Controlling Monomer Feeding Rate to Achieve Highly Crystalline Covalent Triazine Frameworks

The development of catalysts and auxiliaries for the synthesis of covalent organic frameworks

Covalent organic frameworks (COFs) have recently seen significant advancements. Large quantities of structurally & functionally oriented COFs with a wide range of applications, such as gas adsorption, catalysis, separation, and drug delivery, have been explored. Recent achievements in this field are primarily focused on advancing synthetic methodologies, with catalysts playing a crucial role in achieving highly crystalline COF materials, particularly those featuring novel linkages and chemistry. A series of reviews have already been published over the last decade, covering the fundamentals, synthesis, and applications of COFs. However, despite the pivotal role that catalysts and auxiliaries play in forming COF materials and adjusting their properties (e.g., crystallinity, porosity, stability, and morphology), limited attention has been devoted to these essential components. In this Critical Review, we mainly focus on the state-of-the-art progress of catalysts and auxiliaries applied to the synthesis of COFs. The catalysts include four categories: acid catalysts, base catalysts, transition-metal catalysts, and other catalysts. The auxiliaries, such as modulators, oxygen, and surfactants, are discussed as well. This is then followed by the description of several specific applications derived from the utilization of catalysts and auxiliaries. Lastly, a perspective on the major challenges and opportunities associated with catalysts and auxiliaries is provided.

A Dual-Active Covalent Triazine Framework Film for Efficient Visible-Light-Driven Hydrogen Peroxide Production

Photocatalytic hydrogen peroxide production from water and oxygen offers a clean and sustainable alternative to the conventional energy-intensive anthraquinone oxidation method. Compared to powdered covalent triazine frameworks (CTFs), the film morphology of CTFs provides better connectivity in 2D, yielding several advantages: more efficient connections between active sites, reduced electron-hole pair recombination, increased resistance to superoxide radical induced corrosion, and decreased light scattering. Leveraging these benefits, it has incorporated dual active sites for both the oxygen reduction reaction (ORR) and the water oxidation reaction (WOR) into a CTF film system. This dual-active CTF film demonstrated an exceptional hydrogen peroxide production rate of 19 460 µmol h⁻¹ m⁻2 after 1 h and 17 830 µmol h⁻¹ m⁻2 after 5 h under visible light irradiation (≥420 nm) without the need for sacrificial agents.

Construction of High-Performance Anode of Potassium-Ion Batteries by Stripping Covalent Triazine Frameworks with Molten Salt

Covalent triazine frameworks (CTFs) are promising battery electrodes owing to their designable functional groups, tunable pore sizes, and exceptional stability. However, their practical use is limited because of the difficulty in establishing stable ion adsorption/desorption sites. In this study, a melt-salt-stripping process utilizing molten trichloro iron (FeCl3) is used to delaminate the layer-stacked structure of fluorinated covalent triazine framework (FCTF) and generate iron-based ion storage active sites. This process increases the interlayer spacing and uniformly deposits iron-containing materials, enhancing electron and ion transport. The resultant melt-FeCl3-stripped FCTF (Fe@FCTF) shows excellent performance as a potassium ion battery with a high capacity of 447 mAh g-1 at 0.1 A g-1 and 257 mAh g-1 at 1.6 A g-1 and good cycling stability. Notably, molten-salt stripping is also effective in improving the CTF's Na+ and Li+ storage properties. A stepwise reaction mechanism of K/Na/Li chelation with C═N functional groups is proposed and verified by in situ X-ray diffraction testing (XRD), ex-situ X-ray photoelectron spectroscopy (XPS), and theoretical calculations, illustrating that pyrazines and iron coordination groups play the main roles in reacting with K+/Na+/Li+ cations. These results conclude that the Fe@FCTF is a suitable anode material for potassium-ion batteries (PIBs), sodium-ion batteries (SIBs), and lithium-ion batteries (LIBs).

Efficient integration of covalent triazine frameworks (CTFs) for augmented photocatalytic efficacy: A review of synthesis, strategies, and applications

Heterogeneous photocatalysis emerges as an exceptionally appealing technological avenue for the direct capture, conversion, and storage of renewable solar energy, facilitating the generation of sustainable and ecologically benign solar fuels and a spectrum of other pertinent applications. Heterogeneous nanocomposites, incorporating Covalent Triazine Frameworks (CTFs), exhibit a wide-ranging spectrum of light absorption, well-suited electronic band structures, rapid charge carrier mobility, ample resource availability, commendable chemical robustness, and straightforward synthetic routes. These attributes collectively position them as highly promising photocatalysts with applicability in diverse fields, including but not limited to the production of photocatalytic solar fuels and the decomposition of environmental contaminants. As the field of photocatalysis through the hybridization of CTFs undergoes rapid expansion, there is a pressing and substantive need for a systematic retrospective analysis and forward-looking evaluation to elucidate pathways for enhancing performance. This comprehensive review commences by directing attention to diverse synthetic methodologies for the creation of composite materials. And then it delves into a thorough exploration of strategies geared towards augmenting performance, encompassing the introduction of electron donor-acceptor (D-A) units, heteroatom doping, defect Engineering, architecture of Heterojunction and optimization of morphology. Following this, it systematically elucidates applications primarily centered around the efficient generation of photocatalytic hydrogen, reduction of carbon dioxide through photocatalysis, and the degradation of organic pollutants. Ultimately, the discourse turns towards unresolved challenges and the prospects for further advancement, offering valuable guidance for the potent harnessing of CTFs in high-efficiency photocatalytic processes.

