Combinatorial Synthesis of Covalent Organic Framework Particles with Hierarchical Pores and Their Catalytic Application

Sponge-like inorganic-organic S-scheme heterojunction for efficient photocatalytic hydrogen evolution

Covalent organic frameworks (COFs)-based S-scheme heterojunction photocatalysts have gained considerable attention for photocatalytic hydrogen evolution. However, challenges such as limited interfacial contact and low stability persist, primarily due to uneven inorganic semiconductor coverage on the COFs surface. Therefore, constructing inorganic-organic S-scheme heterojunction photocatalysts via the in-situ growth of COFs on inorganic semiconductor surfaces shows great promise. Herein, we successfully developed a sponge-like TiO2@BTTA S-scheme heterojunction with a tight contact interface by in-situ growing COF (referred to as BTTA) on the surface of sponge-like TiO2 (referred to as ST). Density Functional Theory (DFT) calculations confirmed that the ST@BTTA hybrids exhibit the optimal adsorption and desorption capabilities for H2O and H2 molecules, respectively. Notably, the ST@BTTA-120 S-scheme heterojunction photocatalyst demonstrates an outstanding hydrogen production rate under simulated sunlight irradiation, surpassing pristine ST and BTTA by factors of 10.3 and 2.6, respectively. The enhanced photocatalytic performance is attributed to improved solar energy utilization efficiency, a larger specific surface area, and an increased interfacial contact area between ST and BTTA. X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) analyses further verify the S-scheme carrier transfer mechanism in the ST@BTTA hybrids. This research provides a valuable method for designing efficient S-scheme heterojunction photocatalysts with closely integrated interfaces for photocatalytic hydrogen production via water splitting.

Highly Crystalline and Flexible Covalent Organic Frameworks: Advancing Efficient Iodine Adsorption

Flexible covalent organic frameworks (COFs) offer distinct advantages in elasticity and adaptability over rigid COFs, but these benefits often come at the expense of crystallinity due to challenges in polymerization, complicating both synthesis and structural characterization. Current research primarily employs single flexible monomers, which limits the tunability of these frameworks. In this study, we introduce two highly crystalline, flexible COFs, ZCST-102 and ZCST-103, constructed from dual flexible monomers. These COFs exhibit large channels, permanent porosity, high chemical stability, and exceptional crystallinity, along with enhanced structural flexibility. Notably, they achieve an iodine vapor adsorption capacity of up to 4.71 g ⋅ g-1. X-ray photoelectron spectroscopy, Raman spectroscopy, and electron paramagnetic resonance spectroscopy further elucidate the interactions between iodine and the framework structures. This work emphasizes the value of incorporating flexible building blocks to maintain crystallinity while imparting functional versatility, advancing the design of dynamic porous materials.

Covalent organic framework nanomaterials: Syntheses, architectures, and applications

Covalent Organic Frameworks (COFs) are characterized by high thermochemical stability, low backbone density, well-controlled physical and chemical properties, large specific surface volume and porosity, permanently open pore structure, and various synthesis strategies. These remarkable attributes confer COFs with significant potential for a myriad of applications ranging from catalysis technology, gas separation and storage, optoelectronic materials, environmental and energy sciences, and biomedical development. There are many synthetic design methods for COF materials, and dynamic covalent chemistry is the scientific basis of COF materials-oriented design, which gives the error correction ability of the covalent assembly process, and is the key to obtaining crystallization and stability at the same time. However, "crystallinity" and "stability" in the synthesis and preparation of COF materials are often like "You can't have your cake and eat it, too": on the one hand, the reversible covalent bonds used in the synthesis of highly crystalline COF framework are easy to decompose under extreme conditions, which greatly limits its application scenarios; On the other hand, although highly stable COF materials can be prepared by using irreversible covalent bonds, it is usually poor crystalline and difficult to have high performance. In addition, the strict deoxygenation operation required for synthesizing COF materials also limits its macro preparation and large-scale application. Therefore, the synthesis strategy and efficient preparation of highly stable and crystalline COF materials are a major obstacle to the practical application of this field. This paper describes the four structures of COF materials, as well as their synthesis methods, electrical energy-storing electrocatalysis, and significant environmental protection applications. The future directions, prospects, and possible barriers to the development of these materials are envisioned in.

