein we report that oxygen plasma treatment generates structural defects on MIL-125(Ti)-NH₂ , leading to an increase in its photocatalytic activity. Characterization data indicate that plasma-treated materials retain most of their initial crystallinity, while exhibiting somewhat lower surface area and pore volume. XPS and FT-IR spectroscopy reveal that oxygen plasma induces MIL-125(Ti)-NH₂ partial terephthalate decarboxylation and an increase in the Ti-OH population. Thermogravimetric analyses confirm the generation of structural defects by oxygen plasma and allowed an estimation of the resulting experimental formula of the treated MIL-125(Ti)-NH₂ solids. SEM analyses show that oxygen plasma treatment of MIL-125(Ti)-NH₂ gradually decreases its particle size. Importantly, diffuse reflectance UV/Vis spectroscopy and valence band measurements demonstrate that oxygen plasma treatment alters the MIL-125(Ti)-NH₂ band gap and, more significantly, the alignment of highest occupied and lowest unoccupied crystal orbitals. An optimal oxygen plasma treatment to achieve the highest efficiency in water splitting with or without methanol as sacrificial electron donor under UV/Vis or simulated sunlight was determined. The optimized plasma-treated MIL-125(Ti)-NH₂ photocatalyst acts as a truly heterogeneous photocatalyst and retains most of its initial photoactivity and crystallinity upon reuse.

Metalation of Catechol-Functionalized Defective Covalent Organic Frameworks for Lewis Acid Catalysis

Covalent organic frameworks (COFs) have emerged as a fascinating crystalline porous material and are widely used in the field of catalysis. However, developing simple approaches to fabricate conjugated COFs with specific functional groups remains a significant challenge. In this study, the construction of defective COF-LZU1 with Lewis acid sites embedded into the frameworks is fulfilled by a facile solvent-assisted ligand exchange method. A monodentate ligand, protocatechualdehyde, is successfully introduced into the skeleton of COF-LZU1, which endows the defects in the structure of COF-LZU1 via replacement of the original coordinated benzene-1,3,5-tricarbaldehyde ligand. As-synthesized defective COF-LZU1 decorated with protocatechualdehyde is rich of free hydroxy groups for chelating with active metal ions. Specifically, after combining with Fe³⁺ , the defective COF-LZU1 shows excellent activity in catalytic alcoholysis of epoxides under mild conditions. The method reported here will open up the opportunity to incorporate different functional groups into COFs and enrich the strategies for creating new types of porous catalysts.

Active-Site Modulation in an Fe-Porphyrin-Based Metal-Organic Framework through Ligand Axial Coordination: Accelerating Electrocatalysis and Charge-Transport Kinetics

The construction of artificial solar fuel generating systems requires the heterogenization of large quantities of catalytically active sites on electrodes. In that sense, metal–organic frameworks (MOFs) have been utilized to assemble unpreceded concentration of electrochemically active molecular catalysts to drive energy-conversion electrocatalytic reactions. However, despite recent advances in MOF-based electrocatalysis, so far no attempt has been made to exploit their unique chemical modularity in order to tailor the electrocatalytic function of MOF-anchored active sites at the molecular level. Here, we show that the axial coordination of electron-donating ligands to active MOF-installed Fe-porphyrins dramatically alters their electronic properties, accelerating the rates of both redox-based MOF conductivity and the electrocatalytic oxygen reduction reaction (ORR). Additionally, electrochemical characterizations show that in multiple proton-coupled electron transfer reactions MOF-based redox hopping is not the only factor that limits the overall electrocatalytic rate. Hence, future efforts to enhance the efficiency of electrocatalytic MOFs should also consider other important kinetic parameters such as the rate of proton-associated chemical steps as well as mass-transport rates of counterions, protons, and reactants toward catalytically active sites.