Two-dimensional Covalent Organic Frameworks Research Articles

Hierarchical nanocomposites of redox covalent organic frameworks nanowires anchored on graphene sheets for super stability supercapacitor

Covalent organic frameworks (COFs) have shown significant potential as ideal electrode materials for supercapacitors (SCs) due to their highly accessible surface areas and tunable active sites. However, their poor electrochemical properties hinder their application prospects in electrochemical energy storage devices. Herein, we successfully synthesized two-dimensional COF (TpPa-(OH)2) nanowires anchored on the surface of reduced graphene oxide (TpPa-(OH)2/rGO) using a facile hydrothermal method in aqueous solution. The anchored COFs nanowires effectively prevent graphene sheets from stacking, thereby enhancing the electronic double layer capacitor of rGO and introducing pseudo-capacitance contributed by the redox COFs. Additionally, rGO's inherent conductivity facilitates electron transfer and ion migration, improving electrochemical properties. By regulating the content ratio of graphene and COF, the optimal TpPa-(OH)2/rGO shows obvious COFs nanowires structure anchored on graphene sheets and exhibits exceptional electrochemical performance, with a high specific capacitance of 371.1 F/g at 0.5 A/g and excellent cycle stability of 93% after 20,000 cycles at 10 A/g. Furthermore, we assembled symmetric SCs (SSCs) using TpPa-(OH)2/rGO-3, achieving a specific capacitance of 197.1 F/g at 0.2 A/g and delivering a maximum energy density of 16.6 Wh/kg at a power density of 158.7 W/kg. These results unambiguously indicate the significant potential of TpPa-(OH)2/rGO as a high-performance electrode material in energy-storage devices.

Dynamic Insights into the Growth Mechanisms of 2D Covalent Organic Frameworks on Graphene Surfaces.

Producing high-quality two-dimensional (2D) covalent organic frameworks (COFs) is crucial for industrial applications. However, this remains significantly challenging with current synthetic techniques. A deep understanding of the intermolecular interactions, reaction temperature, and oligomers is essential to facilitate the growth of highly crystalline COF films. Herein, molecular dynamics simulations were employed to explore the growth of 2D COFs from monomer assemblies on graphene. Our results showed that chain growth reactions dominated the COF surface growth and that van der Waals (vdW) interactions were important in enhancing the crystallinity through monomer preorganization. Moreover, appropriately tuning the reaction temperature improved the COF crystallinity and minimized the effects of amorphous oligomers. Additionally, the strength of the interface between the COF and the graphene substrate indicated that the adhesion force was proportional to the crystallinity of the COF. This work reveals the mechanisms for nucleation and growth of COFs on surfaces and provides theoretical guidance for fabricating high-quality 2D polymer-based crystalline nanomaterials.

High-Strength Polycrystalline Covalent Organic Framework with Abnormal Thermal Transport Insensitive to Grain Boundary.

Grain boundaries (GBs) in two-dimensional (2D) covalent organic frameworks (COFs) unavoidably form during the fabrication process, playing pivotal roles in the physical characteristics of COFs. Herein, molecular dynamics simulations were employed to elucidate the fracture failure and thermal transport mechanisms of polycrystalline COFs (p-COFs). The results revealed that the tilt angle of GBs significantly influences out-of-plane wrinkles and residual stress in monolayer p-COFs. The tensile strength of p-COFs can be enhanced and weakened with the tilt angle, which exhibits an inverse relationship with the defect density. The crack always originates from weaker heptagon rings during uniaxial tension. Notably, the thermal transport in p-COFs is insensitive to the GBs due to the variation of minor polymer chain length at defects, which is abnormal for other 2D crystalline materials. This study contributes insights into the impact of GBs in p-COFs and offers theoretical guidance for structural design and practical applications of advanced COFs.