A Facile Micelle-Assisted Self-Assembly Method to Covalent Organic Framework Helical Nanoarchitectures

We present a micelle-assisted self-assembly strategy for synthesizing covalent organic framework (COF) helical hollow nanoribbons by using achiral monomers and surfactants. The process involves polymerization of nanowires within rod-like micelle cores, followed by their attachment to form helical nanoribbons and solvothermal crystallization to create hollow COF architectures. This method allows for the controllable synthesis of COF helical nanostructures with tunable pitch and morphology and can be extended to other COF helical architectures by variation of the amine monomer. This strategy provides new insights into designing COF helical nanostructures using achiral building blocks within micellar systems.

Synthesis of Sulfonyl Two-Dimensional Covalent Organic Frameworks for Supercapacitor Applications

Covalent organic frameworks (COFs) are attracting more attention for energy storage applications. COFs possess unique structural properties, such as highly ordered pore structures, abundant functionalization sites, and tunable chemical properties, making them ideal candidates for the development of novel energy storage materials. In this work, we synthesized sulfonyl two-dimensional (2D) covalent organic frameworks (SLD-COFs) using 4,4'-sulfonyldiphenylamine (SLD). SLD-COFs have a remarkable conjugated structure, which includes imine groups forming large π-bonds, and the conjugated structure can provide consecutive electron conduction paths, which enables SLD-COF to transfer charges more efficiently, thus improving the electrical conductivity. Additionally, the sulfonyl groups introduce redox-active sites, which participate in the redox process during electrochemical reactions and generate a pseudocapacitive effect. For a current of 0.5 A/g, the specific capacitance of the SLD-COF material was 31.5 F/g in an acidic electrolyte and 41.7 F/g in an alkaline electrolyte. The structural flexibility and good electrochemical properties of the COFs make them a potentially essential component of energy storage applications. Meanwhile, the capacitance retention of SLD-COFs reaches 78.3% after 1000 GCD cycles at a current density of 1 A/g, which indicates its good cycling stability.

Cocatalyst Embedded Ce-BDC-CeO2 S-Scheme Heterojunction Hollowed-Out Octahedrons With Rich Defects for Efficient CO2 Photoreduction

Constructing heterojunction photocatalysts with optimized architecture and components is an effective strategy for enhancing CO2 photoreduction by promoting photogenerated carrier separation, visible light absorption, and CO2 adsorption. Herein, defect-rich photocatalysts (Ni2P@Ce-BDC-CeO2 HOOs) with S-scheme heterojunction and hollowed-out octahedral architecture are prepared by decomposing Ce-BDC octahedrons embedded with Ni2P nanoparticles and subsequent lactic acid etching for CO2 photoreduction. The hollowed-out octahedral architecture with multistage pores (micropores, mesopores, and macropores) and oxygen vacancy defects are simultaneously produced during the preparation process. The S-scheme heterojunction boosts the quick transfer and separation of photoinduced charges. The formed hollowed-out multi-stage pore structure is favorable for the adsorption and diffusion of CO2 molecules and gaseous products. As expected, the optimized photocatalyst exhibits excellent performance, producing CO with a yield of 61.6 µmol h-1 g-1, which is four times higher than that of the original Ce-BDC octahedrons. The X-ray photoelectron spectroscopy, scanning Kelvin probe, and electron spin resonance spectroscopy characterizations confirm the S-schematic charge-transfer route. The key intermediate species during the CO2 photoreduction process are detected by in situ Fourier transform infrared spectroscopy to support the proposed mechanism for CO2 photoreduction. This work presents a synthetic strategy for excellent catalysts with potential prospects in photocatalytic applications.

