Band Gap Engineering in Solvochromic 2D Covalent Organic Framework Photocatalysts for Visible Light-Driven Enhanced Solar Fuel Production from Carbon Dioxide

Covalent Organic Frameworks with Constrained Palladium Nanoclusters Lock the Enol-Keto Tautomerism Enhancing Hydrogen Peroxide Photosynthesis

The production of hydrogen peroxide from natural seawater is green and sustainable. However, the efficiency of photocatalytic hydrogen peroxide production is low due to the degradation and poisoning of photocatalysts by the abundant ions in natural seawater. Precisely controlling the structure of photocatalysts to enhance their corrosion resistance and minimize the impact of external particles is a challenging task. Here, a novel molecular engineering strategy is reported in which confined Pd nanoclusters locked keto structures to enhance photocatalytic hydrogen peroxide production. Both experimental and theoretical findings reveals that Pd nanoclusters, when confined within a keto structure, not only bolster the stability of the photocatalyst but also augment the efficiency of photogenerated charge carriers' separation and transport. This dual enhancement significantly boosts the photocatalytic performance for hydrogen peroxide synthesis. One of the photocatalysts, TAPT-2KtTb Pd COF, exhibits an impressive photocatalytic hydrogen peroxide production rate of 2676.3 µmol g-1 h-1, which is one of the highest values reported so far. Integral photocatalyzed H2O2 synthesis using confined Pd nanoclusters locked keto conformation COF provides a new insight for developing innovative photocatalysts for H2O2 synthesis.

Toward Pore Size-Selective Photoredox Catalysis Using Bifunctional Microporous 2D Triazine-Based Covalent Organic Frameworks

The design and synthesis of photoactive metal-free 2D materials for selective heterogeneous photoredox catalysis continue to be challenging due to issues related to nonrecyclability, and limited photo- and chemical stability. Herein, we report the photocatalytic properties of a triazine-based porous COF, TRIPTA, which is found to be capable of facilitating both SET (single electron transfer) for photocatalytic reductive debromination of phenacyl bromide in absence of oxygen and generation of reactive oxygen species (ROS) for benzylamine photo-oxidation in the presence of oxygen, respectively, under visible light irradiation. Inspired by the latter results, we further systematically investigated different-sized benzylamine substrates in this single-component reaction and compared the results with an analogous COF (Micro-COF-2) exhibiting a larger pore size. We observed a marked improvement in the conversion of larger-sized substrates with the latter COF, thereby demonstrating angstrom-level pore size-selective photocatalytic activity of COFs.

Tunable Isometric Donor-Acceptor Wurster-Type Covalent Organic Framework Photocathodes

Covalent organic frameworks (COFs) offer remarkable versatility, combining ordered structures, high porosity, and tailorable functionalities in nanoscale reaction spaces. Herein, we report the synthesis of a series of isostructural, photoactive Wurster-type COFs achieved by manipulating the chemical and electronic nature of the Wurster aromatic amine building blocks. A series of donor-acceptor-donor (D-A-D) Wurster building block molecules was synthesized by incorporating heteroaromatic acceptors with varying strengths between triphenylamine donor groups. These tailored building blocks were integrated into a 2D COF scaffold, resulting in highly crystalline structures and similar morphologies across all COFs. Remarkably, this structural uniformity was also achieved in the synthesis of homogeneous and oriented thin films. Steady-state photoluminescence revealed a tunable red-shift in film emission exceeding 100 nm, demonstrating effective manipulation of their optical properties. Furthermore, photoelectrochemical (PEC) water splitting studies exhibited a doubled current density (8.1 μA cm-2 at 0.2 VRHE) for the COF with the strongest acceptor unit. These findings highlight the potential of Wurster D-A-D COFs in photoelectrochemical water splitting devices and pave the way for further exploration of chemical functionality-reactivity-property relationships in this promising class of photoactive materials.

Photocatalytic applications of covalent organic frameworks: synthesis, characterization, and utility

Photocatalysis has emerged as an energy efficient and safe method to perform organic transformations, and many semiconductors have been studied for use as photocatalysts. Covalent organic frameworks (COFs) are an established class of crystalline, porous materials constructed from organic units that are easily tunable. COFs importantly display semiconductor properties and respectable photoelectric behaviour, making them a strong prospect as photocatalysts. In this review, we summarize the design, synthetic methods, and characterization techniques for COFs. Strategies to boost photocatalytic performance are also discussed. Then the applications of COFs as photocatalysts in a variety of reactions are detailed. Finally, a summary, challenges, and future opportunities for the development of COFs as efficient photocatalysts are entailed.

