Covalent Organic Framework Films Research Articles

Metal ion-catalyzed interfacial polymerization of functionalized covalent organic framework films for efficient separation

Covalent organic frameworks (COFs), with high porosity, uniform and tunable pore size, which are advantageous to prepare defect-free membranes with high separation performance and high permeability, and have achieved efficient applications in the field of separation membranes in numerous directions. According to our knowledge, the transition metal ion (Cu2+) which is used in Schiff base condensation reaction has been rarely described. Herein, we report the interfacial polymerization of ternary amines and binary aldehyde monomers via the catalyst Cu(ClO4)2·6H2O. In this work, we report the fabrication of covalent organic framework films from four different functional structure monomers by using the transition metal ion-catalyzed interfacial polymerization at room temperature. The four as-synthesized films are flexible, continuous, defect-free and have large areas, which have excellent performance in the direction of dye separation as well as solvent penetration. After the COF films were transferred to the polyethersulfone support, the composite films exhibited enhance the rejection of methylene blue (98.43%), high-water flux (81.46 L m−2 h−1 bar−1) and solvent flux (ethanol (13.03 L m−2 h−1 bar−1), acetonitrile (50.38 L m−2 h−1 bar−1)). The large area, tunable pore size, and tailored molecular composite membranes show potential for nanofiltration applications.

A sweat-responsive covalent organic framework film for material-based liveness detection and sweat pore analysis

Covalent organic frameworks have shown considerable application potential and exceptional properties in the construction of stimulus-responsive materials. Here, we designed a sweat-responsive covalent organic framework film for material-based fingerprint liveness detection. When exposed to human sweat, the COFTPDA-TFPy film can transform from yellow to red. The COFTPDA-TFPy film, when touched by living fingers, can produce the naked-eye-identified fingerprint pattern through the sweat-induced color change, while artificial fake fingerprints cannot. This technique, which we named material-based liveness detection, can thus intuitively discern living fingers from fake fingerprints with a 100% accuracy rate. Additionally, the distribution of sweat pores on human skin can also be collected and analyzed by shortening the contact time. By merely washing them with ethanol, all the samples can be utilized again. This work inventively accomplished material-based liveness detection and naked-eye-identified sweat pore analysis and highlighted their potential for use in clinical research and personal identification.

Donor–Acceptor Covalent Organic Frameworks Films with Ultralow Band Gaps to Enhanced Third-Order Nonlinear Optical Properties

Covalent organic frameworks (COFs) have attracted much attention in porous materials because of their structural tunability and stability. However, the problems of low transmittance and large light scattering due to the difficulty of powder phase processing have severely limited their practical applications in third-order nonlinear optics (NLO). Herein, we report the first successful synthesis of a series of donor–acceptor (D–A) COFs films (thickness of ∼100 nm) with uniform transparency and narrow optical band gap by the solvothermal and solid–liquid interface methods for third-order NLO applications. Most importantly, the designed PT–PA film exhibits excellent NLO performance, including a normalized transmission value of ∼2.55 at the beam focus, a modulation depth (As) of 139%, and a high nonlinear absorption coefficient (β) of 1.83 × 106 cm GW–1 at the pulse energy of 1 μJ, which is superior to most recently reported third-order NLO materials. The excellent third-order NLO properties are mainly attributed to the homogeneous transparency of the COFs films and the extended-ordered conjugated structure and narrow optical band gap of the COFs films, which enhance the charge transfer in their structures and thus yield excellent third-order NLO behavior. This work provides an option for the development of high-performance NLO optical devices and opens a new path for the future development of COFs films in the optical fields.

Covalent organic framework films grown on spongy g-C3N4 for efficient photocatalytic hydrogen production

Photocatalytic hydrogen production promisingly mitigates and ameliorates the status quo of energy shortage and decarbonization of the energy supply. Given this, a photocatalyst was prepared by modifying spongy g-C3N4 with covalent organic frameworks (COFs). As an electron acceptor with a large specific surface, COFs can trap more transition electrons, leading to an increase in the photogenerated carriers involved in the surface reaction and increasing the active sites of the photocatalyst. The S-scheme heterojunction structure formed in g-C3N4/COFs composite catalyst triggers a decrease in the recombination rate of photogenerated electron-hole pairs as well as an increase in photogenerated electrons. The photocatalytic hydrogen production rate reached the maximum value of 7788.10 μmol·h−1·g−1, almost 7 times that of g-C3N4. The AQE of the catalyst reached 29.6 %. The electron transfer path from g-C3N4 to COFs was verified by density functional theory (DFT) calculation.

