A Review of MOFs and Their Composites-Based Photocatalysts: Synthesis and Applications

Photocatalysis is considered to be a green and environment-friendly technology since it can convert solar energy into other types of chemical energies. Over the past several years, metal-organic frameworks (MOFs)-based photocatalysts have received remarkable research interest due to their unique morphology, high photocatalytic performance, good chemical stability, easy synthesis, and low cost. In this review, the synthetic strategies of developing MOFs-based photocatalysts are first introduced. Second, the recent progress in the fabrication of various types of MOFs composites photocatalysts is summarized. Third, the different applications including hydrogen evolution reaction, oxygen evolution reaction, overall water splitting, nitrogen reduction reaction, carbon dioxide reduction reaction as well as photodegradation of organic pollutants of MOFs-based photocatalysts are summed up. Finally, the challenges and some suggestions for the future development of MOFs- and their composites-based photocatalysts are also highlighted. It is expected that this report will help researchers to systematically devise and develop highly efficient photocatalysts based on MOFs and their composites.     

Tunable Structured Metal Oxides for Biocatalytic Therapeutics

The past few decades have witnessed the flourish of creating metal oxides for biocatalytic therapeutics due to their structural diversities, feasible modifications, tunable catalytic sites, and low cost when compared to their natural enzyme counterparts. Here, in this timely review, the most recent progress and future trends in engineering tunable structured metal oxides and decoding their structure-reactivity relationships for biocatalytic therapeutics is comprehensively summarized. At first, the fundamental activities, evaluations, and mechanisms of metal oxide-based biocatalysts are carefully disclosed. Subsequently, the merits, design methods, and state-of-art achievements of different types of nanostructured and biofunctionalized metal oxides are thoroughly discussed. Thereafter, it provides detailed comments on the catalytic center modulation strategies to engineer metal oxides for efficient reactive oxygen species (ROS)-catalysis, including atomic catalytic site engineering, heterostructures, and support effects. Furthermore, the representative applications of these ROS-catalytic metal oxides have been systematically summarized, such as catalytic disinfections, cancer therapies, ROS scavenging and anti-inflammations, biocatalytic sensors, as well as corresponding toxicities. Finally, current challenges and future perspectives are also highlighted. It is believed that this review can provide cutting-edge and multidisciplinary instruction for the future design of ROS-catalytic metal oxides and stimulate their widespread utilization in broad therapeutic applications.     

“More is Different:” Synergistic Effect and Structural Engineering in Double-Atom Catalysts

Double-atom catalysts (DACs) have emerged as a novel frontier in heterogeneous catalysis because the synergistic effect between adjacent active sites can promote their catalytic activity while maintaining high atomic utilization efficiency, good selectivity, and high stability originating from the atomically dispersed nature. In this review, the recent progress in both experimental and theoretical research on DACs for various catalytic reactions is focused. Specifically, the central tasks in the design of DACs—manipulating the synergistic effect and engineering atomic and electronic structures of catalysts—are systematically reviewed, along with the prevailing experimental, characterization, and computational modeling approaches. Furthermore, the practical applications of DACs in water splitting, oxygen reduction reaction, nitrogen reduction reaction, and carbon dioxide reduction reaction are addressed. Finally, the future challenges for DACs are summarized and an outlook on the further investigations of DACs toward heterogeneous catalysis in high-performance energy and environmental applications is provided.     