Polyphenol-Based Bicontinuous Porous Spheres Via Amine-Mediated Polymerization-Induced Fusion Assembly

Bicontinuous porous materials, which possess 3D interconnected network and pore channels facilitating the mass diffusion to the interior of materials, have demonstrated their promising potentials in a large variety of research fields. However, facile construction of such complex and delicate structures is still challenging. Here, an amine-mediated polymerization-induced fusion assembly strategy is reported for synthesizing polyphenol-based bicontinuous porous spheres with various pore structures. Specifically, the fusion of pore-generating template observed by TEM promotes the development of bicontinuous porous networks that are confirmed by 3D reconstruction. Furthermore, the resultant bicontinuous porous carbon particles after pyrolysis, with a diameter of ≈600 nm, a high accessible surface area of 359 m2 g-1, and a large pore size of 40-150 nm manifest enhanced performance toward the catalytic degradation of sulfamethazine in water decontamination. The present study expands the toolbox of interfacial tension-solvent-dependent porous spheres while providing new insight into their structure-property relationships.

One-pot fabrication and evaluation of β-ketoenamine covalent organic frameworks@silica composite microspheres as reversed-phase/hydrophilic interaction mixed-mode stationary phase for high performance liquid chromatography

Covalent organic frameworks (COFs) show promise as a stationary phase in high performance liquid chromatography (HPLC). However, there are only a few COFs-based stationary phases developed for HPLC separation so far. Therefore, it is crucial to not only develop more varieties of COFs-type stationary phases for HPLC separation, but also to explore the retention mechanism of solutes on these stationary phases. In this paper, a new in-situ growth method was developed to prepare β-ketoenamine COF-TpPa-1@SiO2 composite microspheres, using spherical silica as the core material and COF-TpPa-1 fabricated by covalent conjugation of 1,3,5-triformylphloroglucinol (Tp) and p-phenylenediamine (Pa-1) as the COF shells. The resulting microspheres exhibit uniform morphology, good monodispersity, large specific surface area, narrow size distribution, and high stability. Due to diverse functional groups in the structure of COF-TpPa-1, the microspheres can offer multiple interactions, such as hydrophobic, π-π stacking and electron-donor-acceptor (EDA) between COFs and analytes. As a result, the COF-TpPa-1@SiO2 composite microspheres can be used as a mixed-mode stationary phase for HPLC separation. The chromatographic performance and retention mechanism of the COF-TpPa-1@SiO2 packed column were investigated by separating polar and non-polar solutes, as well as isomers, in various HPLC modes, including reversed-phase liquid chromatography (RPLC), hydrophilic interaction chromatography (HILIC), and RPLC/HILIC mixed-mode chromatography. The results showed successful separation of non-polar alkylbenzene homologues, polycyclic aromatic hydrocarbons (PAHs), and polar amines and phenols in RPLC mode. The "U-shaped" curves of retention factor with the ACN concentration in mobile phase for four nucleobases indicated that the solute retention on the column followed a mixed mode mechanism of RPLC/HILIC. Compared to a traditional C18 column, the COF-TpPa-1@SiO2 column exhibited superior separation efficiency, stability, repeatability and reproducibility in the separation of analytes with different polarities. The column enhanced the aromatic, shape and planar selectivity for PAHs and isomers through π-π interaction and improved the separation efficiency for electron-deficient compounds due to EDA effect. At last, the column was successfully used to separate and detect the residues of 5 phenylurea herbicides (PUHs) in soil. All these results indicate the potential of COFs for chromatography applications.

Hydrazone-Linked Covalent Organic Frameworks

Hydrazone-linked covalent organic frameworks (COFs) with structural flexibility, heteroatomic sites, post-modification ability and high hydrolytic stability have attracted great attention from scientific community. Hydrazone-linked COFs, as a subclass of Schiff-base COFs, was firstly reported in 2011 by Yaghi's group and later witnessed prosperous development in various aspects. Their adjustable structures, precise pore channels and plentiful heteroatomic sites of hydrazone-linked structures possess much potential in diverse applications, for example, adsorption/separation, chemical sensing, catalysis and energy storage, etc. Up to date, the systematic reviews about the reported hydrazone-linked COFs are still rare. Therefore, in this review, we will summarize their preparation methods, characteristics and related applications, and discuss the opportunity or challenge of hydrazone-linked COFs. We hope this review could provide new insights about hydrazone-linked COFs for exploring more appealing functions or applications.