Photons, Excitons, and Electrons in Covalent Organic Frameworks

Covalent organic frameworks (COFs) are created by the condensation of molecular building blocks and nodes to form two-dimensional (2D) or three-dimensional (3D) crystalline frameworks. The diversity of molecular building blocks with different properties and functionalities and the large number of possible framework topologies open a vast space of possible well-defined porous architectures. Besides more classical applications of porous materials such as molecular absorption, separation, and catalytic conversions, interest in the optoelectronic properties of COFs has recently increased considerably. The electronic properties of both the molecular building blocks and their linkage chemistry can be controlled to tune photon absorption and emission, to create excitons and charge carriers, and to use these charge carriers in different applications such as photocatalysis, luminescence, chemical sensing, and photovoltaics. In this Perspective, we will discuss the relationship between the structural features of COFs and their optoelectronic properties, starting with the building blocks and their chemical connectivity, layer stacking in 2D COFs, control over defects and morphology including thin film synthesis, exploring the theoretical modeling of structural, electronic, and dynamic features of COFs, and discussing recent intriguing applications with a focus on photocatalysis and photoelectrochemistry. We conclude with some remarks about present challenges and future prospects of this powerful architectural paradigm.

Covalent Organic Frameworks for Photocatalytic Reduction of Carbon Dioxide: A Review

Covalent organic frameworks (COFs) are crystalline networks with extended backbones cross-linked by covalent bonds. Due to the semiconductive properties and variable metal coordinating sites, along with the rapid development in linkage chemistry, the utilization of COFs in photocatalytic CO2RR has attracted many scientists' interests. In this Review, we summarize the latest research progress on variable COFs for photocatalytic CO2 reduction. In the first part, we present the development of COF linkages that have been used in CO2RR, and we discuss four mechanisms including COFs as intrinsic photocatalysts, COFs with photosensitive motifs as photocatalysts, metalated COF photocatalysts, and COFs with semiconductors as heterojunction photocatalysts. Then, we summarize the principles of structural designs including functional building units and stacking mode exchange. Finally, the outlook and challenges have been provided. This Review is intended to give some guidance on the design and synthesis of diverse COFs with different linkages, various structures, and divergent stacking modes for the efficient photoreduction of CO2.

Large Area Film of Highly Crystalline, Cleavable, and Transferable Semi-Conducting 2D-Imine Covalent Organic Framework on Dielectric Glass Substrate

Two dimensional (2D) imine-based covalent organic framework (COF), 2D-COF, is a newly emerging molecular 2D polymer with potential applications in thin film electronics, sensing, and catalysis. It is considered an ideal candidate due to its robust 2D nature and precise tunability of the electronic and functional properties. Herein, we report a scalable facile synthesis of 2D imine-COF with control over film thickness (ranging from 100 nm to a few monolayers) and film dimension reaching up to 2 cm on a dielectric (glass) substrate. Highly crystalline 2D imine polymer films are formed by maintaining a quasi-equilibrium (very slow, ∼15 h) in Schiff base condensation reaction between p-phenylenediamine (PDA) and benzene-1,3,5-tricarboxaldehyde (TCA) molecules. Free-standing thin and ultrathin films of imine-COF are obtained using sonication exfoliation of 2D-COF polymer. Insights into the microstructure of thin/ultrathin imine-COF are obtained using scanning and transmission electron microscopy (SEM and TEM) and atomic force microscopy (AFM), which shows high crystallinity and 2D layered structure in both thin and ultrathin films. The chemical nature of the 2D polymer was established using X-ray photoelectron spectroscopy (XPS). Optical band gap measurements also reveal a semiconducting gap. This is further established by electronic structure calculation using density functional theory (DFT), which reveals a semiconductor-like band structure with strong dispersion in bands near conduction and valence band edges. The structural characteristics (layered morphology and microscopic structure) of 2D imine-COF show significant potential for its application in thin film device fabrication. In addition, the electronic structure shows strong dispersion in the frontier bands, making it a potential semiconducting material for charge carrier transportation in electronic devices.