Advanced Characterization Techniques for Identifying the Key Active Sites of Gas‐Involved Electrocatalysts

Highly efficient electrocatalysts play an integral part in developing renewable energy conversion and storage technologies. Despite considerable efforts devoted to synthesizing electrocatalysts with superior performance, the identification of active moieties and understanding of reaction mechanisms under practical conditions still remain elusive. Herein, the substantial progresses in unraveling the local electronic and atomic structure optimizations of nanocatalysts for gas‐involved electrocatalysis, disclosing real active sites, and clarifying their relationships with intrinsic activities by combining advanced characterization techniques with computational simulations are summarized. The continuous development of in situ and ex situ characterization tools, particularly at multi‐scale resolution, to monitor or even directly observe the active center structure is systematically discussed, which is divided into four main categories based on the type of active sites: atomically dispersed active sites, vacancies, heteroatom doping sites, and edge sites. Current challenges and perspectives in both fundamental area and industrial application are finally proposed for the future research direction of next‐generation electrode materials. The aim of this review is to provide mechanistic insights into the real catalytically active structure with the assistance of newly developed characterization techniques, guiding the rational design and structure engineering of advanced functional materials with outstanding activity, selectivity, and durability.     

Enabling Efficient Photocatalytic Hydrogen Evolution via In Situ Loading of Ni Single Atomic Sites on Red Phosphorus Quantum Dots

Currently, red phosphorus (RP) based catalysts have shown great potential for photocatalysis due to several important intrinsic advantages. The integration of single atomic sites and RP becomes a promising solution, which has rarely been discussed. Herein, a brand-new type of photocatalyst is proposed by in situ loading Ni single atoms on the P vacancy defects of the RP quantum dots (Ni-RPQD), achieving the successful attempt of combining single atomic catalyst (SAC), RP, and QDs for the first time. The Ni-P sites act as electron antennas, which attract the photocarriers to the solid-liquid interface and activate protons to initiate an efficient hydrogen production process, resulting in a high hydrogen production rate, which is 224 times higher than that of the original RPQD and is also superior to most reported RP-based photocatalysts and competitive with the non-noble metal-based SAC photocatalysts. Theoretical explorations reveal that the atomically dispersed Ni atoms significantly lower the energy barrier for electron transfer during photocatalysis. This results in enhanced adsorption and fast dissociation of water molecules for more efficient H₂ generation. This study offers a significant and new direction for future developments of advanced and stable photocatalysts for water splitting.     

Transforming Damage into Benefit: Corrosion Engineering Enabled Electrocatalysts for Water Splitting

Producing high-purity hydrogen from water electrocatalysis is essential for the flourishing hydrogen energy economy. It is of critical importance to develop low-cost yet efficient electrocatalysts to overcome the high activation barriers during water electrocatalysis. Among the various approaches of catalyst preparation, corrosion engineering that employs the autogenous corrosion reactions to achieve electrocatalysts has emerged as a burgeoning strategy over the past few years. Benefiting from the advantages of simple synthesis, effective regulation, easy scale-up production, and extremely low cost, corrosion engineering converts the harmful corrosion process into the useful catalyst preparation, achieving the goal of “transforming damage into benefit.” Herein, the concept of corrosion engineering, fundamental reaction mechanisms, and affecting factors are firstly introduced. Then, recent progresses on corrosion engineering for fabricating electrocatalysts toward water splitting are summarized and discussed. Specific attentions are devoted to the formation mechanisms, catalytic performances, and structure–activity relations of these catalysts as well as the approaches employed for performance improvements. At last, the current challenges and future exploiting directions are proposed for achieving highly active and durable electrocatalysts. It is envisioned to shed light on the multidisciplinary corrosion engineering that is closely associated with corrosion and material science for energy and environmental applications.     

Defect Engineering of MOF-Based Membrane for Gas Separation

Metal–organic frameworks (MOFs) are highly versatile materials that have been identified as promising candidates for membrane-based gas separation applications due to their uniformly narrow pore windows and virtually unlimited structural and chemical features. Defect engineering of MOFs has opened new opportunities for manipulating MOF structures, providing a simple yet efficient approach for enhancing membrane separation. However, the utilization of this strategy to tailor membrane microstructures and enhance separation performance is still in its infancy. Thus, this summary aims to provide a guideline for tailoring defective MOF-based membranes. Recent developments in defect engineering of MOF-based membranes will be discussed, including the synthesis strategies for defective MOFs, the effects of defects on the gas adsorption properties, gas transport mechanisms, and recently reported defective MOF-based membranes. Furthermore, the emerging challenges and future prospects will be outlined. Overall, defect engineering offers an exciting opportunity to improve the performance of MOF-based gas membranes. However, there is still a long way to go to fully understand the influence of defects on MOF properties and optimize the design of MOF-based membranes for specific gas separation applications. Nonetheless, continued research in this field holds great promise for the development of next-generation membrane-based gas separation technologies.     