Antiaromatic Covalent Organic Frameworks Based on Dibenzopentalenes

Despite their inherent instability, 4n π systems have recently received significant attention due to their unique optical and electronic properties. In dibenzopentalene (DBP), benzanellation stabilizes the highly antiaromatic pentalene core, without compromising its amphoteric redox behavior or small HOMO-LUMO energy gap. However, incorporating such molecules in organic devices as discrete small molecules or amorphous polymers can limit the performance (e.g., due to solubility in the battery electrolyte solution or low internal surface area). Covalent organic frameworks (COFs), on the contrary, are highly ordered, porous, and crystalline materials that can provide a platform to align molecules with specific properties in a well-defined, ordered environment. We synthesized the first antiaromatic framework materials and obtained a series of three highly crystalline and porous COFs based on DBP. Potential applications of such antiaromatic bulk materials were explored: COF films show a conductivity of 4 × 10-8 S cm-1 upon doping and exhibit photoconductivity upon irradiation with visible light. Application as positive electrode materials in Li-organic batteries demonstrates a significant enhancement of performance when the antiaromaticity of the DBP unit in the COF is exploited in its redox activity with a discharge capacity of 26 mA h g-1 at a potential of 3.9 V vs. Li/Li+. This work showcases antiaromaticity as a new design principle for functional framework materials.

A self-standing three-dimensional covalent organic framework film

Covalent crystals such as diamonds are a class of fascinating materials that are challenging to fabricate in the form of thin films. This is because spatial kinetic control of bond formation is required to create covalently bonded crystal films. Directional crystal growth is commonly achieved by chemical vapor deposition, an approach that is hampered by technical complexity and associated high cost. Here we report on a liquid-liquid interfacial approach based on physical-organic considerations to synthesize an ultrathin covalent crystal film. By distributing reactants into separate phases using hydrophobicity, the chemical reaction is confined to an interface that orients the crystal growth. A molecular-smooth interface combined with in-plane isotropic conditions enables the synthesis of films on a centimeter size scale with a uniform thickness of 13 nm. The film exhibits considerable mechanical robustness enabling a free-standing length of 37 µm, as well as a clearly anisotropic chemical structure and crystal lattice alignment.

A Heterostructure Photoelectrode Based on Two-Dimensional Covalent Organic Framework Film Decorated TiO2 Nanotube Arrays for Enhanced Photoelectrochemical Hydrogen Generation.

The well-defined heterostructure of the photocathode is desirable for photoelectrochemically producing hydrogen from aqueous solutions. Herein, enhanced heterostructures were fabricated based on typical stable covalent organic framework (TpPa-1) films and TiO2 nanotube arrays (NTAs) as a proof-of-concept model to tune the photoelectrochemical (PEC) hydrogen generation by tailoring the photoelectrode microstructure and interfacial charge transport. Ultrathin TpPa-1 films were uniformly grown on the surface of TiO2 NTAs via a solvothermal condensation of building blocks by tuning the monomer concentration. The Pt1@TpPa-1/TiO2-NTAs photoelectrode with single-atom Pt1 as a co-catalyst demonstrated improved visible-light response, enhanced photoconductance, lower onset potential, and decreased Tafel slope value for hydrogen evolution. The hydrogen evolution rate of the Pt1@TpPa-1/TiO2-NTAs photoelectrode was five times that of Pt1@TpPa-1 under AM 1.5 simulated sunlight irradiation and the bias voltage of 0 V. A lower overpotential was recorded as 77 mV@10 mA cm-2 and a higher photocurrent density as 1.63 mA cm-2. The hydrogen evolution performance of Pt1@TpPa-1/TiO2-NTAs photoelectrodes may benefit from the well-matched band structures, effective charge separation, lower interfacial resistance, abundant interfacial microstructural sites, and surficial hydrophilicity. This work may raise a promising way to design an efficient PEC system for hydrogen evolution by tuning well-defined heterojunctions and interfacial microstructures.