Regioisomerism in Thienothiophene-Based Covalent Organic Frameworks─A Tool for Band-Gap Engineering

The craft of tuning optical properties is well-established for crystalline inorganic and hybrid solids. However, a far greater challenge is to tune the optical properties of organic materials systematically by design. We now introduce a synthesis concept that enables us to alter the optical properties of crystalline covalent organic frameworks (COFs) systematically using isomeric structures of thienothiophene-based building blocks (T23/32T) combined with a variety of tetratopic aromatic amines, e.g., the Wurster moiety (W-NH2). This concept is demonstrated for the synthesis of COFs in bulk and film forms and provides highly crystalline and porous isomeric COFs featuring predesigned photophysical properties. The band gap of the framework can be tuned continuously and precisely by chemically doping the pristine W23TT COF with its related constitutional isomer building block. Density-functional theory investigations of COF model compounds indicate that the extent of π-conjugation is among the key characteristics enabling the band-gap engineering.

Decoding Excimer Formation in Covalent-Organic Frameworks Induced by Morphology and Ring Torsion

A thorough and quantitative understanding of the fate of excitons in covalent-organic frameworks (COFs) after photoexcitation is essential for their augmented optoelectronic and photocatalytic applications via precise structure tuning. The synthesis of a library of COFs having identical chemical backbone with impeded conjugation, but varied morphology and surface topography to study the effect of these physical properties on the photophysics of the materials is herein reported. The variation of crystallite size and surface topography substantified different aggregation pattern in the COFs, which leads to disparities in their photoexcitation and relaxation properties. Depending on aggregation, an inverse correlation between bulk luminescence decay time and exciton binding energy of the materials is perceived. Further transient absorption spectroscopic analysis confirms the presence of highly localized, immobile, Frenkel excitons (of diameter 0.3-0.5 nm) via an absence of annihilation at high density, most likely induced by structural torsion of the COF skeletons, which in turn preferentially relaxes via long-lived (nanosecond to microsecond) excimer formation (in femtosecond scale) over direct emission. These insights underpin the importance of structural and topological design of COFs for their targeted use in photocatalysis.

Mechanism insight into double S-scheme heterojunctions and atomic vacancies with tunable band structures for notably enhanced light-driven enrofloxacin decomposition

Building narrow band gap semiconductors and fast separation of photogenerated electron-hole (e--h+) structures are of great significance for photocatalytic process. In this contribution, the CeO2-x/C3-yN4/Ce(CO3)(OH) double S-scheme heterojunctions with atomic vacancies tunable band gap (2.54 eV) have been designed and fabricated as a boost photocatalyst for enrofloxacin (ENR) photodegradation. Compared with the control samples, the experimental results indicate that the typical sample (CeO2-x/C3-yN4/Ce(CO3)(OH)-2) achieves the highest ENR photodegradation efficiency (93.6 %) in 240 min under a pH of 6, and the possible photodegradation pathways are also proposed. The superior performance is ascribed to the CeO2-x/C3-yN4/Ce(CO3)(OH) double S-scheme heterojunctions for selective recombination of photogenerated electrons with weak-reduction ability in conduction band (CB) of CeO2-x, C3-yN4 and the photogenerated holes with weak-oxidation nature in valance band (VB) of C3-yN4, Ce(CO3)(OH), which increase the retention rate of photogenerated electrons in CB of Ce(CO3)(OH) and photogenerated holes in VB of CeO2-x to degrade ENR. This is the first systematic study of CeO2-x/C3-yN4/Ce(CO3)(OH) double S-scheme heterojunctions for ENR photodegradation.

Modulating β-Keto-enamine-Based Covalent Organic Frameworks for Photocatalytic Atom-Transfer Radical Addition Reaction

The atom-transfer radical addition (ATRA) reaction simultaneously forges carbon-carbon and carbon-halogen bonds. However, frequently-used photosensitizers such as precious transition metal complexes, or organic dyes have limitations in terms of their potential toxicity and recyclability. Three β-ketoenamine-linked covalent organic frameworks (COFs) from 1,3,5-triformylphloroglucinol and 1,4-phenylenediamines with variable transient photocurrent and photocatalytic activity have been prepared. A COF bearing electron-deficient Cl atoms displayed the highest photocatalytic activity toward the ATRA reaction of polyhalogenated alkanes to give halogenated olefins under visible light at room temperature. This heterogeneous photocatalyst exhibited good functional group tolerance and could be recycled without significant loss of activity.