Recent Advance of Atomically Dispersed Dual-Metal Sites Carbocatalysts: Properties,Synthetic Materials,Catalytic Mechanisms,and Applications in Persulfate-Based Advanced Oxidation Process

Recently, atomically dispersed dual-metal sites carbocatalysts (DMSCs) make a wave in the field of persulfate-based advanced oxidation processes (PS-AOPs) in light of their ≈100% atomic utilization efficiency, high density of active sites, and superior catalytic activity. This review aims to provide a state-of-the-art overview on the development of DMSCs for activating PS. Initially, the types and properties of DMSCs are summarized, as well as the role of doping different heteroatoms is discussed. Subsequently, the properties of different carbon carriers and the methods for the synthesis of DMSCs are outlined. After that, the mechanism and application of DMSCs for the activation of PS toward organic contaminants degradation are revealed. Particularly, the mechanism of nonradical pathway is described, and the necessity of coupling DMSCs-based PS-AOPs to other processes for practical water treatment is emphasized. Finally, the formidable challenges and future research directions of DMSCs are proposed. This review is expected to provide insight into the preparation of DMSCs in the field of nanomaterials and to broaden the path for their environmental applications.     

Creating Atomic Ordering in Electrocatalysis

Catalysis always proceeds in a chaotic fashion. Therefore, identifying the working principles of heterogeneous catalysts is a challenging task. Creating atomic order in heterogeneous catalysts simplifies this task and also offers new opportunities for rationally designing active sites to manipulate catalytic performance. The recent rapid advances in heterogeneous electrocatalysis have led to exciting progress in the construction of atomically ordered materials. Here, the latest progress in electrocatalysts with the periodic atomic arrangement, including intermetallic compounds with long-range order and metal atom-array catalysts with short-range order is summarized. The synthesis principles and the intriguing physical and chemical properties of these electrocatalysts are discussed. Furthermore, the compelling prospects of atomically ordered catalysts in the frontier of catalyst research are outlined.     

Atomic‐Scale CoOx Species in Metal–Organic Frameworks for Oxygen Evolution Reaction

The activity of electrocatalysts strongly depends on the number of active sites, which can be increased by downsizing electrocatalysts. Single‐atom catalysts have attracted special attention due to atomic‐scale active sites. However, it is a huge challenge to obtain atomic‐scale CoO_x catalysts. The Co‐based metal–organic frameworks (MOFs) own atomically dispersed Co ions, which motivates to design a possible pathway to partially on‐site transform these Co ions to active atomic‐scale CoO_x species, while reserving the highly porous features of MOFs. In this work, for the first time, the targeted on‐site formation of atomic‐scale CoO_x species is realized in ZIF‐67 by O₂ plasma. The abundant pores in ZIF‐67 provide channels for O₂ plasma to activate the Co ions in MOFs to on‐site produce atomic‐scale CoO_x species, which act as the active sites to catalyze the oxygen evolution reaction with an even better activity than RuO₂.     

