Supported Noble‐Metal Single Atoms for Heterogeneous Catalysis

Single‐atom catalysts (SACs), with atomically distributed active metal sites on supports, serve as a newly advanced material in catalysis, and open broad prospects for a wide variety of catalytic processes owing to their unique catalytic behaviors. To construct SACs with precise structures and high density of accessible single‐atom sites, while preventing aggregation to large nanoparticles, various strategies for their chemical synthesis have been recently developed by improving the distribution and chemical bonding of active sites on supports, which results in excellent activity and selectivity in a variety of catalytic reactions. Noble‐metal‐based SACs are discussed, and their structural properties, chemical synthesis, and catalytic applications are highlighted. The structure–activity relationships and the underlying catalytic mechanisms are addressed, including the influences of surface species and reducibility of supports on the activity and stability, impact of the unique structural and electronic properties of single‐atom centers modulated by metal/support interactions on catalytic activity and selectivity, and how the modified catalytic mechanism obtained by inhibiting the multiatoms involves catalytic pathways. Finally, the prospects and challenges for development in this field are highlighted.     

Multiscale Design of Graphyne‐Based Materials for High‐Performance Separation Membranes

By varying the number of acetylenic linkages connecting aromatic rings, a new family of atomically thin graph‐n‐yne materials can be designed and synthesized. Generating immense scientific interest due to its structural diversity and excellent physical properties, graph‐n‐yne has opened new avenues toward numerous promising engineering applications, especially for separation membranes with precise pore sizes. Having these tunable pore sizes in combination with their excellent mechanical strength to withstand high pressures, free‐standing graph‐n‐yne is theoretically posited to be an outstanding membrane material for separating or purifying mixtures of either gases or liquids, rivaling or even dramatically exceeding the capabilities of current, state‐of‐art separation membranes. Computational modeling and simulations play an integral role in the bottom‐up design and characterization of these graph‐n‐yne materials. Thus, here, the state of the art in modeling α‐, β‐, γ‐, δ‐, and 6,6,12‐graphyne nanosheets for synthesizing graph‐2‐yne materials and 3D architectures thereof is discussed. Different synthesis methods are described and a broad overview of computational characterizations of graph‐n‐yne's electrical, chemical, and thermal properties is provided. Furthermore, a series of in‐depth computational studies that delve into the specifics of graph‐n‐yne's mechanical strength and porosity, which confer superior performance for separation and desalination membranes, are reviewed.     

Silk‐Derived 2D Porous Carbon Nanosheets with Atomically‐Dispersed Fe‐Nx‐C Sites for Highly Efficient Oxygen Reaction Catalysts

Controlled synthesis of highly efficient, stable, and cost‐effective oxygen reaction electrocatalysts with atomically‐dispersed Me–N_x–C active sites through an effective strategy is highly desired for high‐performance energy devices. Herein, based on regenerated silk fibroin dissolved in ferric chloride and zinc chloride aqueous solution, 2D porous carbon nanosheets with atomically‐dispersed Fe–N_x–C active sites and very large specific surface area (≈2105 m² g^?1) are prepared through a simple thermal treatment process. Owing to the 2D porous structure with large surface area and atomic dispersion of Fe–N_x–C active sites, the as‐prepared silk‐derived carbon nanosheets show superior electrochemical activity toward the oxygen reduction reaction with a half‐wave potential (E_1/2) of 0.853 V, remarkable stability with only 11 mV loss in E_1/2 after 30 000 cycles, as well as good catalytic activity toward the oxygen evolution reaction. This work provides a practical and effective approach for the synthesis of high‐performance oxygen reaction catalysts towards advanced energy materials.     