A Dual-Function AgNW@COF SERS Membrane for Organic Pollutant Removal and Simultaneous Concentration Determination

Surface enhanced Raman spectroscopy (SERS) is a highly sensitive analytical detection method commonly employed in biochemical and environmental analysis. Nevertheless, the rapid movement of analytes and interfering components in flow systems can impact the real-time, online detection capability of Raman spectroscopy. To address this issue, we developed an innovative approach utilizing covalent organic framework (COF), a robust porous material with excellent water stability, to coat the surface of Ag nanowire (AgNW) for synthesizing AgNW@COF hybrid. The regular pores of the COF serve to effectively eliminate large interfering molecules while facilitating the efficient transport of specific analytes to SERS hot spots. Additionally, the fluid flow-induced scouring effect aids in excluding interfering molecules from the surface of AgNW. By incorporating AgNW@COF into a bifunctional filter membrane with simultaneous filtration and sensing capabilities, we had achieved efficient purification of organic pollutants as well as real-time identification of pollutant species and concentration.

Meta-Structured Covalent Organic Framework Nanocoatings with Active and Angle-Independent Structural Coloration

High-performance multifunctional nanocoatings not only protect and enhance substrate materials but also offer additional functionalities. This demands a sophisticated coordination of the coating's inherent properties and microstructural features. Here, a multifunctional active nanocoating via meta-structural engineering of covalent organic framework (COF) deposition materials is presented. This COF nanocoating, characterized by well-defined micropores (1-2 nm), meta-structured textures (30-300 nm), tailored thickness (100-300 nm), and good uniformness, showcases a unique combination of angle-independent structural coloration and ultrafast responsiveness to gaseous stimuli. Remarkably, it demonstrates good compatibility with a wide range of inert substrate materials, from rigid ones like glass and metal to flexible elastomers and nanomaterial films of various shapes and sizes. This versatility enables the facile development of devices that can optically report information about their environments. Examples include chemically active coatings with ultrafast (≈10 ms) color-changing behaviors and programmable actuation behaviors upon exposure to gaseous stimuli, and mechanically active coatings that can detect substrate strain up to 50% yet maintain structural robustness and consistent coloration hue. It is believed that meta-structural engineering of COF nanocoatings on inert substrates can enable them to respond to environmental stimuli, potentially indicating a new trend in developing multifunctional materials and smart devices.

Constructing ordered and tunable extrinsic porosity in covalent organic frameworks via water-mediated soft-template strategy

As one of the most attractive methods for the synthesis of ordered hierarchically porous crystalline materials, the soft-template method has not appeared in covalent organic frameworks (COFs) due to the incompatibility of surfactant self-assembly and guided crystallization process of COF precursors in the organic phase. Herein, we connect the soft templates to the COF backbone through ionic bonds, avoiding their crystallization incompatibilities, thus introducing an additional ordered arrangement of soft templates into the anionic microporous COFs. The ion exchange method is used to remove the templates while maintaining the high crystallinity of COFs, resulting in the construction of COFs with ordered hierarchically micropores/mesopores, herein named OHMMCOFs (OHMMCOF-1 and OHMMCOF-2). OHMMCOFs exhibit significantly enhanced functional group accessibility and faster mass transfer rate. The extrinsic porosity can be adjusted by changing the template length, concentration, and ratio. Cationic guanidine-based COFs (OHMMCOF-3) are also constructed using the same method, which verifies the scalability of the soft-template strategy. This work provides a path for constructing ordered and tunable extrinsic porosity in COFs with greatly improved mass transfer efficiency and functional group accessibility.