Covalent Organic Framework Thin-Film Photodetectors from Solution-Processable Porous Nanospheres

The synthesis of homogeneous covalent organic framework (COF) thin films on a desired substrate with decent crystallinity, porosity, and uniform thickness has great potential for optoelectronic applications. We have used a solution-processable sphere transmutation process to synthesize 300 ± 20 nm uniform COF thin films on a 2 × 2 cm2 TiO2-coated fluorine-doped tin oxide (FTO) surface. This process controls the nucleation of COF crystallites and molecular morphology that helps the nanospheres to arrange periodically to form homogeneous COF thin films. We have synthesized four COF thin films (TpDPP, TpEtBt, TpTab, and TpTta) with different functional backbones. In a close agreement between the experiment and density functional theory, the TpEtBr COF film showed the lowest optical band gap (2.26 eV) and highest excited-state lifetime (8.52 ns) among all four COF films. Hence, the TpEtBr COF film can participate in efficient charge generation and separation. We constructed optoelectronic devices having a glass/FTO/TiO2/COF-film/Au architecture, which serves as a model system to study the optoelectronic charge transport properties of COF thin films under dark and illuminated conditions. Visible light with a calibrated intensity of 100 mW cm-2 was used for the excitation of COF thin films. All of the COF thin films exhibit significant photocurrent after illumination with visible light in comparison to the dark. Hence, all of the COF films behave as good photoactive substrates with minimal pinhole defects. The fabricated out-of-plane photodetector device based on the TpEtBr COF thin film exhibits high photocurrent density (2.65 ± 0.24 mA cm-2 at 0.5 V) and hole mobility (8.15 ± 0.64 ×10-3 cm2 V-1 S-1) compared to other as-synthesized films, indicating the best photoactive characteristics.

Covalent organic framework film-based laser desorption/ionization mass spectrometry for rapid and sensitive quantification of homocysteine in human serum.

Homocysteine (hcy) is a metabolite in the human body and an important determinant of cardiovascular health. To assist in the assessment of human cardiovascular safety, a rapid and accurate quantitative analysis method must be developed. Laser desorption/ionization mass spectrometry (LDI-MS) has been widely used in biological, chemical and medical analysis due to its excellent characteristics of high throughput, low sample consumption and high speed. Herein, a LDI-MS method based on covalent organic framework (COF) film was developed for rapid and sensitive determination of hcy in human serum using an isotope-labeled internal standard. We tried to cultivate COFs on indium-tin oxide (ITO) substrate at room temperature to form thin film, which was used in LDI-MS. Compared with the traditional organic matrices, the COF film showed clean background and high signal response for the detection of hcy. In addition, using COF film as the substrate that can analyze a series of small molecules such as amino acids, bisphenols (Bps), estrogens, drugs, etc., with high signal intensity and clean background. We evaluated the limit of detection (LOD) and the reproducibility of this method. Finally, the calibration curve was made for the quantification of serum hcy using isotope labeled internal standard, and was applied to the rapid determination of hcy in clinical human serum. COF film-assisted LDI-MS had higher response signal and cleaner background compared to four conventional organic matrices. The LOD of the method for hcy was 0.5 μmol/L, equivalent to 500 fmol. This method exhibited excellent performance for small molecule compounds including amino acids, Bps, drugs, and estrogens. The reproducibilities of this method for shot-to-shot and dot-to-dot were proved to be good. This method was applied to the rapid analysis of hcy in clinical human serum, showing good correlation with those obtained by hospital testing. The COF film-based LDI-MS method showed simple sample preparation, short analysis time, clean background and good reproducibility for hcy analysis. As an important indicator of human health, the detection of serum hcy is of great significance to human health.

Improved resistive switching performance through donor–acceptor structure construction in memristors based on covalent organic framework films

The impact of acceptors on memory performance was explored based on two COF film-based devices with different donor–acceptor structures.

Towards high-performance nonlinear optical materials through embedding a D-A system into β-ketoenamine-linked COFs.

Two covalent organic framework (COF) films supported by a glass substrate were obtained by solvothermal reaction of an electron donor with electron acceptor 1,3,5-triformylbenzene (TF) or 2,4,6-triformylphloroglucinol (TFP), respectively. The TFP-BD film exhibits a nonlinear absorption coefficient of -3.01 × 105 cm GW-1. The TFP-BD film can aggregate electrons around the connected monomer through the D-A effect due to its highly polar and electronegative carbonyl oxygen atoms, thereby modulating the electronic structure of the COFs. This work provides a novel approach for the structural modulation of optical materials with strong nonlinearity.

Interface-confined synthesis of a nonplanar redox-active covalent organic framework film for synaptic memristors.