Superlative Porous Organic Polymers for Photochemical and Electrochemical CO2 Reduction Applications: From Synthesis to Functionality

To mimic the carbon cycle at a kinetically rapid pace, the sustainable conversion of omnipresent CO2 to value-added chemical feedstock and hydrocarbon fuels implies a remarkable prototype for utilizing released CO2. Porous organic polymers (POPs) have been recognized as remarkable catalytic systems for achieving large-scale applicability in energy-driven processes. POPs offer mesoporous characteristics, higher surface area, and superior optoelectronic properties that lead to their relatively advanced activity and selectivity for CO2 conversion. In comparison to the metal organic frameworks, POPs exhibit an enhanced tendency toward membrane formation, which governs their excellent stability with regard to remarkable ultrathinness and tailored pore channels. The structural ascendancy of POPs can be effectively utilized to develop cost-effective catalytic supports for energy conversion processes to leapfrog over conventional noble metal catalysts that have nonlinear techno-economic equilibrium. Herein, we precisely surveyed the functionality of POPs from scratch, classified it, and provided a critical commentary of its current methodological advancements and photo/electrochemical achievements in the CO2 reduction reaction.

Strategies for Enhancing the Photocatalytic and Electrocatalytic Efficiency of Covalent Triazine Frameworks for CO2 Reduction

Converting carbon dioxide (CO2) into fuel and high-value-added chemicals is considered a green and effective way to solve global energy and environmental problems. Covalent triazine frameworks (CTFs) are extensively utilized as an emerging catalyst for photo/electrocatalytic CO2 reduction reaction (CO2RR) recently recognized for their distinctive qualities, including excellent thermal and chemical stability, π-conjugated structure, rich nitrogen content, and a strong affinity for CO2, etc. Nevertheless, single-component CTFs have the problems of accelerated recombination of photoexcited electron-hole pairs and restricted conductivity, which limit their application for photo/electrocatalytic CO2RR. Therefore, emphasis will then summarize the strategies for enhancing the photocatalytic and electrocatalytic efficiency of CTFs for CO2RR in this paper, including atom doping, constructing a heterojunction structure, etc. This review first illustrates the synthesis strategies of CTFs and the advantages of CTFs in the field of photo/electrocatalytic CO2RR. Subsequently, the mechanism of CTF-based materials in photo/electrocatalytic CO2RR is described. Lastly, the challenges and future prospects of CTFs in photo/electrocatalytic CO2RR are addressed, which offers a fresh perspective for the future development of CTFs in photo/electrocatalytic CO2RR.

Increasing the Accessibility of Internal Catalytic Sites in Covalent Organic Frameworks by Introducing a Bicontinuous Mesostructure

Introducing continuous mesochannels into covalent organic frameworks (COFs) to increase the accessibility of their inner active sites has remained a major challenge. Here, we report the synthesis of COFs with an ordered bicontinuous mesostructure, via a block copolymer self-assembly-guided nanocasting strategy. Three different mesostructured COFs are synthesized, including two covalent triazine frameworks and one vinylene-linked COF. The new materials are endowed with a hierarchical meso/microporous architecture, in which the mesochannels exhibit an ordered shifted double diamond (SDD) topology. The hierarchically porous structure can enable efficient hole-electron separation and smooth mass transport to the deep internal of the COFs and consequently high accessibility of their active catalytic sites. Benefiting from this hierarchical structure, these COFs exhibit excellent performance in visible-light-driven catalytic NO removal with a high conversion percentage of up to 51.4 %, placing them one of the top reported NO-elimination photocatalysts. This study represents the first case of introducing a bicontinuous structure into COFs, which opens a new avenue for the synthesis of hierarchically porous COFs and for increasing the utilization degree of their internal active sites.