Understanding Photocatalytic Overall Water Splitting of β-Ketoamine COFs through the N-C Site Synergistic Mechanism

Overcoming the sluggish reaction kinetics of the oxygen evolution reaction (OER) is a determining factor for the practical application of photocatalysts for overall water splitting. Two-dimensional covalent organic frameworks (2D-COFs) offer an ideal platform for catalyst design in the field of overall water splitting for their exceptional chemical tunability and high efficiency of light capture. In this work, four β-ketoamine 2D-COFs, consisting of 1,3,5-triformylphloroglucinol (Tp) groups and different linkers with pyridine segments, were constructed and optimized. By means of first-principles calculations, the band structures, free energy changes of photocatalytic hydrogen evolution reaction (HER) and OER, and charge density distributions were calculated and investigated systemically to discuss the visible-light response, overall water splitting activities on active sites, and the characteristic of charge transfer and separation. The protonated pyridine N derived from the double-H2O closed-ring H-bond adsorption model could efficiently induce N-C sites' synergistic effect between pyridine N and its ortho-position C to minimize the OER energy barrier and to enhance the charge transfer and separation. A N-C site synergistic mechanism has been proposed to provide a comprehensive explanation for the experimental results and a new strategy to design novel 2D-COF photocatalysts for overall water splitting.

The Keto-Switched Photocatalysis of Reconstructed Covalent Organic Frameworks for Efficient Hydrogen Evolution

The keto-switched photocatalysis of covalent organic frameworks (COFs) for efficient H2 evolution was reported for the first time by engineering, at a molecular level, the local structure and component of the skeletal building blocks. A series of imine-linked BT-COFs were synthesized by the Schiff-base reaction of 1, 3, 5-benzenetrialdehyde with diamines to demonstrate the structural reconstruction of enol to keto configurations by alkaline catalysis. The keto groups of the skeletal building blocks served as active injectors, where hot π-electrons were provided to Pt nanoparticles (NPs) across a polyvinylpyrrolidone (PVP) insulting layer. The characterization results, together with density functional theory calculations, indicated clearly that the formation of keto-injectors not only made the conduction band level more negative, but also led to an inhomogeneous charge distribution in the donor-acceptor molecular building blocks to form a strong intramolecular built-in electric field. As a result, visible-light photocatalysis of TP-COFs-1 with one keto group in the skeletal building blocks was successfully enabled and achieved an impressive H2 evolution rate as high as 0.96 mmol g-1  h-1 . Also, the photocatalytic H2 evolution rates of the reconstructed BT-COFs-2 and -3 with two and three keto-injectors were significantly enhanced by alkaline post-treatment.

Photoreforming for Lignin Upgrading: A Critical Review

Photoreforming of lignocellulosic biomass to simultaneously produce gas fuels and value-added chemicals has gradually emerged as a promising strategy to alleviate the fossil fuels crisis. Compared to cellulose and hemicellulose, the exploitation and utilization of lignin via photoreforming are still at the early and more exciting stages. This Review systematically summarizes the latest progress on the photoreforming of lignin-derived model components and "real" lignin, aiming to provide insights for lignin photocatalytic valorization from fundamental to industrial applications. Considering the complexity of lignin physicochemical properties, related analytic methods are also introduced to characterize lignin photocatalytic conversion and product distribution. We finally put forward the challenges and perspective of lignin photoreforming, hoping to provide some guidance to valorize biomass into value-added chemicals and fuels via a mild photoreforming process in the future.

Benzotrithiophene-based Covalent Organic Framework Photocatalysts with Controlled Conjugation of Building Blocks for Charge Stabilization

Covalent organic frameworks have recently shown high potential for photocatalytic hydrogen production. However, their structure-property-activity relationship has not been sufficiently explored to identify a research direction for structural design. Herein, we report the design and synthesis of four benzotrithiophene (BTT)-based covalent organic frameworks (COFs) with different conjugations of building units, and their photocatalytic activity for hydrogen production. All four BTT-COFs had slipped parallel stacking patterns with high crystallinity and specific surface areas. The change in the degree of conjugation was found to rationally tune the rate of photocatalytic hydrogen evolution. Based on the experimental and calculation results, the tunable photocatalytic performance could be mainly attributed to the electron affinity and charge trapping of the electron accepting units. This study provides important insights for designing covalent organic frameworks for efficient photocatalysts.

