Out-of-Plane Single-Copper-Site Catalysts for Room-Temperature Benzene Oxidation.

Crafting single-atom catalysts (SACs) that possess "just right" modulated electronic and geometric structures, granting accessible active sites for direct room-temperature benzene oxidation is a coveted objective. However, achieving this goal remains a formidable challenge. Here, we introduce an innovative in situ phosphorus-immitting strategy using a new phosphorus source (phosphorus nitride, P3N5) to construct the phosphorus-rich copper (Cu) SACs, designated as Cu/NPC. These catalysts feature locally protruding metal sites on a nitrogen (N)-phosphorus (P)-carbon (C) support (NPC). Rigorous analyses, including X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS), validate the coordinated bonding of nitrogen and phosphorus with atomically dispersed Cu sites on NPC. Crucially, systematic first-principles calculations, coupled with the climbing image nudged-elastic-band (CI-NEB) method, provide a comprehensive understanding of the structure-property-activity relationship of the distorted Cu-N2P2 centers in Cu/NPC for selective oxidation of benzene to phenol production. Interestingly, Cu/NPC has shown more energetically favorable C-H bond activation compared to the benchmark Cu/NC SACs in the direct oxidation of benzene, resulting in outstanding benzene conversion (50.3 %) and phenol selectivity (99.3 %) at room temperature. Furthermore, Cu/NPC achieves a remarkable turnover frequency of 263 h-1 and mass-specific activity of 35.2 mmol g-1 h-1, surpassing the state-of-the-art benzene-to-phenol conversion catalysts to date.

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.

Cobalt-Porphyrin-Based Covalent Organic Frameworks with Donor-Acceptor Units as Photocatalysts for Carbon Dioxide Reduction.

Covalent organic frameworks (COFs) have emerged as a promising platform for photocatalysts. Their crystalline porous nature allows comprehensive mechanistic studies of photocatalysis, which have revealed that their general photophysical parameters, such as light absorption ability, electronic band structure, and charge separation efficiency, can be conveniently tailored by structural modifications. However, further understanding of the relationship between structure-property-activity is required from the viewpoint of charge-carrier transport, because the charge-carrier property is closely related to alleviation of the excitonic effect. In the present study, COFs composed of a fixed cobalt (Co) porphyrin (Por) centered tetraamine as an acceptor unit with differently conjugated di-carbaldehyde based donor units, such as benzodithiophene (BDT), thienothiophene (TT), or phenyl (TA), were synthesized to form Co-Por-BDT, Co-Por-TT, or Co-Por-TA, respectively. Their photocatalytic activity for reducing carbon dioxide into carbon monoxide was in the order of Co-Por-BDT>Co-Por-TT>Co-Por-TA. The results indicated that the excitonic effect, associated with their charge-carrier densities and π-conjugation lengths, was a significant factor in photocatalysis performance.

Hexaazatriphenylene-Based Two-Dimensional Conductive Covalent Organic Framework with Anisotropic Charge Transfer.

The development of covalent organic frameworks (COFs) with efficient charge transport is of immense interest for applications in optoelectronic devices. To enhance COF charge transport properties, electroactive building blocks and dopants can be used to induce extended conduction channels. However, understanding their intricate interplay remains challenging. We designed and synthesized a tailor-made COF structure with electroactive hexaazatriphenylene (HAT) core units and planar dioxin (D) linkages, denoted as HD-COF. With the support of theoretical calculations, we found that the HAT units in the HD-COF induce strong, eclipsed π-π stacking. The unique stacking of HAT units and the weak in-plane conjugation of dioxin linkages leads to efficient anisotropic charge transport. We fabricated HD-COF films to minimize the grain boundary effect of bulk COFs, which resulted in enhanced conductivity. As a result, the HD-COF films showed an electrical conductivity as high as 1.25 S cm-1 after doping with tris(4-bromophenyl)ammoniumyl hexachloroantimonate.