Synthesis of nanoflower-shaped covalent organic framework fluorescent probe for sensitive detection of aluminum ions

The widespread application of aluminum has led to numerous adverse effects on organisms due to aluminum pollution. In this study, we synthesized a two-dimensional fluorescent covalent organic framework (TAPB-DMTP-COF) material with nanoflower morphology for fluorescence detection of aluminum ions. The coordination binding of Al3+ with the nitrogen atoms on the imine bonds and the oxygen atoms on the methoxy groups restricts intramolecular rotation in TAPB-DMTP-COF, which inhibits the photo-induced electron transfer (PET) in the COF molecule and triggers the fluorescence emission "turn on" phenomena. A fluorescence method for Al3+ detection was established, and it was found that the method exhibits high sensitivity, selectivity, a wide detection range from 4.0 to 100.0 μM, and a low limit of detection (LOD) of 38.5 nM. This work provided new ideas for the convenient and rapid detection of aluminum ions in environmental analysis and expanded the application of fluorescence COFs in analytical sensing.

Tuning Topology of Two-Dimensional Trifluoromethyl-Functionalized Covalent Organic Framework for Efficient Separation of Isomers

Tuning Topology of Two-Dimensional Trifluoromethyl-Functionalized Covalent Organic Framework for Efficient Separation of Isomers

On topological analysis of two-dimensional covalent organic frameworks via M-polynomial

Covalent organic frameworks (ZnP-COFs) made of zinc-porphyrin have become effective materials with a variety of uses, including gas storage and catalysis. To simulate the structural and electrical features of ZnP-COFs, this study goes into the computation of polynomials utilizing degree-based indices. We gave a methodical study of these polynomial computations using Excel, illustrating the complex interrelationships between the various indices. Degree-based indices provide valuable insights into the connectivity of vertices within a network. M-polynomials, on the other hand, offer a mathematical framework for representing and studying the properties of 2D COFs. By encoding structural information into a polynomial form, M-polynomials facilitate the calculation of various topological indices, including the Wiener index, Zagreb indices, and more. The different behavior of ZnP-COFs based on degree-based indices was illustrated graphically, and this comparison provided insightful information for prospective applications and the construction of innovative ZnP-COF structures. Moreover, we discuss the relevance of these techniques in the broader context of materials science and the design of functional covalent organic frameworks.

Single-atom platinum with asymmetric coordination environment on fully conjugated covalent organic framework for efficient electrocatalysis

Two-dimensional (2D) covalent organic frameworks (COFs) and their derivatives have been widely applied as electrocatalysts owing to their unique nanoscale pore configurations, stable periodic structures, abundant coordination sites and high surface area. This work aims to construct a non-thermodynamically stable Pt-N2 coordination active site by electrochemically modifying platinum (Pt) single atoms into a fully conjugated 2D COF as conductive agent-free and pyrolysis-free electrocatalyst for the hydrogen evolution reaction (HER). In addition to maximizing atomic utilization, single-atom catalysts with definite structures can be used to investigate catalytic mechanisms and structure-activity relationships. In this work, in-situ characterizations and theoretical calculations reveal that a nitrogen-rich graphene analogue COF not only exhibits a favorable metal-support effect for Pt, adjusting the binding energy between Pt sites to H* intermediates by forming unique Pt-N2 instead of the typical Pt-N4 coordination environment, but also enhances electron transport ability and structural stability, showing both conductivity and stability in acidic environments.

Metal-free thiophene-based covalent organic frameworks achieve efficient photocatalytic hydrogen evolution by accelerating interfacial charge transfer