Aromatic Amine-Functionalized Covalent Organic Frameworks (COFs) for CO2/N2 Separation

CO₂ is a prominent example for an exhaust gas, and it is known for its high impact on global warming. Therefore, carbon capture from CO₂ emissions of industrial processes is increasingly important to halt and prevent the disruptive consequences of global warming. Covalent organic frameworks (COFs) as porous nanomaterials have been shown to selectively adsorb CO₂ in high quantities and with high CO₂/N₂ selectivity. Interactions with amines are recognized to selectively adsorb CO₂ and help capture it from exhaust emissions. Herein, a novel COF (Me₃TFB-(NH₂)₂BD), which was not accessible via a direct condensation reaction, was synthetized by dynamic linker exchange starting with Me₃TFB-BD. Despite the linker exchange, the porosity of the COF was largely maintained, resulting in a high BET surface area of 1624 ± 89 m²/g. The CO₂ and N₂ adsorption isotherms at 273 and 295 K were studied to determine the performance in carbon capture at flue gas conditions. Me₃TFB-(NH₂)₂BD adsorbs 1.12 ± 0.26 and 0.72 ± 0.07 mmol/g of CO₂ at 1 bar and 273 and 295 K, respectively. The COF shows a high CO₂/N₂ IAST selectivity under flue gas conditions (273 K:83 ± 11, 295 K: 47 ± 11). The interaction of the aromatic amine groups with CO₂ is based on physisorption, which is expected to make the regeneration of the material energy efficient.

Rational modification of hydroxy-functionalized covalent organic frameworks for enhanced photocatalytic hydrogen peroxide evolution

Covalent organic frameworks (COFs), a class of flexibly tunable crystalline materials, have fascinating potential in photocatalytic hydrogen peroxide (H2O2) evolution under visible light irradiation. However, achieving efficient catalytic activity by tuning the composition of COFs and the linkages of building blocks is still a challenge. Herein, four imine-linked COFs with different numbers of hydroxy-functionalized are constructed to unveil the latent structure-activity relationship between the reversibility of bonding in supramolecular chemistry and the photocatalytic H2O2 performance. As the optimized material, TAPT-HTA-COF (1H-COF) containing single hydroxy group in aldehyde node exhibits a highest ordered structure and conjugation degree along and across the plane in the extended frameworks originating from the flexibly reversible iminol-to-ketoenamine tautomerism than others, which broadens the visible light absorption and accelerates the dissociation of photogenerated carriers in 1H-COF. These merits ensure that 1H-COF has the highest H2O2 yield (44.5 μmol L-1) and O2 two-electron reduction pathway among the four COFs under visible light irradiation (λ > 420 nm, 10 vol% isopropanol aqueous solution). At the same time, the long-range ordered framework of 1H-COF is well preserved during the photocatalytic H2O2 evolution process assisted by the proton-induced tautomerization. This work facilitates the design and development of COF-based photocatalysts in the evolution of H2O2.

Tuning UV Absorption in Imine-Linked Covalent Organic Frameworks via Methylation

Covalent organic frameworks (COFs) are porous materials with high surface areas, making them interesting for a large variety of applications including energy storage, gas separation, photocatalysis, and chemical sensing. Structural variation plays an important role in tuning COF properties. Next to the type of the building block core, bonding directionality, and linking chemistry, substitution of building blocks provides another level of synthetic control. Thorough characterization and comparison of various substitution patterns is relevant for the molecular engineering of COFs via rational design. To this end, we have systematically synthesized and characterized multiple combinations of several methylated and non-methylated building blocks to obtain a series of imine-based COFs. This includes the experimental assignment of the COF structure by solid-state NMR. By comparing the properties of all COFs, the following trends were found: (1) upon methylation of the aldehyde nodes, COFs show increased Brunauer-Emmett-Teller surface areas, reduced pore collapse, blue-shifted absorbance spectra, and ∼0.2 eV increases in their optical band gaps. (2) COFs with dimethylated amine linkers show a lower porosity. (3) In tetramethylated amine linkers, the COF porosity even further decreases, the absorbance spectra are clearly red-shifted, and smaller optical band gaps are obtained. Our study shows that methyl substitution patterns on COF building blocks are a handle to control the UV absorbance of the resulting frameworks.