MOF‐Confined Sub‐2 nm Atomically Ordered Intermetallic PdZn Nanoparticles as High‐Performance Catalysts for Selective Hydrogenation of Acetylene

Controllable synthesis of ultrasmall atomically ordered intermetallic nanoparticles is a challenging task, owing to the high temperature commonly required for the formation of intermetallic phases. Here, a metal–organic framework (MOF)‐confined co‐reduction strategy is developed for the preparation of sub‐2 nm intermetallic PdZn nanoparticles, by employing the well‐defined porous structures of calcinated ZIF‐8 (ZIF‐8C) and an in situ co‐reduction therein. HAADF‐STEM, HRTEM, and EDS characterizations reveal the homogeneous dispersion of these sub‐2 nm intermetallic PdZn nanoparticles within the ZIF‐8C frameworks. XRD, XPS, and EXAFS measurements further confirm the atomically ordered intermetallic phase nature of these sub‐2 nm PdZn nanoparticles. Selective hydrogenation of acetylene evaluation results show the excellent catalytic properties of the sub‐2 nm intermetallic PdZn, which result from the energetically more favorable path for acetylene hydrogenation and ethylene desorption over the ultrasmall particles than over larger‐sized intermetallic PdZn as revealed by density functional theory (DFT) calculations. Moreover, this protocol is also extendable for the preparation of sub‐2 nm intermetallic PtZn nanoparticles and is expected to provide a novel methodology in synthesizing ultrasmall atomically ordered intermetallic nanomaterials by rationally functionalizing MOFs.     

Atomically Dispersed Binary Co‐Ni Sites in Nitrogen‐Doped Hollow Carbon Nanocubes for Reversible Oxygen Reduction and Evolution

With the inspiration of developing bifunctional electrode materials for reversible oxygen electrocatalysis, one strategy of heteroatom doping is proposed to fabricate dual metal single‐atom catalysts. However, the identification and mechanism functions of polynary single‐atom structures remain elusive. Atomically dispersed binary Co‐Ni sites embedded in N‐doped hollow carbon nanocubes (denoted as CoNi‐SAs/NC) are synthesized via proposed pyrolysis of dopamine‐coated metal‐organic frameworks. The atomically isolated bimetallic configuration in CoNi‐SAs/NC is identified by combining microscopic and spectroscopic techniques. When employing as oxygen electrocatalysts in alkaline medium, the resultant CoNi‐SAs/NC hybrid manifests outstanding catalytic performance for bifunctional oxygen reduction/evolution reactions, boosting the realistic rechargeable zinc–air batteries with high efficiency, low overpotential, and robust reversibility, superior to other counterparts and state‐of‐the‐art precious‐metal catalysts. Theoretical computations based on density functional theory demonstrate that the homogenously dispersed single atoms and the synergistic effect of neighboring Co‐Ni dual metal center can optimize the adsorption/desorption features and decrease the overall reaction barriers, eventually promoting the reversible oxygen electrocatalysis. This work not only sheds light on the controlled synthesis of atomically isolated advanced materials, but also provides deeper understanding on the structure–performance relationships of nanocatalysts with multiple active sites for various catalytic applications.     

Architecture of β‐Graphdiyne‐Containing Thin Film Using Modified Glaser–Hay Coupling Reaction for Enhanced Photocatalytic Property of TiO2

β‐Graphdiyne (β‐GDY) is a member of 2D graphyne family with zero band gap, and is a promising material with potential applications in energy storage, organic electronics, etc. However, the synthesis of β‐GDY has not been realized yet, and the measurement of its intrinsic properties remains elusive. In this work, β‐GDY‐containing thin film is successfully synthesized on copper foil using modified Glaser–Hay coupling reaction with tetraethynylethene as precursor. The as‐grown carbon film has a smooth surface and is continuous and uniform. Electrical measurements reveal the conductivity of 3.47 × 10^?6 S m^?1 and the work function of 5.22 eV. TiO₂@β‐GDY nanocomposite is then prepared and presented with an enhancement of photocatalytic ability compared to pure TiO₂.     

Exploring Approaches for the Synthesis of Few‐Layered Graphdiyne

Graphdiyne (GDY) is an emerging carbon allotrope in the graphyne (GY) family, demonstrating extensive potential applications in the fields of electronic devices, catalysis, electrochemical energy storage, and nonlinear optics. Synthesis of few‐layered GDY is especially important for both electronic applications and structural characterization. This work critically summarizes the state‐of‐art of GDY and focuses on exploring approaches for few‐layered GDY synthesis. The obstacles and challenges of GDY synthesis are also analyzed in detail. Recently developed synthetic methods are discussed such as i) the copper substrate‐based method, ii) the chemical vapor deposition (CVD) method, iii) the interfacial construction method, and iv) the graphene‐templated method. Throughout the discussion, the superiorities and limitations of different methods are analyzed comprehensively. These synthetic methods have provided considerable inspiration approaching synthesis of few‐layered or single‐layered GDY film. The work concludes with a perspective on promising research directions and remaining barriers for layer‐controlled and morphology‐controlled synthesis of GDY with higher crystalline quality.     