The development of novel synthetic methodologies and unprecedented structures of covalent organic framework (COF) films is of great importance for exploring their potential applications in optoelectronic devices, sensors, and membrane separation. From the point of view of monomer selection, rigid building blocks are always the first choice for synthesizing crystalline COF films. However, the preparation of COF films with flexible building units remains challenging. Herein, by introducing flexible triphenylamine-based building units, a nonplanar COF film (TFPA-TAPA film) is fabricated via liquid-liquid interface-confined synthesis at room temperature and atmospheric pressure. The growth mechanism of the flexible building units at the liquid-liquid interface is related to the transformation of strip-type slices into free-standing COF films by dynamic covalent chemistry. As a proof-of-concept, the as-fabricated Al/TFPA-TAPA/ITO device shows excellent multilevel storage and history-dependent memristive switching behavior. The synaptic potentiation/depression, human learning and memorization functions, as well as the transition from short-term synaptic plasticity to long-term plasticity, are successfully emulated by using this synaptic memristor.

Shape‐Controlled Synthesis of Covalent Organic Frameworks Enabled by Polymerization‐Induced Phase Separation

The shape and morphology modulations of covalent organic frameworks (COFs) are both difficult, but are of significanceto tackle to realize high-performance and practical applications. Here, a two-step method is reported that separates the phase separation and crystallization processes for the shape-controlled synthesis of COFs. The insight into the polymerization-induced phase separation (PIPS) allows for the flexible shaping of COFs into column, rod, film and others, as well as for constructing hierarchically porous structure. The as-synthesized COF monoliths are crack-free, no powder detaching, and show 214MPa of compressive modulus. The resulting good permeability and mechanical flexibility enable COF films to apply for flow-through adsorption and extraction of pollutants at high flow rates. This work successfully resolves the contradiction between PIPS and crystallization, offering a general approach for scalable production of COFs with desired shapes, sizes, and morphologies.

Functional Covalent Organic Framework Films Based on Surface and Interfacial Chemistry for Molecular Separations.

Covalent organic frameworks (COFs) are promising crystalline porous materials with highly tunable structures and functionalities. In the last decade, COF films have been synthesized and used as multifunctional materials for a diverse range of separation applications. However, there are still challenges in the scaling-up preparation of COF films with benchmarked performance for precise molecular separations. Recently, research has turned its attention to preparing functional COF films with an appropriate aperture size/functionality, facile preparation process, and superior stability. In this Perspective, we outline the recent advances in designing and preparing functional COF films based on surface and interfacial chemistry. On top of that, current obstacles and opportunities in the scaling-up preparation of functional COF films and their industrial applications are proposed and discussed. This Perspective strives to inspire the development of functional COF films with tailored structures and functionalities and promote their practical applications in diverse molecular separation processes.

Large‐area Free‐standing Metalloporphyrin‐based Covalent Organic Framework Films by Liquid‐air Interfacial Polymerization for Oxygen Electrocatalysis

AbstractSynthesizing large‐area free‐standing covalent organic framework (COF) films is of vital importance for their applications but is still a big challenge. Herein, we reported the synthesis of large metalloporphyrin‐based COF films and their applications for oxygen electrocatalysis. The reaction of meso‐benzohydrazide‐substituted metal porphyrins with tris‐aldehyde linkers afforded free‐standing COF films at the liquid‐air interface. These films can be scaled up to 3000 cm2 area and display great mechanical stability and structural integrity. Importantly, the Co‐porphyrin‐based films are efficient for electrocatalytic O2 reduction and evolution reactions. A flexible, all‐solid‐state Zn‐air battery was assembled using the films and showed high performance with a charge–discharge voltage gap of 0.88 V at 1 mA cm−2 and high stability under bent conditions (0° to 180°). This work thus presents a strategy to synthesize functionalized COF films with high quality for uses in flexible electronics.

Large-area Free-standing Metalloporphyrin-based Covalent Organic Framework Films by Liquid-air Interfacial Polymerization for Oxygen Electrocatalysis.

Synthesizing large-area free-standing covalent organic framework (COF) films is of vital importance for their applications but is still a big challenge. Herein, we reported the synthesis of large metalloporphyrin-based COF films and their applications for oxygen electrocatalysis. The reaction of meso-benzohydrazide-substituted metal porphyrins with tris-aldehyde linkers afforded free-standing COF films at the liquid-air interface. These films can be scaled up to 3000 cm2 area and display great mechanical stability and structural integrity. Importantly, the Co-porphyrin-based films are efficient for electrocatalytic O2 reduction and evolution reactions. A flexible, all-solid-state Zn-air battery was assembled using the films and showed high performance with a charge-discharge voltage gap of 0.88 V at 1 mA cm-2 and high stability under bent conditions (0° to 180°). This work thus presents a strategy to synthesize functionalized COF films with high quality for uses in flexible electronics.

Enabling Solution Processable COFs through Suppression of Precipitation during Solvothermal Synthesis.