Photocatalysis is widely recognized as an efficient and environmentally friendly pathway for the conversion of solar energy into chemical energy. In recent years, designing efficient hydrogen evolution to obtain green clean energy has been one of the major challenges faced by researchers. Covalent organic frameworks (COFs) are considered as new and effective photocatalyst due to the inherent porosity, high crystallinity and structural adjustability. In this study, two metal-free thiophene-based covalent organic frameworks (JLNU-308 and JLNU-309) were constructed by solvothermal method through effective modulation sites. Because JLNU-COFs has a narrow band gap and good absorption capacity in the spectral range, the interfacial charge transfer speed is accelerated to solve the problem of low charge separation efficiency. Remarkably, JLNU-309 displays an impressive hydrogen evolution rate of 2868.57 μmol g−1 h−1 under visible light irradiation (λ > 420 nm). The superior hydrogen evolution reaction (HER) activity is due to the triazine units, which have high visible light absorption capacity, one-dimensional nanochannels of two-dimensional COFs and synergy with charge mobility. The photoelectrochemical measurements and calculation of density functional theory (DFT) supports the accuracy of the experiment. This work opens an avenue for constructing functional covalent organic frameworks for environmentally relevant critical applications. In addition, the simple synthesis process, excellent activity and high chemical stability indicate that JLNU-COFs are promising photocatalytic hydrogen production materials.

High Sodium Ion Storage by Multifunctional Covalent Organic Frameworks for Sustainable Sodium Batteries.

Rechargeable sodium batteries hold great promise for circumventing the increasing demand for lithium-ion batteries (LIBs) and the limited supply of lithium. However, efficient sodium ion storage remains a great impediment in this field. In this study, we report the designed synthesis of a multifunctional two-dimensional covalent organic framework featuring hexaazatrinaphthalene cores linked by imidazole moieties and demonstrate its effective performance in sodium ion storage. Benzimidazole-linked covalent organic framework (BCOF-1) was synthesized by a condensation reaction between hexaazatrinaphthalenehexamine (HATNHA) and terephthalaldehyde (TA) and exhibited a high theoretical specific capacity of 392 mA h g-1. BCOF-1 crystallizes, forming eclipsed AA stacking and mesoporous hexagonal one-dimensional channels with high surface area (840 m2 g-1), facilitating fast ionic mobility and charge transfer and enabling high-rate capability at high current rates. BCOF-1 exhibits pseudocapacitive-like behavior with a high specific capacity of 387 mA h g-1, an energy density of 302 W h kg-1 at 0.1 C, and a power density of 682 W kg-1 at 5 C. Our results demonstrate that redox-active COFs have the desired structural and electronic merits to advance the use of organic electrodes in sodium-ion storage toward sustainable and efficient batteries.

Modulated synthesis of stoichiometric and sub-stoichiometric two-dimensional covalent organic frameworks for enhanced ethylene purification

Controlled synthesis of two-dimensional covalent organic frameworks (2D COFs), including stoichiometric and sub-stoichiometric variations, is a topic of growing interest due to its potential in gas separation applications. In this study, we successfully synthesized three distinct 2D COFs by carefully adjusting solvent compositions and monomer ratios during the synthesis of [4 + 4] type COFs. These included a stoichiometric [4 + 4] type COF and two sub-stoichiometric [4 + 2] type COFs, featuring unreacted amino or formyl groups. The resulting COFs exhibit different gas adsorption and separation properties. Specifically, sub-stoichiometric COF-DA with residual amino groups shows comparable adsorption capacity for C2H2, C2H4, and CO2 to stoichiometric COF-DAPy. In contrast, sub-stoichiometric COF-Py with residual formyl groups displays enhanced adsorption selectivity for C2H2/C2H4 and C2H2/CO2 separation, with the C2H2/C2H4 selectivity being the highest among reported COFs, attributed to increased pore polarity resulting from the presence of formyl groups. This study not only offers an additional example of sub-stoichiometric COF synthesis but also advocates for further exploration of sub-stoichiometric COF materials, particularly in the field of gas adsorption and separation.