Interface‐Directed Self‐Assembly of Cell‐Laden Microgels

Cell‐laden hydrogels show great promise for creating engineered tissues. However, a major shortcoming with these systems has been the inability to fabricate structures with controlled micrometer‐scale features on a biologically relevant length scale. In this Full Paper, a rapid method is demonstrated for creating centimeter‐scale, cell‐laden hydrogels through the assembly of shape‐controlled microgels or a liquid–air interface. Cell‐laden microgels of specific shapes are randomly placed on the surface of a high‐density, hydrophobic solution, induced to aggregate and then crosslinked into macroscale tissue‐like structures. The resulting assemblies are cell‐laden hydrogel sheets consisting of tightly packed, ordered microgel units. In addition, a hierarchical approach creates complex multigel building blocks, which are then assembled into tissues with precise spatial control over the cell distribution. The results demonstrate that forces at an air–liquid interface can be used to self‐assemble spatially controllable, cocultured tissue‐like structures.     

Efficient and Layer‐Dependent Exciton Pumping across Atomically Thin Organic–Inorganic Type‐I Heterostructures

The fundamental light–matter interactions in monolayer transition metal dichalcogenides might be significantly engineered by hybridization with their organic counterparts, enabling intriguing optoelectronic applications. Here, atomically thin organic–inorganic (O–I) heterostructures, comprising monolayer MoSe₂ and mono‐/few‐layer single‐crystal pentacene samples, are fabricated. These heterostructures show type‐I band alignments, allowing efficient and layer‐dependent exciton pumping across the O–I interfaces. The interfacial exciton pumping has much higher efficiency (>86 times) than the photoexcitation process in MoSe₂, although the pentacene layer has much lower optical absorption than MoSe₂. This highly enhanced pumping efficiency is attributed to the high quantum yield in pentacene and the ultrafast energy transfer between the O–I interface. Furthermore, those organic counterparts significantly modulate the bindings of charged excitons in monolayer MoSe₂ via their precise dielectric environment engineering. The results open new avenues for exploring fundamental phenomena and novel optoelectronic applications using atomically thin O–I heterostructures.     

Atomically Precise Prediction of 2D Self‐Assembly of Weakly Bonded Nanostructures: STM Insight into Concentration‐Dependent Architectures

A joint experimental and computational study is reported on the concentration‐dependant self‐assembly of a flat C₃‐symmetric molecule on a graphite surface. As a model system a tripodal molecule, 1,3,5‐tris(pyridin‐3‐ylethynyl)benzene, has been chosen, which can adopt either C_3h or C_s symmetry when planar, as a result of pyridyl rotation along the alkynyl spacers. Density functional theory (DFT) simulations of 2D nanopatterns with different surface coverage reveal that the molecule can generate different types of self‐assembled motifs. The stability of fourteen 2D patterns and the influence of concentration are analyzed. It is found that ordered, densely packed monolayers and 2D porous networks are obtained at high and low concentrations, respectively. A concentration‐dependent scanning tunneling microscopy (STM) investigation of this molecular self‐assembly system at a solution/graphite interface reveals four supramolecular motifs, which are in perfect agreement with those predicted by simulations. Therefore, this DFT method represents a key step forward toward the atomically precise prediction of molecular self‐assembly on surfaces and at interfaces.     