Covalent organic frameworks (COFs) are crystalline, nanoporous materials of interest for various applications, but current COF synthetic routes lead to insoluble aggregates which precludes processing for practical implementation. Here, we report a COF synthesis method that produces a stable, homogeneous suspension of crystalline COF nanoparticles that enables the preparation of COF monoliths, membranes, and films using conventional solution-processing techniques. Our approach involves the use of a polar solvent, diacid catalyst, and slow reagent mixing procedure at elevated temperatures which altogether enable access to crystalline COF nanoparticle suspension that does not aggregate or precipitate when kept at elevated temperatures. On cooling, the suspension undergoes a thermoreversible gelation transition to produce crystalline and highly porous COF materials. We further show that the modified synthesis approach is compatible with various COF chemistries, including both large- and small-pore imine COFs, hydrazone-linked COFs, and COFs with rhombic and hexagonal topologies, and in each case, we demonstrate that the final product has excellent crystallinity and porosity. The final materials contain both micro- and macropores, and the total porosity can be tuned through variation of sample annealing. Dynamic light scattering measurements reveal the presence of COF nanoparticles that grow with time at room temperature, transitioning from a homogeneous suspension to a gel. Finally, we prepare imine COF membranes and measure their rejection of polyethylene glycol (PEG) polymers and oligomers, and these measurements exhibit size-dependent rejection and adsorption of PEG solutes. This work demonstrates a versatile processing strategy to create crystalline and porous COF materials using solution-processing techniques and will greatly advance the development of COFs for various applications.

Oriented Thiophene‐Extended Benzotrithiophene Covalent Organic Framework Thin Films: Directional Electrical Conductivity (Adv. Funct. Mater. 47/2022)

Covalent Organic Frameworks In article number 2205949, Dana Dina Medina, Ralf Thomas Weitz, Timothy Clark and co-workers describe the synthesis of covalent organic frameworks (COFs) based on a thiophene-extended benzotrithiophene (BTT) building block. Oriented thin films of the BTT COFs show room-temperature in-plane electrical conductivity of up to 10−4 S m−1 and a trap-dominated charge transport via a hopping mechanism. Moreover, electrical conductivity of the COF films is directional with a strong preference for the in-plane over the out-of-plane direction.

An ultrathin double-layer covalent organic framework/zwitterionic microporous polymer functional separator for high-performance lithium-sulfur battery

The development of efficient lithium-sulfur (Li-S) battery depends on a well-designed functional separator that can effectively limit the transport of polysulfides while facilitating the migration of lithium ions. Herein, a multifunctional separator is fabricated with commercial polypropylene membrane (PP) as bottom porous support, a covalent organic framework (COF) film as gutter layer, and an ultrathin zwitterionic microporous polymer (QTB-SA) layer as functional barrier. In such a configuration, the COF gutter layer plays a role in adjusting the surface property of PP support without obviously increasing the transport resistance of lithium ions, while the QTB-SA functional barrier enables the formation of confined ionic channels due to its intrinsic microporosity and zwitterionic nature. Thus, selective ion transport concerning this separator can be expected owing to the availability of electrostatic interaction with lithium polysulfides. The experimental results show that the QTB-SA/COF/PP-based Li-S battery possesses high prolonged cycling behavior (capacity decay rate of 0.087 % per cycle at 1C over 500 cycles) and attractive rate capability up to 5C. Moreover, the Li-S battery can deliver a decent areal capacity of 3.4 mAh cm−2 after 100 cycles with a raised sulfur loading (4.5 mg cm−2).

A novel study on COF-based semi-solid electrolyte for spinel LiNi0.5Mn1.5O4 targeting transition metals migration

Cobalt-free battery is one of the development trends of lithium-ion batteries. High-voltage LiNi0.5Mn1.5O4 (LNMO) with high theoretical energy density is one of the most promising Co-free cathode materials in the further. However, the side reaction between the LNMO electrode and electrolyte at high voltage or high temperature and the corresponding by-products (e.g. Mn2+/Mn3+ and Ni2+) severely deteriorate the battery performance. In this work, an anionic covalent organic framework (COF)–based semi-solid electrolyte was constructed by synthesizing a free-standing COF film with a small amount of liquid electrolyte. Further, the experimental results proved that the cycling and C-rate performance of LNMO battery is improved by using the semi-solid electrolyte. This can be proposed that the anionic COF is capable of absorbing strongly the Mn3+/Mn2+ and Ni2+ through Coulomb interaction, restraining their destructive migration to the anode. Therefore, this work will be beneficial to the commercialization of LNMO cathode material.