A new nitrogen-rich imine-linked neutral covalent organic framework: Synthesis and high-efficient adsorption of organic dyes

Nowadays, water pollution caused by organic dyes has received increasing attentions due to serious harm to public health and ecosystem, and it is of great significance to design new porous materials with high adsorption capacity to remove organic dye pollutants from wastewater. Herein, a new two-dimensional N-rich imine-linked covalent organic framework (termed as BPA-TAPA COF) has been constructed through Schiff-base reaction between 2, 2′-bipyridine-5,5′-dicarboxaldehyde (BPA) and tri(4-aminophenyl)amine (TAPA) under solvothermal conditions, and showed good crystallinity, high thermal stability and certain porosity. Then, it was used as a functional adsorbent for removal of organic dyes, which exhibited good adsorption performances for both cationic and anionic dyes, namely malachite green (MG), Janus green B (JGB), Congo red (CR), and acid orange (AO). Significantly, the adsorption capacities of BPA-TAPA COF for MG and CR reached up to 1070 and 2016 mg·g−1, in which the latter ranked as the highest in comparison to those of the other documented functional COFs. The removal efficiency of BPA-TAPA COF toward CR (100 ppm) in aqueous solution reached to 99.0% within 180 min. Hydrogen bonds and π-π stacking attractions between COF and dye molecules played crucial roles in adsorption of organic dyes. This work uncovers that neutral COFs with nitrogen-rich functional sites can be used as new adsorbents for removal of both cationic and anionic organic dyes and might provide new information for developing effective adsorbents with high adsorption capacity.

A pH-regulated fluorescence covalent organic framework for quantitative water content detection in methanol

In this paper, we designed and synthesized a two-dimensional fluorescent covalent organic framework (TAPB-DMTP-COF) for the precise determination of H2O content in methanol. The COF was synthesized using two typical monomers by grinding method, which significantly reduced the synthesis time. By adjusting the pH value of the COF suspension to 4.0, the portion of the COF material structure is disrupted, thereby mitigating π-π stacking and resolving the aggregation-caused quenching (ACQ) effect. Consequently, the non-fluorescent TAPB-DMTP-COF exhibited blue-purple fluorescence emission in methanol. At the same time, it is observed that in the presence of H2O, there is a red shift in the maximum fluorescence emission peak of TAPB-DMTP-COF, which correlates with the H2O content within a specific range. Notably, this redshift demonstrates a linear relationship with H2O content from 4% to 80% in methanol. Our work presents novel insights for efficient analysis and detection of H2O content in methanol and could be used for H2O detection in other organic solvents.

Understanding the effect of layer spacing on water desalination through two-dimensional covalent organic framework membranes via molecular dynamics simulation

The desalination performance of covalent organic framework (COF) membrane can be tuned through adjusting its layer spacing, which can improve the water permeance while maintaining high ion rejection rate. However, the mechanism of the influence of COF membrane spacing on its seawater desalination performance is still unclear. In this work, the molecular dynamics (MD) simulation were performed to systematically investigate the influence of the layer spacing on desalination performance of AB-stacked MPCOF, including 0.34 nm, 0.44 nm, 0.54 nm and 0.64 nm. The findings indicated a decrease in the resistance of water molecules passing through the AB stacked MPCOF membrane, leading to a significant increase in water permeance as the layer spacing increases. The interlayer spacing of the COF membrane also significantly influenced its interactions with ions. When the layer spacing exceeded 0.44 nm, the ion rejection rate began to decrease. As a result, the AB-stacked MPCOF membranes with 0.44 nm possessed the superior desalination performance, which was much better than other conventional membranes. Our work explored the mechanisms of the effects of COF layer spacing on desalination performance, which might provide valuable suggestions for the optimization of subsequent COF desalination membranes.

Iron-decorated covalent organic framework as efficient catalyst for activating peroxydisulfate to degrade 2,4-dichlorophenol: Performance and mechanism insight