Local Electronic Structure of Molecular Heterojunctions in a Single‐Layer 2D Covalent Organic Framework

The synthesis of a single‐layer covalent organic framework (COF) with spatially modulated internal potentials provides new opportunities for manipulating the electronic structure of molecularly defined materials. Here, the fabrication and electronic characterization of COF‐420: a single‐layer porphyrin‐based square‐lattice COF containing a periodic array of oriented, type II electronic heterojunctions is reported. In contrast to previous donor–acceptor COFs, COF‐420 is constructed from building blocks that yield identical cores upon reticulation, but that are bridged by electrically asymmetric linkers supporting oriented electronic dipoles. Scanning tunneling spectroscopy reveals staggered gap (type II) band alignment between adjacent molecular cores in COF‐420, in agreement with first‐principles calculations. Hirshfeld charge analysis indicates that dipole fields from oriented imine linkages within COF‐420 are the main cause of the staggered electronic structure in this square grid of atomically–precise heterojunctions.     

Graphynes for Water Desalination and Gas Separation

Selective transport of mass through membranes, so‐called separation, is fundamental to many industrial applications, e.g., water desalination and gas separation. Graphynes, graphene analogs yet containing intrinsic uniformly distributed pores, are excellent candidates for highly permeable and selective membranes owing to their extreme thinness and high porosity. Graphynes exhibit computationally determined separation performance far beyond experimentally measured values of commercial state‐of‐the‐art polyamide membranes; they also offer advantages over other atomically thin membranes like porous graphene in terms of controllability in pore geometry. Here, recent progress in proof‐of‐concept computational research into various graphynes for water desalination and gas separation is discussed, and their theoretically predicted outstanding permeability and selectivity are highlighted. Challenges associated with the future development of graphyne‐based membranes are further analyzed, concentrating on controlled synthesis of graphyne, maintenance of high structural stability to withstand loading pressures, as well asthe demand for accurate computational characterization of separation performance. Finally, possible directions are discussed to align future efforts in order to push graphynes and other 2D material membranes toward practical separation applications.     

Mechanochemical Synthesis of γ‐Graphyne with Enhanced Lithium Storage Performance

γ‐Graphyne is a new nanostructured carbon material with large theoretical Li⁺ storage due to its unique large conjugate rings, which makes it a potential anode for high‐capacity lithium‐ion batteries (LIBs). In this work, γ‐graphyne‐based high‐capacity LIBs are demonstrated experimentally. γ‐Graphyne is synthesized through mechanochemical and calcination processes by using CaC₂ and C₆Br₆. Brunauer–Emmett–Teller, atomic force microscopy, X‐ray photoelectron spectroscopy, solid‐state ¹³C NMR and Raman spectra are conducted to confirm its morphology and chemical structure. The sample presents 2D mesoporous structure and is exactly composed of sp and sp²‐hybridized carbon atoms as the γ‐graphyne structure. The electrode shows high Li⁺ storage (1104.5 mAh g^?1 at 100 mA g^?1) and rate capability (435.1 mAh g^?1 at 5 A g^?1). The capacity retention can be up to 948.6 (200 mA g^?1 for 350 cycles) and 730.4 mAh g^?1 (1 A g^?1 for 600 cycles), respectively. These excellent electrochemical performances are ascribed to the mesoporous architecture, large conjugate rings, enlarged interplanar distance, and high structural integrity for fast Li⁺ diffusion and improved cycling stability in γ‐graphyne. This work provides an environmentally benign and cost‐effective mechanochemical method to synthesize γ‐graphyne and demonstrates its superior Li⁺ storage experimentally.     

Dislocation‐Free and Atomically Flat GaN Hexagonal Microprisms for Device Applications

III‐nitrides are considered the material of choice for light‐emitting diodes (LEDs) and lasers in the visible to ultraviolet spectral range. The development is hampered by lattice and thermal mismatch between the nitride layers and the growth substrate leading to high dislocation densities. In order to overcome the issue, efforts have gone into selected area growth of nanowires (NWs), using their small footprint in the substrate to grow virtually dislocation‐free material. Their geometry is defined by six tall side‐facets and a pointed tip which limits the design of optoelectronic devices. Growth of dislocation‐free and atomically smooth 3D hexagonal GaN micro‐prisms with a flat, micrometer‐sized top‐surface is presented. These self‐forming structures are suitable for optical devices such as low‐loss optical cavities for high‐efficiency LEDs. The structures are made by annealing GaN NWs with a thick radial shell, reforming them into hexagonal flat‐top prisms with six equivalents either m‐ or s‐facets depending on the initial heights of the top pyramid and m‐facets of the NWs. This shape is kinetically controlled and the reformation can be explained with a phenomenological model based on Wulff construction that have been developed. It is expected that the results will inspire further research into micron‐sized III‐nitride‐based devices.     

