Synthesis of a Porous [14]Annulene Graphene Nanoribbon and a Porous [30]Annulene Graphene Nanosheet on Metal Surfaces

Metal-Free Graphene-Based Derivatives as Oxygen Reduction Reaction Electrocatalysts in Energy Conversion and Storage Systems: An Overview

Oxygen reduction reaction (ORR) is one of the most important reactions in electrochemical energy storage and conversion devices. To overcome the slow kinetics, minimize the overpotential, and make this reaction feasible, efficient, and stable, electrocatalysts are needed. Metal-free graphene-based systems are considered promising and cost-effective ORR catalysts with adjustable structures. This review is meant to give a rational overview of the graphene-based metal-free ORR electrocatalysts, illustrating the huge amount of related research developed particularly in the field of fuel cells and metal-air batteries, with particular attention to the synthesis procedures. The novelty of this review is that, beyond general aspects regarding the synthesis and characterization of graphene, above 90% of the various graphene (doped and undoped species, composites)-based ORR electrocatalysts have been reported, which represents an unprecedented thorough collection of both experimental and theoretical studies. Hundreds of references are included in the review; therefore, it can be considered as a vademecum in the field.

On-surface synthesis of organometallic nanorings linked by unconventional intermediates of the Ullmann reaction

Ullmann coupling has been one of the most important organic reactions for the formation of an aryl-aryl bond, which is of great significance in medicinal chemistry, natural product synthesis, and optoelectronic material fabrication. However, the associated reaction mechanism has not been determined with certainty and has mostly relied on theoretical calculations, since the identification of reaction intermediates lacked experimental evidence. Herein, we report the visualization of an unprecedented C-Cu-Br-Cu-C bonded intermediate state of Ullmann coupling by means of on-surface synthesis. These intermediates tend to form nanorings on a Cu(111) surface, as thermodynamically stable structures. Advanced techniques, including scanning tunneling microscopy, non-contact atomic force microscopy, and synchrotron radiation photoemission spectroscopy, together with density functional theory calculations, were used to scrutinize the structural assignments and intermediate transition process at the sub-molecular level. The C-Cu-Br-Cu-C structure is confirmed to be the precursor state of the conventional C-Cu-C intermediate during an on-surface Ullmann reaction, since their coexistence and transformation were observed experimentally. Our findings offer insights into revisiting and understanding the reaction mechanism of Ullmann coupling.

Atomic Hydrogen Promotes Polyparaphenylene Fusion into Graphene Nanoribbons on Au(111)

Graphene nanoribbons (GNRs) hold significant potential for applications in semiconductor electronics. Compared to precursor design, the lateral fusion of polymers offers a versatile and flexible approach to producing GNRs with desirable properties. Developing effective methods to enhance lateral fusion is therefore highly desirable. In this study, the beneficial effects of atomic hydrogen (AH) are reported in promoting the lateral fusion of polyparaphenylene (PPP) chains into GNRs on Au(111). When PPP chains are treated with AH at 570 K, the threshold temperature for GNR formation by lateral fusion is reduced to 590 K, a substantial decrease from the 650 K required for the formation of GNR by postannealing without AH. The promoting effect of AH is attributed to the superhydrogenation of PPP chains at 570 K, which 1) locally disrupts π-aromaticity, and 2) facilitates closer proximity of PPP chains. Both effects aid in the dehydrogenative C-C coupling of PPP chains. Scanning tunneling microscopy (STM) and synchrotron radiation photoemission spectroscopy (SRPES) are used to identify reaction products and monitor reaction pathways. The demonstrated role of AH in this study may be extended to other systems involving the lateral fusion of polycyclic aromatic hydrocarbon (PAH)-based polymers.

On-Surface Synthesis of Two-Dimensional Carbon-Based Networks via Hierarchical Ullmann Coupling Reactions

The recent developed bottom-up on-surface synthesis offers unprecedent opportunities for the fabrication of two-dimensional (2D) carbon-based networks with atomic precision. Hierarchical coupling approach has been proposed as an efficient strategy for improving the corresponding reaction selectivity and quality of target structures. Herein, we report the synthesis of a nitrogen-doped carbon-based network on Ag(100) utilizing a hierarchical Ullmann coupling strategy. The accurate identification of reaction intermediates and products by scanning tunneling microscopy allows us to unravel the reaction mechanism. The synthetic process of 2D carbon-based networks is kinetics-driven, relying on the competition between dechlorination and C-C coupling. We expect that our discussion on the mechanism of hierarchical coupling may shed light on the rational design and precise synthesis of 2D carbon-based networks on surfaces.

Controlling the Selectivity of Reaction Products by Transmetalation on a Ag(111) Substrate

On-surface synthesis has shown great promise in the precise bottom-up preparation of molecular nanostructures. Apart from the direct C-C coupling reaction pathway, an alternative strategy is to exploit the metal-organic interactions provided by integrated metals for preassembly, which exhibit high reversibility and can anchor specific conformations of molecular precursors, thus allowing the precise construction of nanostructures with improved reaction selectivity. Previous studies have mainly been devoted to the construction of target reaction products through the incorporation of metal atoms, ranging from intrinsic to extrinsic atoms on metal substrates and, more recently, to their cooperative effects. However, the formation of different covalent nanostructures by competitive interactions between intrinsic and extrinsic adatoms remains elusive. Herein, we controlled the selectivity of covalent reaction products from isomerically specific trans-chains to cis-rings, resulting from the transmetalation of intrinsic Ag adatoms to extrinsic Na atoms on a Ag(111) substrate. Our results exhibit the competitive interactions between intrinsic and extrinsic metal atoms in real space and demonstrate their influence on the selectivity of reaction products, which should broaden the regulatory strategies for on-surface synthesis that shed light on the controllable and selective synthesis of target covalent nanostructures.