Conductive and Ultrastable Covalent Organic Framework/Carbon Hybrid as an Ideal Electrocatalytic Platform.

Developing covalent organic frameworks (COFs) with good electrical conductivity is essential to widen their range of practical applications. Thermal annealing is known to be a facile approach for enhancing conductivity. However, at higher temperatures, most COFs undergo amorphization and/or thermal degradation because of the lack of linker rigidity and physicochemical stability. Here, we report the synthesis of a conductive benzoxazole-linked COF/carbon hybrid material (BCOF-600C) by simple thermal annealing. The fused-aromatic benzoxazole and biphenyl building units endow the resulting COF with excellent physicochemical stability against high temperatures and strong acids/bases. This allows heat treatment to further enhance electrical conductivity with minimal structural alteration. The robust crystalline structure with periodically incorporated nitrogen atoms allowed platinum (Pt) atoms to be atomically integrated into the channel walls of BCOF-600C. The resulting electrocatalyst with well-defined active sites exhibited superior catalytic performance toward hydrogen evolution in acidic media.

Crystalline Porphyrazine-Linked Fused Aromatic Networks with High Proton Conductivity.

Fused aromatic networks (FANs) have been studied in efforts to overcome the low physicochemical stability of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), while preserving crystallinity. Herein, we describe the synthesis of a highly stable and crystalline FAN (denoted as Pz-FAN) using pyrazine-based building blocks to form porphyrazine (Pz) linkages via an irreversible reaction. Unlike most COFs and FANs, which are synthesized from two different building blocks, the new Pz-FAN is formed using a single building block by self-cyclotetramerization. Controlled and optimized reaction conditions result in a highly crystalline Pz-FAN with physicochemical stability. The newly prepared Pz-FAN displayed a high magnitude (1.16×10-2  S cm-1 ) of proton conductivity compared to other reported FANs and polymers. Finally, the Pz-FAN-based membrane was evaluated for a proton-exchange membrane fuel cell (PEMFC), which showed maximum power and current densities of 192 mW cm-2 and 481 mA cm-2 , respectively.

Extreme Enhancement of Carbon Hydrogasification via Mechanochemistry.

Carbon hydrogasification is the slowest reaction among all carbon-involved small-molecule transformations. Here, we demonstrate a mechanochemical method that results in both a faster reaction rate and a new synthesis route. The reaction rate was dramatically enhanced by up to 4 orders of magnitude compared to the traditional thermal method. Simultaneously, the reaction exhibited very high selectivity (99.8 % CH4 , versus 80 % under thermal conditions) with a cobalt catalyst. Our study demonstrated that this extreme increase in reaction rate originates from the continuous activation of reactive carbon species via mechanochemistry. The high selectivity is intimately related to the activation at low temperature, at which higher hydrocarbons are difficult to form. This work is expected to advance studies of carbon hydrogasification, and other solid-gas reactions.

Catalyst- and Solvent-Free Synthesis of a Chemically Stable Aza-Bridged Bis(phenanthroline) Macrocycle-Linked Covalent Organic Framework.

Developing new linkage-based covalent organic frameworks (COFs) is one of the major topics in reticular chemistry. Electrically conductive COFs have enabled applications in energy storage and electrochemical catalysis, which are not feasible using insulating COFs. Despite significant advances, the construction of chemically stable conductive COFs by the formation of new linkages remains relatively unexplored and challenging. Here we report the solvent- and catalyst-free synthesis of a two-dimensional aza-bridged bis(phenanthroline) macrocycle-linked COF (ABBPM-COF) from the thermally induced poly-condensation of a tri-topic monomer and ammonia gas. The ABBPM-COF structure was elucidated using multiple techniques, including X-ray diffraction analysis combined with structural simulation, revealing its crystalline nature with an ABC stacking mode. Further experiments demonstrated its excellent chemical stability in acid/base solutions. Electrical-conductivity measurements showed that the insulating ABBPM-COF becomes a semiconducting material after exposure to iodine vapor.