3D Crown Ether Covalent Organic Framework as Interphase Layer toward High-Performance Lithium Metal Batteries

The practical application of lithium (Li) metal batteries is inhibited by accumulative Li dendrites and continuous active Li consumption during cycling, which results in a low Coulombic efficiency and short lifetime. Constructing artificial solid-electrolyte interphase (SEI) layer in Li anode, such as 2D covalent organic frameworks (COFs), is an effective strategy to restrain the formation of Li dendrites and improve cycling performance. However, the exploration of 3D COFs as protecting layers is rarely reported, because of the preconception that the interconnect pores in 3D COFs eventually cause Li dendrites in disordered direction. 3D crown ether-based COF with ffc topology as interphase layer, in which the crown ether units are arranged in parallel and vertical orientation along the electrode, is demonstrated. The strong coupling effect between the crown ether and Li+ accelerates Li+ diffusion kinetics and enables homogeneous Li+ flux, resulting in a high Li+ transference number of 0.85 and smooth Li deposition in 3D direction. Li/COF-Cu cells display a lower Li-nucleation overpotential (17.4 mV) and high average Coulombic efficiency of ≈98.6% during 340 cycles with COF incorporation. This work gives a new insight into designing COFs for energy storage systems.

Ordered Mesoporous Crystalline Frameworks Toward Promising Energy Applications

Ordered mesoporous crystalline frameworks (MCFs), which possess both functional frameworks and well-defined porosity, receive considerable attention because of their unique properties including high surface areas, large pore sizes, tailored porous structures, and compositions. Construction of novel crystalline mesoporous architectures that allows for rich accessible active sites and efficient mass transfer is envisaged to offer ample opportunities for potential energy-related applications. In this review, the rational synthesis, unique structures, and energy applications of MCFs are the main focus. After summarizing the synthetic approaches, an emphasis is placed on the delicate control of crystallites, mesophases, and nano-architectures by concluding basic principles and showing representative examples. Afterward, the currently fabricated components of MCFs such as metals, metal oxides, metal sulfides, and metal-organic frameworks are described in sequence. Further, typical applications of MCFs in rechargeable batteries, supercapacitors, electrocatalysis, and photocatalysis are highlighted. This review ends with the possible development and synthetic challenges of MCFs as well as a future prospect for high-efficiency energy applications, which underscores a pathway for developing advanced materials.

Hierarchically Porous Covalent Organic Frameworks: Synthesis Methods and Applications

Covalent organic frameworks (COFs) with high porosity have garnered considerable interest for various applications owing to their robust and customizable structure. However, conventional COFs are hindered by their narrow pore size, which poses limitations for applications such as heterogeneous catalysis and guest delivery that typically involve large molecules. The development of hierarchically porous COF (HP-COF), featuring a multi-scale aperture distribution, offers a promising solution by significantly enhancing the diffusion capacity and mass transfer for larger molecules. This review focuses on the recent advances in the synthesis strategies of HP-COF materials, including topological structure design, in-situ templating, monolithic COF synthesis, defect engineering, and crystalline self-transformation. The specific operational principles and affecting factors in the synthesis process are summarized and discussed, along with the applications of HP-COFs in heterogeneous catalysis, toxic component treatment, optoelectronics, and the biomedical field. Overall, this review builds a bridge to understand HP-COFs and provides guidance for further development of them on synthesis strategies and applications.

Covalent Organic Frameworks: Opportunities for Rational Materials Design in Cancer Therapy

Nanomedicines are extensively used in cancer therapy. Covalent organic frameworks (COFs) are crystalline organic porous materials with several benefits for cancer therapy, including porosity, design flexibility, functionalizability, and biocompatibility. This review examines the use of COFs in cancer therapy from the perspective of reticular chemistry and function-oriented materials design. First, the modification sites and functionalization methods of COFs are discussed, followed by their potential as multifunctional nanoplatforms for tumor targeting, imaging, and therapy by integrating functional components. Finally, some challenges in the clinical translation of COFs are presented with the hope of promoting the development of COF-based anticancer nanomedicines and bringing COFs closer to clinical trials.