Structural Conjugation Tuning in Covalent Organic Frameworks Boosts Charge Transfer and Photocatalysis Performances

Structural conjugation greatly affects the optical and electronic properties of the COF photocatalyst. Herein, we show that 2D hydrazone COFs with either π-extended biphenyl (BPh-COF) or acetylene (AC-COF) frameworks demonstrated distinct charge transfer and photocatalytic performances. The two COFs show good crystallinity and decent porosity as their frameworks are enforced by intra/interlayers hydrogen bonding. However, computational and experimental data reveal that AC-COF managed broader visible-light absorption and narrower optical bandgaps and performed efficient photoinduced charge separation and transfer in comparison with BPh-COF, meaning that the ethynyl skeleton with enhanced planarity better improves the π-conjugation of the whole structure. As a result, AC-COF exhibited an ideal bandgap for rapid oxidative coupling of amines under visible-light irradiation. Furthermore, taking advantage of its better charge transfer properties, AC-COF demonstrated considerable enhanced product conversion and notable functional tolerance for metallaphotocatalytic C-O cross-coupling of a wide range of both aryl bromides and chlorides with alcohols. More importantly, besides being recoverable, AC-COF showcased the previously inaccessible etherification of dihaloarene. This report shows a facile approach for manipulating the structure-activity relationship and paves the way for the development of a COF photocatalyst for solar-to-chemical energy conversion.

Increasing Donor-Acceptor Interactions and Particle Dispersibility of Covalent Triazine Frameworks for Higher Crystallinity and Enhanced Photocatalytic Activity

Covalent triazine frameworks (CTFs) have recently emerged as an efficient class of photocatalysts due to their structural diversity and excellent stability. Nevertheless, the synthetic reactions of CTFs have usually suffered from poor reversibility, resulting in a low crystallinity of the materials. Here, we report the introduction of methoxy groups on the monomer 2,5-diphenylthiazolo[5,4-d]thiazole to reinforce interlayer π-π interactions of the resulting donor-acceptor type CTFs, which improved crystallinity, further increasing the visible light absorption range and allowing for efficient separation and transport of carriers. The morphology is strongly correlated to the wettability, which has a significant impact on the mass transfer capacity and photocatalytic activity in the photocatalytic reaction. To further improve crystallinity and photocatalytic activity, CTF-NWU-T3 photocatalysts in a bowl shape were prepared using a SiO2 template. The energy band structure, photocatalytic hydrogen evolution, and pollutant degradation efficiency of involved materials were investigated. The donor-acceptor type CTF-NWU-T3 with a bowl-shaped morphology, synthesized using the template method and the introduction of methoxy groups, exhibited an excellent photocatalytic hydrogen production rate of 32064 μmol·h-1·g-1. This study highlights the significance of improving donor-acceptor interactions and increasing the dispersibility of catalyst particles in dispersion to enhance the photocatalytic activity of heterogeneous photocatalysts.

Regulating the Content of Donor Unit in Donor-Acceptor Covalent Triazine Frameworks for Promoting Photocatalytic H2 Production

Using their own triazine groups as natural receptors, the introduction of various donor units to construct donor-receptor configuration in covalent triazine frameworks (CTFs) has been shown to be an effective strategy to improve photocatalytic activity. In this work, the effect of donor unit content (D-content) on the photoelectric properties and photocatalytic activity of CTFs was thoroughly investigated. Four analogous CTFs with different D-content have been rationally designed and synthesized, in which the bithiophene (Btp) as the donor unit and triazine as the acceptor unit. And CTF-Btp with the highest D-content showed the best photocatalytic activity. The experimental and theoretical results indicated this improvement is attributed to stronger visible light absorption capacity and higher photoinduced charge carrier separation efficiency. This study elucidates the relationship between the structural features of CTFs with varying D-content and their photocatalytic activity, offering a promising strategy for developing efficient photocatalysts.

Polymeric nanoparticles (PNPs) for oral delivery of insulin

Successful oral insulin administration can considerably enhance the quality of life (QOL) of diabetes patients who must frequently take insulin injections. Oral insulin administration, on the other hand, is seriously hampered by gastrointestinal enzymes, wide pH range, mucus and mucosal layers, which limit insulin oral bioavailability to ≤ 2%. Therefore, a large number of technological solutions have been proposed to increase the oral bioavailability of insulin, in which polymeric nanoparticles (PNPs) are highly promising for oral insulin delivery. The recently published research articles chosen for this review are based on applications of PNPs with strong future potential in oral insulin delivery, and do not cover all related work. In this review, we will summarize the controlled release mechanisms of oral insulin delivery, latest oral insulin delivery applications of PNPs nanocarrier, challenges and prospect. This review will serve as a guide to the future investigators who wish to engineer and study PNPs as oral insulin delivery systems.