Revealing Isolated M-N3 C1 Active Sites for Efficient Collaborative Oxygen Reduction Catalysis.

Single atom catalysts (SACs) are of great importance for oxygen reduction, a critical process in renewable energy technologies. The catalytic performance of SACs largely depends on the structure of their active sites, but explorations of highly active structures for SAC active sites are still limited. Herein, we demonstrate a combined experimental and theoretical study of oxygen reduction catalysis on SACs, which incorporate M-N3 C1 site structure, composed of atomically dispersed transition metals (e.g., Fe, Co, and Cu) in nitrogenated carbon nanosheets. The resulting SACs with M-N3 C1 sites exhibited prominent oxygen reduction catalytic activities in both acidic and alkaline media, following the trend Fe-N3 C1 > Co-N3 C1 > Cu-N3 C1 . Theoretical calculations suggest the C atoms in these structures behave as collaborative adsorption sites to M atoms, thanks to interactions between the d/p orbitals of the M/C atoms in the M-N3 C1 sites, enabling dual site oxygen reduction.

Converting Unstable Imine-Linked Network into Stable Aromatic Benzoxazole-Linked One via Post-oxidative Cyclization.

Efficiently converting unstable linkages into stable linkages is an important objective in the chemistry of covalent organic frameworks (COFs), because it enhances stability and preserves crystallinity. Here, an unstable imine-linked COF was converted into a stable aromatic benzoxazole-linked COF (BO-COF) via post-oxidative cyclization, based on chemistry used to form fused-aromatic ladder-like rigid-rod polymers. The structure of the porous BO-COF was confirmed by transmission electron microscopy, infrared and solid-state nuclear magnetic resonance spectroscopies, powder X-ray diffraction patterns, and nitrogen adsorption-desorption isotherms. The efficient post-treatment of an unstable reversible COF converted it into a stable irreversible COF, which had significantly improved thermal and chemical stabilities as well as high crystallinity. This strategy can be universally applied for the synthesis of stable fused-aromatic COFs, expanding their practical applications.

Hyperbranched Macromolecules: From Synthesis to Applications.

Hyperbranched macromolecules (HMs, also called hyperbranched polymers) are highly branched three-dimensional (3D) structures in which all bonds converge to a focal point or core, and which have a multiplicity of reactive chain-ends. This review summarizes major types of synthetic strategies exploited to produce HMs, including the step-growth polycondensation, the self-condensing vinyl polymerization and ring opening polymerization. Compared to linear analogues, the globular and dendritic architectures of HMs endow new characteristics, such as abundant functional groups, intramolecular cavities, low viscosity, and high solubility. After discussing the general concepts, synthesis, and properties, various applications of HMs are also covered. HMs continue being materials for topical interest, and thus this review offers both concise summary for those new to the topic and for those with more experience in the field of HMs.

Construction of Porous Mo3 P/Mo Nanobelts as Catalysts for Efficient Water Splitting.

A novel synthesis strategy is demonstrated to prepare Mo3 P/Mo nanobelts with porous structure for the first time. The growth and formation mechanism of the porous Mo3 P/Mo nanobelt structure was disclosed by varying the contents of H2 /PH3 and the reaction temperature. During the hydrogen evolution reaction (HER) catalysis, the optimized porous Mo3 P/Mo nanobelts exhibited a small overpotential of 78 mV at a current density of 10 mA cm-2 and a low Tafel slope of 43 mV dec-1 , as well as long-term stability in alkaline media, surpassing Pt wire. Density functional theory (DFT) calculations reveal that the H2 O dissociation on the surface of Mo3 P is favorable during the HER.

A Robust 3D Cage-like Ultramicroporous Network Structure with High Gas-Uptake Capacity.