Herein, a novel two-dimensional double-pore covalent organic framework (JLNU-305) was synthesized using N,N,N',N'-tetrakis(4-aminophenyl)-1,4-phenylenediamine (TAPD) and 2,2′-bipyridine-5,5′-dicarboxaldehyde (BPDA). The extended π-π conjugated structure and nitrogen-riched pyridine in JLNU-305 (JLNU = Jilin Normal University) provide abundant binding sites for Fe doping. The obtained JLNU-305-Fe exhibited high and recycled catalytic efficiency for peroxydisulfate (PDS) activation to completely degrade 10 mg/L 2,4-dichlorophenol (2,4-DCP) within 8 min. The JLNU-305-Fe/PDS system showed excellent catalytic activity and cyclic stability. The capture experiments and electron paramagnetic resonance (ESR) analysis indicated that the catalytic behavior of JLNU-305-Fe/PDS is contributed to the synergistic effect between free radicals and non-free radicals. It is the first time to activate PDS for covalent organic frameworks (COFs) being used to degrade 2,4-DCP, which has a great potential for development and practical application in related water environment remediation.

Advancements in two-dimensional covalent organic framework nanosheets for electrocatalytic energy conversion: current and future prospects

Humanity is confronting significant environmental issues due to rising energy demands and the unchecked use of fossil fuels. Thus, the strategic employment of sustainable and environmentally friendly energy sources is becoming increasingly vital. Additionally, addressing challenges, such as low reactivity, suboptimal energy efficiency, and restricted selectivity, requires the development of innovative catalysts. Two-dimensional (2D) covalent organic frameworks (COFs), known for their limitless structural versatility, are proving to be important materials in energy conversion applications. The exceptional properties of 2D COFs, including an organized arrangement resulting in well-defined active sites and π-π stacking interactions, enable breakthroughs in sustainable energy conversion applications. In this study, we comprehensively investigate universal synthesis methods and specific techniques, such as membrane-based deposition, liquid-phase intercalation, and polymerization. Furthermore, we demonstrate energy-conversion applications of 2D COFs as eco-friendly catalysts for electrochemical processes to promote sustainability and scalability by utilizing them in the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, and carbon dioxide reduction reaction. Additionally, we will explore methods for analyzing the physicochemical properties of precisely fabricated 2D COFs. Despite extensive research pertaining to 2D COFs, their practical industrial applications remain limited. Therefore, we propose various perspectives, including enhancing performance, improving synthesis methods, developing binder-free catalysts, expanding catalyst functionality, and advancing full-cell research, to achieve complete industrialization by leveraging their potential.

Control over Charge Separation by Imine Structural Isomerization in Covalent Organic Frameworks with Implications on CO2 Photoreduction.

Two-dimensional covalent organic frameworks (COFs) are an emerging class of photocatalytic materials for solar energy conversion. In this work, we report a pair of structurally isomeric COFs with reversed imine bond directions, which leads to drastic differences in their physical properties, photophysical behaviors, and photocatalytic CO2 reduction performance after incorporating a Re(bpy)(CO)3Cl molecular catalyst through bipyridyl units on the COF backbone (Re-COF). Using the combination of ultrafast spectroscopy and theory, we attributed these differences to the polarized nature of the imine bond that imparts a preferential direction to intramolecular charge transfer (ICT) upon photoexcitation, where the bipyridyl unit acts as an electron acceptor in the forward imine case (f-COF) and as an electron donor in the reverse imine case (r-COF). These interactions ultimately lead the Re-f-COF isomer to function as an efficient CO2 reduction photocatalyst, while the Re-r-COF isomer shows minimal photocatalytic activity. These findings not only reveal the essential role linker chemistry plays in COF photophysical and photocatalytic properties but also offer a unique opportunity to design photosensitizers that can selectively direct charges.

Mechanical interlocking induced emission in two-dimensional covalent organic frameworks

The development of two-dimensional (2D) solid-state luminescent covalent organic frameworks (COFs) remains an intriguing challenge due to the aggregation quenching caused by π-π interactions. In this study, a novel approach utilizing mechanical interlocking was employed to construct two 2D-COFs, namely mTPE-COF-1 and mTPE-COF-2. These COFs exhibited robust solid-state emissive properties. Fully characterization and spectroscopic analysis revealed that the regular interleaving mechanical restraints was able to mitigate the π-π stacking interactions in the two 2D-COFs. As a result, this inhibition inactivates non-radiation deactivation and turns on the luminescence. Theoretical calculations support the promotion of mechanical interlocking for emissions. Furthermore, by coating mTPE-COF-1 and mTPE-COF-2 onto a blue light-emitting diode, a high purity white light can be generated. This study offers a novel approach to design 2D luminescent COFs using mechanical interlocking strategy.