Fractal‐Theory‐Based Control of the Shape and Quality of CVD‐Grown 2D Materials

The precise control of the shape and quality of 2D materials during chemical vapor deposition (CVD) processes remains a challenging task, due to a lack of understanding of their underlying growth mechanisms. The existence of a fractal‐growth‐based mechanism in the CVD synthesis of several 2D materials is revealed, to which a modified traditional fractal theory is applied in order to build a 2D diffusion‐limited aggregation (2D‐DLA) model based on an atomic‐scale growth mechanism. The strength of this model is validated by the perfect correlation between theoretically simulated data, predicted by 2D‐DLA, and experimental results obtained from the CVD synthesis of graphene, hexagonal boron nitride, and transition metal dichalcogenides. By applying the 2D‐DLA model, it is also discovered that the single‐domain net growth rate (SD‐NGR) plays a crucial factor in governing the shape and quality of 2D‐material crystals. By carefully tuning SD‐NGR, various fractal‐morphology high‐quality single‐crystal 2D materials are synthesized, achieving, for the first time, the precise control of 2D‐material CVD growth. This work lays the theoretical foundation for the precise adjustment of the morphologies and physical properties of 2D materials, which is essential to the use of fractal‐shaped nanomaterials for the fabrication of new‐generation neural‐network nanodevices.     

Atomic‐Level Fe‐N‐C Coupled with Fe3C‐Fe Nanocomposites in Carbon Matrixes as High‐Efficiency Bifunctional Oxygen Catalysts

Highly active and durable bifunctional oxygen electrocatalysts are of pivotal importance for clean and renewable energy conversion devices, but the lack of earth‐abundant electrocatalysts to improve the intrinsic sluggish kinetic process of oxygen reduction/evolution reactions (ORR/OER) is still a challenge. Fe‐N‐C catalysts with abundant natural merits are considered as promising alternatives to noble‐based catalysts, yet further improvements are urgently needed because of their poor stability and unclear catalytic mechanism. Here, an atomic‐level Fe‐N‐C electrocatalyst coupled with low crystalline Fe₃C‐Fe nanocomposite in 3D carbon matrix (Fe‐SAs/Fe₃C‐Fe@NC) is fabricated by a facile and scalable method. Versus atomically FeN_x species and crystallized Fe₃C‐Fe nanoparticles, Fe‐SAs/Fe₃C‐Fe@NC catalyst, abundant in vertical branched carbon nanotubes decorated on intertwined carbon nanofibers, exhibits high electrocatalytic activities and excellent stabilities both in ORR (E_1/2, 0.927 V) and OER (E_J=10, 1.57 V). This performance benefits from the strong synergistic effects of multicomponents and the unique structural advantages. In‐depth X‐ray absorption fine structure analysis and density functional theory calculation further demonstrate that more extra charges derived from modified Fe clusters decisively promote the ORR/OER performance for atomically FeN₄ configurations by enhanced oxygen adsorption energy. These insightful findings inspire new perspectives for the rational design and synthesis of economical–practical bifunctional oxygen electrocatalysts.     

Fluorescence Enhancement on Large Area Self‐Assembled Plasmonic‐3D Photonic Crystals

Discontinuous plasmonic‐3D photonic crystal hybrid structures are fabricated in order to evaluate the coupling effect of surface plasmon resonance and the photonic stop band. The nanostructures are prepared by silver sputtering deposition on top of hydrophobic 3D photonic crystals. The localized surface plasmon resonance of the nanostructure has a symbiotic relationship with the 3D photonic stop band, leading to highly tunable characteristics. Fluorescence enhancements of conjugated polymer and quantum dot based on these hybrid structures are studied. The maximum fluorescence enhancement for the conjugated polymer of poly(5‐methoxy‐2‐(3‐sulfopropoxy)‐1,4‐phenylenevinylene) potassium salt by a factor of 87 is achieved as compared with that on a glass substrate due to the enhanced near‐field from the discontinuous plasmonic structures, strong scattering effects from rough metal surface with photonic stop band, and accelerated decay rates from metal‐coupled excited state of the fluorophore. It is demonstrated that the enhancement induced by the hybrid structures has a larger effective distance (optimum thickness ≈130 nm) than conventional plasmonic systems. It is expected that this approach has tremendous potential in the field of sensors, fluorescence‐imaging, and optoelectronic applications.     