A three-dimensional (3D) cage-like organic network (3D-CON) structure synthesized by the straightforward condensation of building blocks designed with gas adsorption properties is presented. The 3D-CON can be prepared using an easy but powerful route, which is essential for commercial scale-up. The resulting fused aromatic 3D-CON exhibited a high Brunauer-Emmett-Teller (BET) specific surface area of up to 2247 m2 g-1 . More importantly, the 3D-CON displayed outstanding low pressure hydrogen (H2 , 2.64 wt %, 1.0 bar and 77 K), methane (CH4 , 2.4 wt %, 1.0 bar and 273 K), and carbon dioxide (CO2 , 26.7 wt %, 1.0 bar and 273 K) uptake with a high isosteric heat of adsorption (H2 , 8.10 kJ mol-1 ; CH4 , 18.72 kJ mol-1 ; CO2 , 31.87 kJ mol-1 ). These values are among the best reported for organic networks with high thermal stability (ca. 600 °C).

Direct Synthesis of a Covalent Triazine-Based Framework from Aromatic Amides.

There have been extensive efforts to synthesize crystalline covalent triazine-based frameworks (CTFs) for practical applications and to realize their potential. The phosphorus pentoxide (P2 O5 )-catalyzed direct condensation of aromatic amide instead of aromatic nitrile to form triazine rings. P2 O5 -catalyzed condensation was applied on terephthalamide to construct a covalent triazine-based framework (pCTF-1). This approach yielded highly crystalline pCTF-1 with high specific surface area (2034.1 m2 g-1 ). At low pressure, the pCTF-1 showed high CO2 (21.9 wt % at 273 K) and H2 (1.75 wt % at 77 K) uptake capacities. The direct formation of a triazine-based COF was also confirmed by model reactions, with the P2 O5 -catalyzed condensation reaction of both benzamide and benzonitrile to form 1,3,5-triphenyl-2,4,6-triazine in high yield.

Triboelectric Energy Harvesting from Highly Conjugated Fused Aromatic Ladder Structure Under Extreme Environmental Conditions.

Practical application of triboelectric nanogenerators (TENGs) has been challenging, particularly, under harsh environmental conditions. This work proposes a novel 3D-fused aromatic ladder (FAL) structure as a tribo-positive material for TENGs, to address these challenges. The 3D-FAL offers a unique materials engineering platform for tailored properties, such as high specific surface area and porosity, good thermal and mechanical stability, and tunable electronic properties. The fabricated 3D-FAL-based TENG reaches a maximum peak power density of 451.2 µW cm-2 at 5 Hz frequency. More importantly, the 3D-FAL-based TENG maintains stable output performance under harsh operating environments, over wide temperature (-45-100 °C) and humidity ranges (8.3-96.7% RH), representing the development of novel FAL for sustainable energy generation under challenging environmental conditions. Furthermore, the 3D-FAL-based TENG proves to be a promising device for a speed monitoring system engaging reconstruction in virtual reality in a snowy environment.

Self-Accommodation Induced Electronic Metal-Support Interaction on Ruthenium Site for Alkaline Hydrogen Evolution Reaction.

Tuning the metal-support interaction of supported metal catalysts has been found to be the most effective approach to modulating electronic structure and improving catalytic performance. But practical understanding of the charge transfer mechanism at the electronic level of catalysis process has remained elusive. Here, it is reported that ruthenium (Ru) nanoparticles can self-accommodate into Fe3 O4 and carbon support (Ru-Fe3 O4 /C) through the electronic metal-support interaction, resulting in robust catalytic activity toward the alkaline hydrogen evolution reaction (HER). Spectroscopic evidence and theoretical calculations demonstrate that electronic perturbation occurred in the Ru-Fe3 O4 /C, and that charge redistribution directly influenced adsorption behavior during the catalytic process. The RuO bond formed by orbital mixing changes the charge state of the surface Ru site, enabling more electrons to flow to H intermediates (H* ) for favorable adsorption. The weak binding strength of the RuO bond also reinforces the anti-bonding character of H* with a more favorable recombination of H* species into H2 molecules. Because of this satisfactory catalytic mechanism, the Ru-Fe3 O4 /C supported nanoparticle catalyst demonstrated better HER activity and robust stability than the benchmark commercial Pt/C benchmark in alkaline media.