Research Progress in Donor-Acceptor Type Covalent Organic Frameworks

Covalent organic frameworks (COFs) are new organic porous materials constructed by covalent bonds, with the advantages of pre-designable topology, adjustable pore size, and abundant active sites. Many research studies have shown that COFs exhibit great potential in gas adsorption, molecular separation, catalysis, drug delivery, energy storage, etc. However, the electrons and holes of intrinsic COF are prone to compounding in transport, and the carrier lifetime is short. The donor-acceptor (D-A) type COFs, which are synthesized by introducing D and A units into the COFs backbone, combine separated electron and hole migration pathway, tunable band gap and optoelectronic properties of D-A type polymers with the unique advantages of COFs and have made great progress in related research in recent years. Here, the synthetic strategies of D-A type COFs are first outlined, including the rational design of linkages and D-A units as well as functionalization approaches. Then the applications of D-A type COFs in catalytic reactions, photothermal therapy, and electronic materials are systematically summarized. In the final section, the current challenges, and new directions for the development of D-A type COFs are presented.

Construction of Type-II Heterojunctions in Crystalline Carbon Nitride for Efficient Photocatalytic H2 Evolution

As an encouraging photocatalyst, crystalline carbon nitride (CCN) exhibits unsatisfactory photocatalytic activity and stability due to its rapid recombination of photo-generative carriers. Herein, high-crystalline g-C3N4 was prepared, including CCN obtained in KCl (K-CCN), LiCl-KCl mixture (Li/K-CCN), and LiCl-NaCl-KCl mixture (Li/Na/K-CCN), via the molten salt strategy using pre-prepared bulk carbon nitride (BCN) as a precursor. The obtained BCN sample was formed by heptazine-based units, which convert into triazine-based units for K-CCN. Heptazine and triazine are two isotypes that co-exist in the Li/K-CCN and Li/Na/K-CCN samples. Compared with BCN and other CCN samples, the as-prepared Li/Na/K-CCN sample exhibited the optimal photocatalytic hydrogen evolution rates (3.38 mmol·g-1·h-1 under simulated sunlight and 2.25 mmol·g-1·h-1 under visible light) and the highest apparent quantum yield (10.97%). The improved photocatalytic performance of the Li/Na/K-CCN sample is mainly attributed to the construction of type-II heterojunction and the institution of the built-in electric field between triazine-based CCN and heptazine-based BCN. This work provides a new strategy for the structural optimization and heterostructure construction of crystalline carbon nitride photocatalysts.

Metallaphotocatalytic Amination of Aryl Chlorides Enabled by Highly Crystalline Acetylene-Based Hydrazone-Linked Covalent Organic Frameworks

Amination of aryl chlorides by metallaphotocatalysis is highly desired but remains practically challenging. Meanwhile, relying on soluble noble-metal photocatalysts suffers from resource scarcity and structural instability which limit their practical application. Here in, a highly crystalline acetylene-based hydrazone-linked covalent organic framewok-1 (AC-COF-1) is reported that enables metallaphotocatalytic amination of aryl chlorides. The non-planar effect of hydrazone linkage and weak interlayer attraction of acetylene bond are minimized by intralayer hydrogen-bonding. As a result, the COF shows not only improved crystallinity and porosity, but also enhanced optical and electronic properties compared to a COF analog without hydrogen-bonding. Notably, dual AC-COF-1/Ni system affords CN coupling products from broad aryl chloride substrates in excellent yields (up to 99%) and good functional tolerance. Furthermore, AC-COF-1 is recoverable and reusable for seven times photocatalysis cycles. This report demonstrates simple approach to tune the structure-activity relationship in COFs at molecular level.

Emerging conjugated polymers for heterogeneous photocatalytic chemical transformation

In recent decades, the efficient utilization of solar energy through heterogeneous photocatalytic chemical transformation has attracted much attention. As emerging metal-free, pure organic and heterogeneous photocatalysts, π-conjugated polymers (CPs) have been used in visible-light-driven chemical transformations due to their stability, high specific surface area, metal-free nature, and high structural designability. In this review, we summarize the synthesis protocols and design strategies for efficient CP-based photocatalysts based on the photocatalytic mechanisms. Then we highlight the key progress in light-driven chemical transformation using CPs developed by our group. Finally, we present the outlook and possible challenges for future progress of the field.

Organic Covalent Interaction-based Frameworks as Emerging Catalysts for Environment and Energy Applications: Current Scenario and Opportunities

The term "covalent organic framework" (COF) refers to a class of porous organic polymeric materials made from organic building blocks that have been covalently bonded. The preplanned and predetermined bonding of the monomer linkers allow them to demonstrate directional flexibility in two- or three-dimensional spaces. COFs are modern materials, and the discovery of new synthesis and linking techniques has made it possible to prepare them with a variety of favorable features and use them in a range of applications. Additionally, they can be post-synthetically altered or transformed into other materials of particular interest to produce compounds with enhanced chemical and physical properties. Because of its tunability in different chemical and physical states, post-synthetic modifications, high stability, functionality, high porosity and ordered geometry, COFs are regarded as one of the most promising materials for catalysis and environmental applications. This study highlights the basic advancements in establishing the stable COFs structures and various post-synthetic modification approaches. Further, the photocatalytic applications, such as organic transformations, degradation of emerging pollutants and removal of heavy metals, production of hydrogen and Conversion of carbon dioxide (CO₂ ) to useful products have also been presented. Finally, the future research directions and probable outcomes have also been summarized, by focusing their promises for specialists in a variety of research fields.