Effect of Nitrogen Atom Introduction on the Photocatalytic Hydrogen Evolution Activity of Covalent Triazine Frameworks: Experimental and Theoretical Study

The construction of high-performance photocatalyst has always been explored. Covalent organic frameworks (COFs), especially keto-amine-linked COFs, have many advantages, such as adjustable bandgaps, π-π stacking structure, excellent response ability to visible light, high specific surface area, high mobility of carrier carriers, good physical and chemical stability, and so on, showing strong potential applications in photocatalytic solar energy conversion and hydrogen production. Two analogous covalent triazine frameworks (CTFs), T3H-CTF and T3N-CTF, have been synthesized via Schiff-base condensation reactions between 2,4,6-trihydroxybenzene-1,3,5-tricarbalehyde (MOP) and the corresponding triazine-based aromatic amines under solvothermal condition. For T3N-CTF, the peripheral aromatic linker to the central triazine unit was the pyridine unit, instead of the benzene unit in the T3H-CTF unit. T3N-CTF had a hydrogen production rate (HPR) of 6485.05 μmol g^-1  h^-1 , much higher than that of T3H-CTF (2028.06 μmol g^-1  h^-1 ). Accordingly, T3N-CTF had a much higher apparent quantum yield (AQY) of 12.2 % than that of T3H-CTF (4.12 %) at 405 nm. The experimental and theoretical results showed that the extended light absorption range, enlarged surface area, and enhanced separation and transportation efficiencies of charge carriers of T3N-CTF compared with T3H-CTF were uniformly induced by the introduction of peripheral nitrogen atoms into the skeleton of former CTF, which eventually boosted the visible-light induced hydrogen evolution reaction (HER). The work suggests a new method for enhancing the intrinsic HER activity by modulating the electronic features of the conjugated COFs by the introduction of pyridinic N atoms.

Recent advances in the tuning of the organic framework materials - The selections of ligands, reaction conditions, and post-synthesis approaches

Organic framework materials, particularly metal-organic frameworks (MOFs), graphene-organic frameworks (GOFs), and covalent organic frameworks (COFs), have led to the revolution across fields including catalysts, sensors, gas capture, and biology mainly owing to their ultra-high surface area-to-volume ratio, on-demand tunable crystal structures, and unique surface properties. While the wet chemistry routes have been the predominant synthesis approach, the crystal phase, morphological parameters, and physicochemical properties of organic framework materials are largely affected by various synthesis parameters and precursors. In this work, we specifically review the influences of synthesis parameters towards crystal structures and chemical compositions of organic framework materials, including selected ligand types and lengths, reaction temperature/solvent/reactant compositions, as well as post-synthesis modification approaches. More importantly, the subsequent impacts on the general electronic, mechanical, surface chemical, and thermal properties as well as the consequent variation in performances towards catalytic, desalination, gas sensing, and gas storage applications are critically discussed. Finally, the current challenges and prospects of organic framework materials are provided.

Acetylene/Vinylene-Bridged π-Conjugated Covalent Triazine Polymers for Photocatalytic Aerobic Oxidation Reactions under Visible Light Irradiation

Solar-driven photocatalytic chemical transformation provides a sustainable strategy to produce valuable feedstock, but designing photocatalysts with high efficiency remains challenging. Herein, two acetylene- or vinylene-bridged π-conjugated covalent triazine polymers, A-CTP-DPA and V-CTP-DPE, were successfully fabricated toward metal-free photocatalytic oxidation under visible light irradiation. Compared to the one without acetylene or vinylene bridge, both resulting polymers exhibited superior activity in photocatalytic selective oxidation of sulfides and oxidative coupling of amines; in particular, A-CTP-DPA delivered an optimal photocatalytic performance. The superior activity was attributed to the broadened spectral response range, effective separation, rapid transportation of photogenerated charge carriers, and abundant active sites for photogenerated electrons due to the existence of the acetylene bridge in the framework. This work highlights the potential of acetylene and vinylene bridges in tuning catalytic efficiency of organic semiconductors, providing a guideline for the design of efficient photocatalysts.