Merging Platinum Single Atoms to Achieve Ultrahigh Mass Activity and Low Hydrogen Production Cost.

Single atom catalysts (SACs) with isolated active sites exhibit the highest reported mass activity for hydrogen evolution catalysis, which is crucial for practical applications. Here, we demonstrate that ultrahigh mass activity can also be achieved by rationally merging the isolated platinum (Pt) active sites in SAC. The catalyst was obtained by the thermodynamically driven diffusing and merging phosphorus-doped carbon (PC) supported Pt single atoms (Pt1@PC) into Pt nanoclusters (PtM@PC). X-ray absorption spectroscopy analysis revealed that the merged nanoclusters exhibit much stronger interactions with the support than the traditional method, enabling more efficient electron transfer. The optimized PtM@PC exhibited an order of magnitude higher mass activity (12.7 A mgPt-1) than Pt1@PC (0.9 A mgPt-1) at an overpotential of 10 mV in acidic media, which is the highest record to date, far exceeding reports for other outstanding SACs. Theoretical study revealed that the collective active sites in PtM@PC exhibit both favorable hydrogen binding energy and fast reaction kinetics, leading to the significantly enhanced mass activity. Despite its low Pt content (2.2 wt %), a low hydrogen production cost of ∼3 USD kg-1 was finally achieved in the full-water splitting at a laboratory scale.

Origamic metal-organic framework toward mechanical metamaterial.

Origami, known as paper folding has become a fascinating research topic recently. Origami-inspired materials often establish mechanical properties that are difficult to achieve in conventional materials. However, the materials based on origami tessellation at the molecular level have been significantly underexplored. Herein, we report a two-dimensional (2D) porphyrinic metal-organic framework (MOF), self-assembled from Zn nodes and flexible porphyrin linkers, displaying folding motions based on origami tessellation. A combined experimental and theoretical investigation demonstrated the origami mechanism of the 2D porphyrinic MOF, whereby the flexible linker acts as a pivoting point. The discovery of the 2D tessellation hidden in the 2D MOF unveils origami mechanics at the molecular level.

Tin Nanoclusters Confined in Nitrogenated Carbon for the Oxygen Reduction Reaction.

The oxygen reduction reaction is essential for fuel cells and metal-air batteries in renewable energy technologies. Developing platinum-group-metal (PGM)-free catalysts with comparable catalytic performance is highly desired for cost efficiency. Here, we report a tin (Sn) nanocluster confined catalyst for the electrochemical oxygen reduction. The catalyst was fabricated by confining 1-1.5 nm sized Sn nanoclusters in situ in microporous nitrogen-doped carbon polyhedra (SnxNC) with an average pore size of 0.7 nm. SnxNC exhibited high catalytic performance in acidic media, including positive onset and half-wave potentials, comparable to those of the state-of-the-art Pt/C and far exceeding those of the Sn single-atom catalyst. Combined structural and theoretical analyses reveal that the confined Sn nanoclusters, which have favorable oxygen adsorption behaviors, are responsible for the high catalytic performance, but not Sn single atoms.

Solution-Processable Semiconducting Conjugated Planar Network.

After the emergence of graphene in the material science field, top-down and bottom-up studies to develop semiconducting organic materials with layered structures became highly active. However, most of them have suffered from poor processability, which hampers device fabrication and frustrates practical applications. Here, we suggest an unconventional approach to fabricating semiconducting devices, which avoids the processability issue. We designed a soluble amorphous network using a solution process to form a thin film on a substrate. We then employed heat treatment to develop a flattened organic structure in the thin film, as an active layer for organic thin-film transistors (TFTs). The fabricated TFTs showed good performance in both horizontal and vertical charge transport, suggesting a versatile and useful approach for the development of organic semiconductors.