Covalent Triazine Frameworks (CTFs): Synthesis, Crystallization, and Photocatalytic Water Splitting

Covalent Triazine Frameworks (CTFs) have received great attention from academia owing to their unique structure characteristics such as nitrogen-rich structure, chemical stability, fully conjugated skeleton and high surface area; all these unique properties make CTFs attractive for widespread applications, especially for photocatalytic applications. In this review, we aim to provide recent advances in the CTFs preparation, and mainly focus on their photocatalytic applications. This review provides a comprehensive and systematic overview of the CTFs' synthetic methods, crystallinity lifting strategies, and their applications for photocatalytic water splitting. Firstly, a brief background including the photocatalytic water splitting and crystallinity are provided. Then, synthetic methods related to CTFs and the strategies for enhancing the crystallinity are summarized and compared. After that, the general photocatalytic mechanism and the strategies to improve the photocatalytic performance of CTFs are discussed. Finally, the perspectives and challenges of fabricating high crystalline CTFs and designing CTFs with excellent photocatalytic performance are discussed, inspiring the development of CTF materials in photocatalytic applications.

Hydrogen Bond Activation by Pyridinic Nitrogen for the High Proton Conductivity of Covalent Triazine Framework Loaded with H3 PO4

Under high temperature anhydrous conditions, it is still a formidable challenge to improve the performance of proton-conducting materials based on H₃ PO₄ and elucidate its proton conduction mechanism. Herein, a highly stable covalent triazine frameworks (CTFs) based on H₃ PO₄ is reported. The more pyridinic nitrogen CTFs contain, the higher proton conductivity is. Compared with H₃ PO₄ @CTF-L with less pyridinic nitrogen, H₃ PO₄ @CTF-H has a higher proton conductivity of 1.6×10^-1  S cm^-1 at 150 °C under anhydrous conditions, which does not decay after about 18 months exposure in air. The high proton conductivity is associated with the formation and breaking of the activated N_triazine ⋯H⁺ ⋯H₂ PO₄ ^- pairs by pyridinic nitrogen of CTFs. The outstanding long-term stability is mainly attributed to the ultra-strong triazine skeleton structure of CTFs.

Insights into Photocatalytic Degradation Pathways and Mechanism of Tetracycline by an Efficient Z-Scheme NiFe-LDH/CTF-1 Heterojunction

Photocatalysis offers a sustainable approach for recalcitrant organic pollutants degradation, yet it is still challenging to seek robust photocatalysts for application purposes. Herein, a novel NiFe layered double hydroxide (LDH)/covalent triazine framework (CTF-1) Z-scheme heterojunction photocatalyst was rationally designed for antibiotics degradation under visible light irradiation. The NiFe-LDH/CTF-1 nanocomposites were readily obtained via in situ loading of NiFe-LDH on CTF-1 through covalent linking. The abundant coupling interfaces between two semiconductor counterparts lay the foundation for the formation of Z-scheme heterostructure, thereby effectively promoting the transfer of photogenerated electrons, inhibiting the recombination of carriers, as well as conferring the nanocomposites with stronger redox ability. Consequently, the optimal photocatalytic activity of the LDH/CTF heterojunction was significantly boosted for the degradation of a typical antibiotic, tetracycline (TC). Additionally, the photodegradation process and the mineralization of TC were further elucidated. These results envision that the LDH/CTF-1 can be a viable photocatalyst for long-term and sustainable wastewater treatment.

2D/2D Inorganic/Organic Hybrid of Lead-Free Cs2AgBiBr6 Double Perovskite/Covalent Triazine Frameworks with Boosted Charge Separation and Efficient CO2 Photoreduction

Heterojunction construction, especially the inorganic/organic hybrids, is regarded as a universal and effective strategy to achieve high-performance photocatalysts. Herein, a 2D/2D inorganic/organic hybrid photocatalyst was constructed by the electrostatic self-assembly of the lead-free double-perovskite of Cs₂AgBiBr₆ nanosheets (NSs) and covalent triazine framework (CTF) NSs. The resultant Cs₂AgBiBr₆/CTF-1 (CABB/CTF-1) hybrid possessed a large surface-to-surface contact area, ensuring intimate interfacial interaction and efficient charge transfer/separation. Meanwhile, the periodical pore structure of CTF-1 endowed the CABB/CTF-1 hybrid with enhanced CO₂ adsorption/activation capacity. Consequently, the 2D/2D CABB/CTF-1 hybrid exhibited a remarkable photocatalytic performance toward CO₂ reduction. Based on the band structure analysis and various characterization techniques, for example, X-ray photoelectron spectra and electron spin resonance, an S-scheme charge transfer mechanism was proposed. This study presents a new protocol for designing 2D/2D inorganic/organic hybrid photocatalytic systems, which hold great potentials in solar fuel applications.