Pristine Metal–Organic Frameworks and their Composites for Renewable Hydrogen Energy Applications

Metal–organic frameworks (MOFs) have emerged as ideal multifunctional platforms for renewable hydrogen (H₂) energy applications owing to their tunable chemical compositions and structures and high porosity. Their advanced component species and porous structure contribute greatly to the enhanced activity, electrical conductivity, photo response, charge-hole separation efficiency, and structural stability of MOF materials, which are promising for practical H₂ economy. In this review, we mainly introduce design strategies for the enhancement of electro-/photochemical behaviors or adsorption performance of porous MOF materials for H₂ production, storage, and utilization from compositional perspective. Following these engineering strategies, the correlation between composition and property-structure-performance of pristine MOFs and their composite with advanced components is illustrated. Finally, challenges and directions of future development of related MOFs and MOF composites for H₂ economy are provided.     

2D Molybdenum Compounds for Electrocatalytic Energy Conversion

The development of advanced nanomaterials is urgent for electrocatalytic energy conversion applications. Recently, 2D nanomaterial-derived heterogeneous electrocatalysts have shown great promise for both fundamental research and practical applications owing to their extremely high surface-to-volume ratio and tunable geometric and electronic properties. Because of their unique electronic structure and physicochemical properties, molybdenum (Mo)-based 2D nanomaterials are emerging as one of the most attractive candidates among the nonprecious materials for electrocatalysts. This review provides a comprehensive overview of the recent advances in the synthesis and modulation of 2D Mo compounds for applications in electrocatalytic energy conversion. The categories based on different compositions and corresponding synthetic approaches of 2D Mo compounds are first introduced. Subsequently, various atomic/plane/synergistic engineering strategies, along with catalytic optimization in the electrochemical process that involves the cycles of water, carbon, and nitrogen, are discussed in detail. Finally, the current challenges and future opportunities for the development of 2D Mo-based electrocatalysts are proposed with the goal of shedding light on these promising 2D nanomaterials for electrocatalytic energy conversion.     

Bioinspired Self-Assembling Materials for Modulating Enzyme Functions

Enzymes tend to malfunction when they work out of their natural cellular environments. Engineering a favorable microenvironment around enzymes has emerged as an effective strategy to finely tune the enzymatic functions and reshape the biocatalytic activities. Supramolecular self-assembly provides a bottom-up approach for spatial arrangement of functional groups and fabrication of materials with tailorable local properties. In this review, the progress in designing, creating, and tailoring the enzyme microenvironments is discussed, with the bioinspired self-assembling materials as the scaffolds built from molecular building blocks. The relationship between the physicochemical properties and the local environments (pH, substrates, or hydration) of the scaffolds, and the catalytic properties of the scaffolded enzymes are focused upon. The power of the self-assembly to regulate the catalytic systems dynamically is also highlighted. In the end, an outlook on the obstacles, possible solutions, and future directions on the microenvironment engineering of enzymes is provided.     

Molecular Engineering to Tune the Ligand Environment of Atomically Dispersed Nickel for Efficient Alcohol Electrochemical Oxidation

Atomically dispersed metals maximize the number of catalytic sites and enhance their activity. However, their challenging synthesis and characterization strongly complicates their optimization. Here, the aim is to demonstrate that tuning the electronic environment of atomically dispersed metal catalysts through the modification of their edge coordination is an effective strategy to maximize their performance. This article focuses on optimizing nickel-based electrocatalysts toward alcohol electrooxidation in alkaline solution. A new organic framework with atomically dispersed nickel is first developed. The coordination environment of nickel within this framework is modified through the addition of carbonyl (CO) groups. The authors then demonstrate that such nickel-based organic frameworks, combined with carbon nanotubes, exhibit outstanding catalytic activity and durability toward the oxidation of methanol (CH₃OH), ethanol (CH₃CH₂OH), and benzyl alcohol (C₆H₅CH₂OH); the smaller molecule exhibits higher catalytic performance. These outstanding electrocatalytic activities for alcohol electrooxidation are attributed to the presence of the carbonyl group in the ligand chemical environment, which enhances the adsorption for alcohol, as revealed by density functional theory calculations. The work not only introduces a new atomically dispersed Ni-based catalyst, but also demonstrates a new strategy for designing and engineering high-performance catalysts through the tuning of their chemical environment.     