Synthesis of highly efficient selenium oxide hybridized g-C3N4 photocatalyst for NADH/NADPH regeneration to facilitate solar-to-chemical reaction

An inexpensive graphitic carbon nitrite (g-C3N4) photocatalyst was hybridized with selenium oxide (SeO2) photocatalyst by a monolayer-dispersed technique. After hybridization of g-C3N4 with SeO2, the NADH/NADPH regeneration efficiency of SeO2 photocatalyst was enhanced under solar light illumination was observed. The photocatalytic activity of SeO2/g-C3N4 photocatalyst under solar light illumination was enhanced by 3-fold higher than g-C3N4 photocatalyst, the solar light photocatalytic activity was produced and the photo-decomposition of SeO2 photocatalyst was completely stifled after hybridized SeO2 photocatalyst by g-C3N4 photocatalyst. The improvement in performance and photo-decomposition inhibition under solar light illumination was persuaded by efficiency separation of photo-persuaded holes from SeO2 to the valence bond (V.B.)/highest occupied molecular orbital (HOMO) of g-C3N4 under solar light illumination, the electron jumped from the V.B. to the conduction band (C.B.)/lowest unoccupied molecular orbital (LUMO) of g-C3N4 could directly insert into the C.B. of SeO2 photocatalyst, synthesized SeO2/g-C3N4 photocatalyst is highly active for NADH/NADPH regeneration under solar light.

Highly hydrophilic covalent organic frameworks as efficient and reusable photocatalysts for oxidative coupling of amines in aqueous solution

Highly hydrophilic 2D-COFs, TFB-XX-DMTH, have been successfully constructed by a three-component in situ assembly strategy and exhibited superior photocatalytic performance in oxidative coupling reactions of benzylamines in aqueous solution.

Ionic Covalent Organic Frameworks for Energy Devices

Covalent organic frameworks (COFs) are a class of porous crystalline materials whose facile preparation, functionality, and modularity have led to their becoming powerful platforms for the development of molecular devices in many fields of (bio)engineering, such as energy storage, environmental remediation, drug delivery, and catalysis. In particular, ionic COFs (iCOFs) are highly useful for constructing energy devices, as their ionic functional groups can transport ions efficiently, and the nonlabile and highly ordered all-covalent pore structures of their backbones provide ideal pathways for long-term ionic transport under harsh electrochemical conditions. Here, current research progress on the use of iCOFs for energy devices, specifically lithium-based batteries and fuel cells, is reviewed in terms of iCOF backbone-design strategies, synthetic approaches, properties, engineering techniques, and applications. iCOFs are categorized as anionic COFs or cationic COFs, and how each of these types of iCOFs transport lithium ions, protons, or hydroxides is illustrated. Finally, the current challenges to and future opportunities for the utilization of iCOFs in energy devices are described. This review will therefore serve as a useful reference on state-of-the-art iCOF design and application strategies focusing on energy devices.

First-Principles Study of Hydrogen Storage of Sc-Modified Semiconductor Covalent Organic Framework-1

At present, the development of new carbon-based nanoporous materials with semiconductor properties and high hydrogen storage capacity has become a research hotspot in the field of hydrogen storage and hydrogen supply. Here, we pioneered the study of the hydrogen storage capacity of a scandium (Sc) atom-modified semiconductor covalent organic framework-1 (COF-1) layer. It was found that the hydrogen storage capacity of the COF-1 structure was significantly enhanced after the modification of the Sc atom. We found that each Sc atom of the modified COF-1 structure can stably adsorb up to four H₂ molecules, and the average adsorption energy of the four hydrogen molecules is -0.284 eV/H₂. Six Sc atoms are stably adsorbed most bilaterally on the cell of the COF-1 unit, which can adsorb 24 H₂ molecules in total. In addition, we have further studied the adsorption and desorption behaviors of H₂ molecules on the 6Sc-COF-1 surface at 300 and 400 K, respectively. It can be found that each Sc atom of the COF-1 unit cell can stably adsorb three H₂ molecules with a hydrogen storage performance of 5.23 wt % at 300 K, which is higher than those of lithium-modified phosphorene (4.4 wt %) and lithium-substituted BHNH sheets (3.16 wt %). At 400 K, all of the adsorbed H₂ molecules are released. This confirms the excellent reversibility of 6Sc-COF-1 in hydrogen storage performance. This research has great significance in the application of fuel cells, surpassing traditional hydrogen storage materials.