Insights into the Crucial Role of Electron and Spin Structures in Heteroatom-Doped Covalent Triazine Frameworks for Removing Organic Micropollutants

The water shortage crisis, characterized by organic micropollutants (OMPs), urgently requires new materials and methods to deal with it. Although heteroatom doping has been developed into an effective method to modify carbon nanomaterials for various heterogeneous adsorption and catalytic oxidation systems, the active source regulated by intrinsic electron and spin structures is still obscure. Here, a series of nonmetallic element-doped (such as P, S, and Se) covalent triazine frameworks (CTFs) were constructed and applied to remove organic pollutants using the adsorption-photocatalysis process. The external mass transfer model (EMTM) and the homogeneous surface diffusion model (HSDM) were employed to describe the adsorption process. It was found that sulfur-doped CTF (S-CTF-1) showed a 25.6-fold increase in saturated adsorption capacity (554.7 μmol/g) and a 169.0-fold surge in photocatalytic kinetics (5.07 h^-1), respectively, compared with the pristine CTF-1. A positive correlation between electron accumulation at the active site (N1 atom) and adsorption energy was further demonstrated with experimental results and theoretical calculations. Meanwhile, the photocatalytic degradation rates were greatly enhanced by forming a built-in electric field driven by spin polarization. In addition, S-CTF-1 still maintained a 98.3% removal of 2,2',4,4'-tetrahydroxybenzophenone (BP-2) micropollutants and 97.6% regeneration after six-cycle sequencing batch treatment in real water matrices. This work established a relation between electron and spin structures for adsorption and photocatalysis, paving a new way to design modified carbon nanomaterials to control OMPs.

Singlet Oxygen and Mobile Hydroxyl Radicals Co-operating on Gas-Solid Catalytic Reaction Interfaces for Deeply Oxidizing NOx

Learning from the important role of porphyrin-based chromophores in natural photosynthesis, a bionic photocatalytic system based on tetrakis (4-carboxyphenyl) porphyrin-coupled TiO₂ was designed for photo-induced treating low-concentration NO_x indoor gas (550 parts per billion), achieving a high NO removal rate of 91% and a long stability under visible-light (λ ≥ 420 nm) irradiation. Besides the great contribution of the conventional ^•O₂^- reactive species, a synergic effect between a singlet oxygen (¹O₂) and mobile hydroxyl radicals (^•OH_f) was first illustrated for removing NO_x indoor gas (¹O₂ + 2NO → 2NO₂, NO₂ + ^•OH_f → HNO₃), inhibiting the production of the byproducts of NO₂. This work is helpful for understanding the surface mechanism of photocatalytic NO_x oxidation and provides a new perspective for the development of highly efficient air purification systems.

In-Situ Prepared NRCPFs as Highly Active Photo-platforms for In-Situ Bond Formation Between Aryldiazonium Salts and Heteroarenes

Covalent perylene frameworks (CPFs) with melamine linkages have newly received risinginterest for a variety of applications because of nitrogen rich content and high stability. Herein we account a new simple strategy to in-situ attain nitrogen rich covalent peryleneframeworks (NRCPFs) as highly active photo-platforms for in-situ bond formation betweenaryldiazonium salts and heteroarenes (C-H bond arylation) through the controlled photoredoxroute.

Crystalline Covalent Triazine Frameworks with Fibrous Morphology via a Low-Temperature Polycondensation of Planar Monomer

The morphology regulation of covalent triazine frameworks (CTFs) is a great challenge, which may be due to the difficulty in controlling its morphology by traditional synthesis methods. Herein, a general approach to fabricate morphology controllable CTFs by a mild polycondensation reaction in mixed solvents without any templating agents is reported. As a proof of concept, a type of crystalline CTFs with distinctive fibrous morphology (MS-F-CTF-1) (MS: Mixed Solvent; F: Fibrous Morphology) is developed by adjusting the ratio of mixed solvents to control the solubility of monomers, so that the nucleation, crystal growth, and subsequent self-assembly are controlled, which facilitates the formation of fibrous morphology. The resultant crystalline MS-F-CTF-1 shows uniform fibrous morphology with a diameter of about 100 nm and a length of several micrometers. Notably, the fibrous morphology of CTFs can efficiently improve the photocatalytic hydrogen evolution performance, in which the hydrogen evolution rate can be boosted by about two times in comparison to block ones.