Pd Single‐Atom Catalysts on Nitrogen‐Doped Graphene for the Highly Selective Photothermal Hydrogenation of Acetylene to Ethylene

The selective hydrogenation of acetylene to ethylene in an ethylene‐rich gas stream is an important process in the chemical industry. Pd‐based catalysts are widely used in this reaction due to their excellent hydrogenation activity, though their selectivity for acetylene hydrogenation and durability need improvement. Herein, the successful synthesis of atomically dispersed Pd single‐atom catalysts on nitrogen‐doped graphene (Pd₁/N‐graphene) by a freeze‐drying‐assisted method is reported. The Pd₁/N‐graphene catalyst exhibits outstanding activity and selectivity for the hydrogenation of C₂H₂ with H₂ in the presence of excess C₂H₄ under photothermal heating (UV and visible‐light irradiation from a Xe lamp), achieving 99% conversion of acetylene and 93.5% selectivity to ethylene at 125 °C. This remarkable catalytic performance is attributed to the high concentration of Pd active sites on the catalyst surface and the weak adsorption energy of ethylene on isolated Pd atoms, which prevents C₂H₄ hydrogenation. Importantly, the Pd₁/N‐graphene catalyst exhibits excellent durability at the optimal reaction temperature of 125 °C, which is explained by the strong local coordination of Pd atoms by nitrogen atoms, which suppresses the Pd aggregation. The results presented here encourage the wider pursuit of solar‐driven photothermal catalyst systems based on single‐atom active sites for selective hydrogenation reactions.     

Graphene Nanoribbons: On-Surface Synthesis and Integration into Electronic Devices

Graphene nanoribbons (GNRs) are quasi-1D graphene strips, which have attracted attention as a novel class of semiconducting materials for various applications in electronics and optoelectronics. GNRs exhibit unique electronic and optical properties, which sensitively depend on their chemical structures, especially the width and edge configuration. Therefore, precision synthesis of GNRs with chemically defined structures is crucial for their fundamental studies as well as device applications. In contrast to top-down methods, bottom-up chemical synthesis using tailor-made molecular precursors can achieve atomically precise GNRs. Here, the synthesis of GNRs on metal surfaces under ultrahigh vacuum (UHV) and chemical vapor deposition (CVD) conditions is the main focus, and the recent progress in the field is summarized. The UHV method leads to successful unambiguous visualization of atomically precise structures of various GNRs with different edge configurations. The CVD protocol, in contrast, achieves simpler and industry-viable fabrication of GNRs, allowing for the scale up and efficient integration of the as-grown GNRs into devices. The recent updates in device studies are also addressed using GNRs synthesized by both the UHV method and CVD, mainly for transistor applications. Furthermore, views on the next steps and challenges in the field of on-surface synthesized GNRs are provided.     

Oxidase‐Like Fe‐N‐C Single‐Atom Nanozymes for the Detection of Acetylcholinesterase Activity

Single‐atom catalysts (SACs) have attracted extensive attention in the catalysis field because of their remarkable catalytic activity, gratifying stability, excellent selectivity, and 100% atom utilization. With atomically dispersed metal active sites, Fe‐N‐C SACs can mimic oxidase by activating O₂ into reactive oxygen species, O₂^?? radicals. Taking advantages of this property, single‐atom nanozymes (SAzymes) can become a great impetus to develop novel biosensors. Herein, the performance of Fe‐N‐C SACs as oxidase‐like nanozymes is explored. Besides, the Fe‐N‐C SAzymes are applied in biosensor areas to evaluate the activity of acetylcholinesterase based on the inhibition toward nanozyme activity by thiols. Moreover, this SAzymes‐based biosensor is further used for monitoring the amounts of organophosphorus compounds.