General framework for enzyme-photo-coupled catalytic system toward carbon dioxide conversion

High emission of carbon dioxide (CO₂) has aroused global concern due to the 'greenhouse effect'. The conversion of CO₂ to valuable chemicals/materials is an indispensable route toward 'carbon neutrality'. Enzyme-photo-coupled catalytic systems (EPCCSs), integrating synthetic library of semiconductor photocatalyst and natural database of enzyme, have emerged as a green and powerful platform toward CO₂ conversion. Herein, we discuss the recent progress in design and application of EPCCSs for CO₂ conversion from the perspective of pathway engineering, reaction engineering and system engineering. We firstly summarize the explored pathways of EPCCSs for converting CO₂ to C₁ and C₂₊ products. Secondly, we discuss the matching of kinetics between photocatalytic and enzymatic reactions in EPCCSs. Thirdly, we unveil the complex interplay between photocatalytic and enzymatic modules, and further demonstrate the strategy of compartmentalization to eliminate the negative interactions. Lastly, we conclude with the perspective on the opportunities and challenges of EPCCSs for CO₂ conversion.

Highly Efficient S-g-CN/Mo-368 Catalyst for Synergistically NADH Regeneration Under Solar Light

Sulfur-doped graphitic carbon nitride (S-g-CN) has gained significant attention in recent years. Sulfur-doped graphitic carbon nitride (S-g-CN) is a promising metal-free photocatalyst because of its band orientation, natural abundance and groundwork. Improved photocatalytic activity of S-g-CN material for solar chemical production persists a hot yet challenging problem. Herein, we provide an adaptable method for the synthesis of S-g-CN nanocomposite decorated with the moiety of giant polyoxometalate (S-g-CN/Mo-368) that subsequently showed highly efficient photocatalytic activity. The as-synthesized S-g-CN/Mo-368 as a recyclable artificial photocatalyst revealed excellent activity for solar chemical production, that is nicotinamide adenine dinucleotide (NADH) regeneration under visible light. The immobilized Mo-368 on the S-g-CN surface increased the visible light adsorption capacity of the S-g-CN/Mo-368 photocatalyst. The visible light absorption activity, morphology, element compositions, particle size and zeta potential of S-g-CN powder and S-g-CN/Mo-368 were thoroughly investigated. From the application point of view, S-g-CN/Mo-368 was applied to determine the solar chemical production (i.e. NADH regeneration) under visible light with a higher yield% of about ~ 94.85%.

Photocatalytic Water Splitting Reaction Catalyzed by Ion-Exchanged Salts of Potassium Poly(heptazine imide) 2D Materials

Potassium poly (heptazine imide) (K-PHI), a crystalline two-dimensional carbon-nitride material, is an active photocatalyst for water splitting. The potassium ions in K-PHI can be exchanged with other ions to change the properties of the material and eventually to design the catalysts. We report here the electronic structures of several ion-exchanged salts of K-PHI (K, H, Au, Ru, and Mg) and their feasibility as water splitting photocatalysts, which were determined by density functional theory (DFT) calculations. The DFT results are complemented by experiments where the performances in the photocatalytic hydrogen evolution reaction (HER) were recorded. We show that due to its narrow band gap, Ru-PHI is not a suitable photocatalyst. The water oxidation potentials are straddled between the band edge potentials of H-PHI, Au-PHI, and Mg-PHI; thus, these are active photocatalysts for both the oxygen and hydrogen evolution reactions, whereas K-PHI is active only for the HER. The experimental data show that these are active HER photocatalysts, in agreement with the DFT results. Furthermore, Mg-PHI has shown remarkable performance in the HER, with a rate of 539 μmol/(h·g) and a quantum efficiency of 7.14% at 410 nm light irradiation, which could be due to activation of the water molecule upon adsorption, as predicted by our DFT calculations.

Modulating the Stacking Model of Covalent Organic Framework Isomers with Different Generation Efficiencies of Reactive Oxygen Species

The development of a facile synthesis and controllable layer stacking approach for covalent organic frameworks (COFs) is an important issue for modulating their properties and realizing their application diversity. Herein, three COF isomers with different stacking models (eclipsed AA, staggered AB, and ABC stacking) were obtained by modulating the reaction temperature and solvent medium. Experimental and theoretical calculations show that the ABC stacking isomer obtained at room temperature is the kinetic product, while the AA stacking isomer prepared by the solvothermal method is a thermodynamic product. Owing to the tautomerism involved in the reaction process, these isomers possess different ratios of enol and keto forms. Thus, they exhibit different generation efficiencies of Type I and Type II reactive oxygen species (ROS). The ABC stacking isomers could be employed as metal-free heterogeneous photocatalysts for visible-light-induced oxidation of amines to imines, owing to the highest generation efficiency of Type I ROS.