Two-Dimensional Covalent Organic Frameworks as Photocatalysts for Solar Energy Utilization

In the context of energy crisis and global warming, developing clean and sustainable energy is receiving increasing attention. Photocatalytic process including water splitting, CO₂ reduction, coenzyme regeneration, etc., provides an ideal way to utilize renewable solar resources. The photocatalyst plays a central role in photocatalytic processes. Organic porous polymers have recently gained extensive attention in photocatalysis. Covalent organic frameworks (COFs), as one of the organic porous polymers, have the characteristics of high crystallinity, porosity and structural designability that make them perfect platforms for photocatalysis. In this minireview, the recent progresses of 2D COFs as photocatalysts were summarized including our recent work. The synthesis of the diversified structures of the COFs including the different linkages was first introduced. Then, the photocatalytic applications of the 2D COFs including photocatalytic hydrogen evolution, CO₂ conversion, coenzyme regeneration and other traditional organic reaction were then discussed. Finally, conclusions and prospects were provided in the last section. This article is protected by copyright. All rights reserved.

Fused-Ring-Linked Covalent Organic Frameworks

The development of linkage chemistry in the research area of covalent organic frameworks (COFs) is fundamentally important for creating robust structures with high crystallinity and diversified functionality. We reach herein a new level of complexity and controllability in linkage chemistry by achieving the first synthesis of fused-ring-linked COFs. A series of bicyclic pyrano[4,3-b]pyridine COFs have been constructed via a cascade protocol involving Schiff-base condensation, intramolecular [4 + 2] cycloaddition, and dehydroaromatization. With a broad scope of Brønsted or Lewis acids as the catalyst, the designed monomers, that is, O-propargylic salicylaldehydes and multitopic anilines, were converted into the fused-ring-linked frameworks in a one-pot fashion. The obtained COFs exhibited excellence in terms of purity, stability, and crystallinity, as comprehensively characterized by solid-state nuclear magnetic resonance (NMR) spectroscopy, powder X-ray diffraction, high-resolution transmission electron microscopy, and so on. Specifically, the highly selective formation (>94%) of pyrano[4,3-b]pyridine linkage was verified by quantitative NMR measurements combined with ¹³C-labeling synthesis. Moreover, the fused-ring linkage possesses fully locked conformation, which benefits to the high crystallinity observed for these COFs. Advancing the linkage chemistry from the formation of solo bonds or single rings to that of fused rings, this study has opened up new possibilities for the concise construction of sophisticated COF structures with high controllability.

Three-Dimensional Crystalline Covalent Triazine Frameworks via a Polycondensation Approach

The growth of crystalline covalent triazine frameworks (CTFs) is still considered as a great challenge due to the less reversible covalent bonds of triazine linkages. The research studies of crystalline CTFs to date have been limited to two-dimensional (2D) structures, and the three-dimensional (3D) crystalline CTFs have never been reported before. Herein we report the design and synthesis of two 3D crystalline CTFs, termed 3D CTF-TPM and 3D CTF-TPA through a reversible/irreversible polycondensation approach. The targeted 3D CTFs adopt ctn topology, and show moderate crystallinity, relatively large surface area (ca. 2000 m²  g^-1 ), and high CO₂ uptake capacity (23.61 wt.%). Moreover, these 3D CTFs exhibit ultrastability in the presence of boiling water, strong acid (1 M HCl) and strong base (1 M NaOH). This contribution represents the first report of 3D crystalline CTFs, which not only extends their structural diversity but also offers a synthetic strategy and structural basis for expanding practical applications of CTF materials.

Graphitic Aza-Fused π-Conjugated Networks: Construction, Engineering, and Task-Specific Applications

2D π-conjugated networks linked by aza-fused units represent a pivotal category of graphitic materials with stacked nanosheet architectures. Extensive efforts have been directed at their fabrication and application since the discovery of covalent triazine frameworks (CTFs). Besides the triazine cores, tricycloquinazoline and hexaazatriphenylene linkages are further introduced to tailor the structures and properties. Diverse related materials have been developed rapidly, and a thorough outlook is necessitated to unveil the structure-property-application relationships across multiple subcategories, which is pivotal to guide the design and fabrication toward enhanced task-specific performance. Herein, the structure types and development of related materials including CTFs, covalent quinazoline networks, and hexaazatriphenylene networks, are introduced. Advanced synthetic strategies coupled with characterization techniques provide powerful tools to engineer the properties and tune the associated behaviors in corresponding applications. Case studies in the areas of gas adsorption, membrane-based separation, thermo-/electro-/photocatalysis, and energy storage are then addressed, focusing on the correlation between structure/property engineering and optimization of the corresponding performance, particularly the preferred features and strategies in each specific field. In the last section, the underlying challenges and opportunities in construction and application of this emerging and promising material category are discussed.