Ultrafine Pt‐Based Nanowires for Advanced Catalysis

At the frontier of electrocatalysis and heterogeneous reactions, significant effort has been devoted to Pt‐based nanomaterials owing to their advantages of tunable morphology and excellent catalytic properties. In contrast to Pt‐based nanocatalysts with other morphologies, nanowire catalysts, especially 1D ultrafine nanowire (NW) structure, are garnering increased attention because of their advantages of high atomic efficiency, intrinsic isotropy, rich high‐index facets, better conductivity, robust structure stability for prohibiting dissolution, ripening, and aggregation. Regardless of these advantages, it is still challenging to realize the precise control of ultrafine Pt‐based NWs in terms of their size, crystal phase structure, and composition. Aiming to synthesize advanced ultrafine Pt‐based NWs catalysts with higher activity, durability, and selectivity toward catalytic reactions, this review summarizes the recently available approaches for improving the catalytic performance of ultrafine Pt‐based NWs with detailed guidance. A summary of recent progress in ultrafine Pt‐based NWs catalysts for advanced catalysis and heterogeneous reactions is also provided. Furthermore, integrated experimental and theoretical studies are reviewed to explain the activity, stability, and selectivity enhancement mechanism. In the final section, the challenges and outlook are also discussed to provide guidance for the rational engineering of efficient ultrafine Pt‐based NWs catalysts for applications in renewable‐energy‐related devices.     

The Role of Defects in Metal–Organic Frameworks for Nitrogen Reduction Reaction: When Defects Switch to Features

The electrochemical nitrogen reduction reaction (NRR), a contributor for producing ammonia under mild conditions sustainably, has recently attracted global research attention. Thus far, the design of highly efficient electrocatalysts to enhance NRR efficiency is a specific focus of the research. Among them, defect engineering of electrocatalysts is considered a significant way to improve electrocatalytic efficiency by regulating the electronic state and providing more active sites that can give electrocatalysts better physicochemical properties. Recently, metal–organic frameworks (MOFs), along with their derivatives, have captured immense interest in electrocatalytic reactions owing to not only their large surface area and high porosity but also the ability to create rich defects in their structures. Hence, they can provide plenty of exposed active sites for electron transfer, NN cleavage, and N₂ adsorption to enhance NRR performance. Herein, the concept, the in situ characterizations techniques for defects, and the most common ways to create defects into MOFs have been summarized. Furthermore, the recent advances of MOF-based electrocatalysts towards NRR have been recapitulated. Ultimately, the major challenges and outlook of defects in MOFs for NRR are proposed. This paper is anticipated to provide critical guidelines for optimizing NRR electrocatalysts.     

Defect Engineering in Carbon‐Based Electrocatalysts: Insight into Intrinsic Carbon Defects

Functionalized carbon nanomaterials, as significant options for renewable energy systems, are widely utilized in diversified electrochemical reactions in virtue of property advantages. The inevitable defect sites in architectures greatly affect physicochemical properties of carbon nanomaterials, thus defect engineering has recently become a vital research orientation of carbon‐based electrocatalysts. The intentionally introduced intrinsic carbon defect sites in the frameworks can directly serve as the potential active sites owing to the altered surface charge state, modulated adsorption free energy of key intermediates, as well as diminished bandgap. Furthermore, the synergistic sites between intrinsic defects and heteroatom dopants/captured atomic metal species can further optimize the electronic structure and adsorption/desorption behavior, making carbon‐based catalysts comparable to commercial precious metal catalysts in electrocatalysis. With pressing research demands, the common configurations, construction strategies, structure–activity relationships, and characterization methods for intrinsic carbon defect‐involved catalytic centers are systematically summarized. Such theoretical and experimental evidences of intrinsic defect‐induced activity can reveal the active centers and relevant catalytic mechanism, thereby providing necessary guidance for the design and construction of highly efficient carbon‐based electrocatalysts and promoting their commercial applications.     