Creating amphiphilic porosity in two-dimensional covalent organic frameworks via steric-hindrance-mediated precision hydrophilic-hydrophobic microphase separation

Creating different pore environments within a covalent organic framework (COF) will lead to useful multicompartment structure and multiple functions, which however has been scarcely achieved. Herein we report designed synthesis of three two-dimensional COFs with amphiphilic porosity by steric-hindrance-mediated precision hydrophilic-hydrophobic microphase separation. Dictated by the different steric effect of the substituents introduced to a monomer, dual-pore COFs with kgm net, in which all hydroxyls locate in trigonal micropores while hydrophobic sidechains exclusively distribute in hexagonal mesopores, have been constructed to form completely separated hydrophilic and hydrophobic nanochannels. The unique amphiphilic channels in the COFs enable the formation of Janus membranes via interface growth. This work has realized the creation of two types of channels with opposite properties in one COF, demonstrating the feasibility of introducing different properties/functions into different pores of heteropore COFs, which can be a useful strategy to develop multifunctional materials.

Solid-Liquid Interfacial Engineered Large-Area Two-Dimensional Covalent Organic Framework Films.

Constructing uniform covalent organic framework (COF) film on substrates for electronic devices is highly desirable. Here, a simple and mild strategy is developed to prepare them by polymerization on a solid-liquid interface. The universality of the method is confirmed by the successful preparation of five COF films with different microstructures. These films have large lateral size, controllable thickness, and high crystalline quality. And COF patterns can also be directly achieved on substrates via hydrophilic and hydrophobic interface engineering, which is in favor of preparing device array. For application studies, the PyTTA-TPA (PyTTA: 4,4',4'',4'''-(1,3,6,8-Tetrakis(4-aminophenyl)pyrene and TPA: terephthalaldehyde) COF film has a high photoresponsivity of 59.79 μA W-1 at 420 nm for photoelectrochemical (PEC) detection. When employed as an active material for optoelectronic synaptic devices for the first attempt, it shows excellent light-stimulated synaptic plasticity properties such as short-term plasticity (STP), long-term plasticity (LTP), and the conversion of STP to LTP, which can be used to simulate biological synaptic functions.

Regenerable and Highly Stable Two-Dimensional Imine-Based Covalent Organic Framework for Simultaneous Rapid Detection and Adsorption of Cu2+ Ions.

The development of efficient fluorescent probes and adsorbents for detecting and removing Cu2+, which pose potential environmental and health risks, is a highly active area of research. However, achieving simultaneously improved fluorescence detection efficiency and enhanced adsorption capacity in a single porous probe remains a significant challenge. In this study, we successfully synthesized a two-dimensional imine-based TAP-COF using 2,4,6-triformylphloroglucinol and tri(4-aminophenyl)amine as raw materials. TAP-COF exhibited excellent properties, including a large specific surface area of 685.65 m2·g-1, exceptional thermal stability (>440 °C), chemical stability, temporal stability, and recyclability. Fluorescence testing revealed that TAP-COF exhibited remarkable specificity and high sensitivity for detecting Cu2+. The fluorescence mechanism, in which the excited state intramolecular proton transfer was impeded by the interaction of Cu2+ with C═O and C-N bonds on TAP-COF upon the addition of Cu2+, was further elucidated through experimental and theoretical methods. Furthermore, the adsorption capacity of TAP-COF toward Cu2+ was investigated, confirming the excellence of TAP-COF as a fluorescent probe and adsorbent for the specific detection and removal of Cu2+. This work holds significant implications for improving environmental and human health concerns associated with Cu2+ contamination.