Triazine 2D Nanosheets as a New Class of Nanomaterials: Crystallinity, Properties and Applications

Based on the recent (2015–2021) literature data, the authors analyze the mutual dependence of crystallinity/amorphism and specific surface area and porosity in covalent triazine frameworks (CTFs), taking into account thermodynamic and kinetic control in the synthesis of these 2D nanosheets. CTFs have now become a promising new class of high-performance porous organic materials. They can be recycled and reused easily, and thus have great potential as sustainable materials. For 2D CTFs, numerous examples are given to support the known rule that the structure and properties of any material with a given composition depend on the conditions of its synthesis. The review may be useful for elder students, postgraduate students, engineers and research fellows dealing with chemical synthesis and modern nanotechnologies based on 2D covalent triazine frameworks.

Dual Rate-Modulation Approach for the Preparation of Crystalline Covalent Triazine Frameworks Displaying Efficient Sodium Storage

A dual rate-modulation approach was implemented for the first time to create crystalline covalent triazine frameworks. Based on a new polycondensation approach, regulating the condensation rate via the exploitation of a modulated aldehyde monomer and addition of an extrinsic inhibitor affords inherent control over the polymer growth and therefore provides tunable crystallinities and porosities for the resulting triazine frameworks. The existence of rich redox-active triazine linkages gives rise to obtaining exceptional sodium storage, where 239 mAh g^-1 at 1.0 A g^-1 is obtained after 200 cycles. We anticipate this new protocol based on the dynamic imine metathesis will facilitate new possibilities for the construction of crystalline covalent triazine frameworks and promote their energy-related applications.

Design strategies for improving the crystallinity of covalent organic frameworks and conjugated polymers: a review

Highly crystalline covalent organic frameworks (COFs) or conjugated polymers (CPs) are very important and highly desirable because these materials would display better performance in diverse devices and provide more structure-property related information. However, how to achieve highly crystalline or single-crystal COFs and CPs is very challenging. Recently, many research studies have demonstrated the possibility of enhancing the crystallinity of COFs and CPs. Thus, it is timely to offer an overview of the important progress in improving the crystallinity of COFs and CPs from the viewpoint of design strategies. These strategies include polycondensation reaction optimization, improving the planarity, fluorine substitution, side chain engineering, and so on. Furthermore, the challenges and perspectives are also discussed to promote the realization of highly crystalline or single-crystal COFs and CPs.

Covalent Triazine-Based Frameworks with Cobalt-Loading for Visible Light-Driven Photocatalytic Water Oxidation

Conjugated polymers have received significant attention as photocatalysts. However, photocatalytic oxygen evolution has only been reported for a few polymers so far. Here, we present a bipyridine-based covalent triazine-based framework...

Synthesis of highly crystalline imine-linked covalent organic frameworks via controlling monomer feeding rates in an open system

Covalent organic frameworks (COFs) have enormous potential in various applications because of their high crystallinity and superior surface area.

The O/S heteroatom effects of covalent triazine frameworks for photocatalytic hydrogen evolution

S/O heterocyclic covalent triazine frameworks (CTFs i.e., CTF-7 and CTF-8) were synthesized using thiophene and furan as building blocks, respectively. The hydrogen evolution rate of CTF-7 is 7430 μmol g^-1 h^-1, which is about 5.6 times that of CTF-8. Due to their low electronegativity, sulfur heteroatoms are more favorable for charge separation than oxygen heteroatoms in CTFs. This work provides a guiding principle for the design of high efficiency photocatalyst structures.

Structure modulation of g-C3N4 in TiO2{001}/g-C3N4 hetero-structures for boosting photocatalytic hydrogen evolution

Structure design of photocatalysts is highly desirable for taking full advantage of their abilities for H₂ evolution. Herein, the highly-efficient TiO₂{001}/g-C₃N₄ (TCN) heterostructures have been fabricated successfully via an in situ ethanol-thermal method. And the structure of g-C₃N₄ in the TCN heterostructures could be exfoliated from bulk g-C₃N₄ to nanosheets, nanocrystals and quantum dots with the increase of the synthetic temperature. Through detailed characterization, the structural evolution of g-C₃N₄ could be attributed to the enhanced temperature of the ethanol-thermal treatment with the shear effects of HF acid. As expected, the optimal TCN-2 heterostructure shows excellent photocatalytic H₂ evolution efficiency (1.78 mmol h⁻¹ g⁻¹) under visible-light irradiation. Except for the formed built-in electric field, the significantly enhanced photocatalytic activity of TCN-2 could be ascribed to the enhanced crystallinity of TiO₂{001} nanosheets and the formed g-C₃N₄ nanocrystals with large surface area, which could extend the visible light absorption, and expedite the transfer of photo-generated charge carriers further. Our work could provide guidance on designing TCN heterostructures with the desired structure for highly-efficient photocatalytic water splitting.

The structure of g-C₃N₄ in the prepared TiO₂{001}/g-C₃N₄ hetero-structures could be modulated from BCN to CN-NS, CN-NC or CN-QD by tuning the synthetic temperature.