Defective Photocathode: Fundamentals,Construction, and Catalytic Energy Conversion

The development of a long-term and sustainable energy economy is one of the most significant technological challenges facing humanity. Photoelectrochemical (PEC) technology is considered as the most attractive route for converting solar energy into chemical energy. However, the slow reaction kinetics of PEC oxidation and reduction greatly hinder its practical application. To address this issue, engineering photoelectrodes with various defects can significantly improve their catalytic performance, which can not only regulate catalyst electronic structure but also promote charge transfer/separation by serving as an active/adsorption/energy storage site. Herein, the defect engineering strategies for photoelectrodes are systematically summarized, focusing on the latest progress in defective photocathode for energy conversion. First, an overview of defect types, basic principles of photocathode, and the positive role of defects in the photocathode are provided. Second, the construction strategies and characterization methods of defective photocathode are summarized. Then, the progress of typical energy conversion applications, including hydrogen production, CO₂ reduction, and nitrogen reduction over defective photocathode, is reviewed, highlighting the crucial role of defects in high catalytic performance. Finally, the challenges and future prospects of defective photocathode are discussed, aiming to bring new opportunities for the development of photocathode through defect engineering.     

Laser-Assisted Printing of Electrodes Using Metal–Organic Frameworks for Micro-Supercapacitors

Direct laser scribing, an advanced printing technique, has been recently developed to enable the carbonization of carbonaceous precursors in a rapid, precise, and cost-effective manner. Herein, it is reported that metal−organic frameworks (MOFs) can be converted into patterned derived carbon with desired structural features using a CO₂ infrared laser system. Metal species in MOFs play a key role in the morphology, porous structure, and crystallinity of the resulting laser-induced products by studying six representative MOFs. Diverse features such as ordered porous structure and continuous network microstructure can be obtained in the laser-induced MOF-derived carbon, which is influenced by the melting and boiling points of metals and their magnetic and catalytic behaviors. Furthermore, a core–shell structured composite (MOF-199@ZIF-67) has been designed and prepared for the fabrication of 12-interdigital electrodes derived from the composite by laser-assisted printing. The as-obtained electrodes with highly porous and hierarchical structure show an enhanced specific capacitance for micro-supercapacitors (MSCs). This work provides a complementary heat treatment method to produce MOF-derived carbon nanomaterials with desired structural features and patterns for MSCs and micro-device-related applications.     

Understanding of Neighboring Fe-N4-C and Co-N4-C Dual Active Centers for Oxygen Reduction Reaction

Single atomic dispersed M-N-C (M = Fe, Co, Ni, Cu, etc.) composites display excellent performance for catalytic reactions. However, the analysis and understanding of neighboring M-N-C centers at the atomic level are still insufficient. Here, FeCo-N-doped hollow carbon nanocages (FeCo-N-HCN) with neighboring Fe-N₄-C and Co-N₄-C dual active centers as efficient catalysts are reported. Spherical aberration-corrected high angle annular dark-field scanning transmission electron microscopy, small area (1 nm²) electron energy loss spectroscopy, and X-ray absorption spectroscopy data analysis and fitting prove the neighboring Fe-N₄-C and Co-N₄-C dual active structure in FeCo-N-HCN. Experimental tests and density functional theory calculation results reveal that the FeCo-N-HCN catalyst displays better catalytic activity than Fe single-metal catalyst for oxygen reduction reaction (ORR), which is attributed to the synergistic effect of Fe-N₄-C and Co-N₄-C dual active centers reducing the reaction energy barriers for ORR. Although the catalytic performance of the FeCo-N-HCN catalyst is not comparable to the-state-of-art catalysts reported due to the low metal contents (Fe: 1.96 wt% and Co: 1.31 wt%), these results can refresh the understanding of neighboring M-N-C centers at the atomic level and provide guidance for the design of catalysts in the future.