Lying or Standing of Thiophene on a Surface Determines the Reaction Difference

Adsorption configurations of molecules on a surface play an important role in the on-surface reaction. In the on-surface synthesis reaction, most of the molecules prefer the lying adsorption configuration to maximize the interaction between the molecule and substrate. In this work, we report an on-surface study of 2,3,4,5-tetrabromothiophene by scanning tunneling microscopy, density functional theory, and X-ray photoelectron spectroscopy. Due to different interactions between thiophene and metal surfaces, lying or standing configurations of 2,3,4,5-tetrabromothiophene can be selected by the choice of metal substrates. Moreover, a catalytic role of the metal substrate in the molecular reaction with lying and standing adsorption configurations is demonstrated at the molecular level. This work broadens the understanding of thiophene's configurations in surface reactions and the product diversity driven by adsorption configurations. It also offers a guiding framework for synthesizing multifunctional materials by thiophene derivatives.

The Effects of Pore Defects in π-Extended Pentadecabenzo[9]helicene

The introduction of precise pore defects into nanocarbon structures results in the emergence of distinct physicochemical characteristics. However, there is a lack of research on non-planar chiral nanographene involving precise pore defects. Herein, we have developed two analogues to the π-extended pentadecabenzo[9]helicene (EP9H) containing embedded pore defects. Each molecules, namely extended dodecabenzo[7]helicene (ED7H; 1) or extended nonabenzo[5]helicene (EN5H; 2), exhibits dual-state emission. Significantly, the value of |glum| of 1 is exceptionally high at 1.41×10-2 in solution and BCPL as 254 M-1 cm-1. In PMMA film, |glum| of 1 is 8.56×10-3, and in powder film, it is 5.00×10-3. This study demonstrates that nanocarbon molecules with pore defects exhibit dual-state emission properties while maintaining quite good chiral luminescence properties. It was distinguished from the aggregation-caused quenching (ACQ) effect corresponding to the nanocarbon without embedded defect. Incorporating pore defects into chiral nanocarbon molecules also simplifies the synthesis process and enhances the solubility of the resulting product. These findings suggest that the introduction of pore defects can be a viable approach to improve nanocarbon molecules.

Recent Research Progress of Porous Graphene and Applications in Molecular Sieve, Sensor, and Supercapacitor

Porous graphene, including 2D and 3D porous graphene, is widely researched recently. One of the most attractive features is the proper utilization of graphene defects, which combine the advantages of both graphene and porous materials, greatly enriching the applications of porous graphene in biology, chemistry, electronics, and other fields. In this review, the defects of graphene are first discussed to provide a comprehensive understanding of porous graphene. Then, the latest advancements in the preparation of 2D and 3D porous graphene are presented. The pros and cons of these preparation methods are discussed in detail, providing a direction for the fabrication of porous graphene. Moreover, various superior properties of porous graphene are described, laying the foundation for their promising applications. Owing to its abundant morphology, wide distribution of pore size, and remarkable properties benefited from porous structure, porous graphene can not only promote molecular diffusion and electron transfer but also expose more active sites. Consequently, a serious of applications containing gas sieving, liquid separation, sensors, and supercapacitors, are presented. Finally, the challenges confronted during preparation and characterization of porous graphene are discussed, offering guidance for the future development of porous graphene in fabrication, characterization, properties, and applications.

On-surface synthesis of porous graphene nanoribbons mediated by phenyl migration

Advancements in the on-surface synthesis of atomically precise graphene nanostructures are propelled by the introduction of innovative precursor designs and reaction types. Until now, the latter has been confined to cross-coupling and cyclization reactions that involve the cleavage of specific atoms or groups. In this article, we elucidate how the migration of phenyl substituents attached to graphene nanoribbons can be harnessed to generate arrays of [18]-annulene pores at the edges of the nanostructures. This sequential pathway is revealed through a comprehensive study employing bond-resolved scanning tunneling microscopy and ab-initio computational techniques. The yield of pore formation is maximized by anchoring the graphene nanoribbons at steps of vicinal surfaces, underscoring the potential of these substrates to guide reaction paths. Our study introduces a new reaction to the on-surface synthesis toolbox along with a sequential route, altogether enabling the extension of this strategy towards the formation of other porous nanostructures.

Bottom-up Synthesis and Characterization of Porous 12-Atom-Wide Armchair Graphene Nanoribbons

Although several porous carbon/graphene nanoribbons (GNRs) have been prepared, a direct comparison of the electronic properties between a nonporous GNR and its periodically perforated counterpart is still missing. Here, we report the synthesis of porous 12-atom-wide armchair-edged GNRs from a bromoarene precursor on a Au(111) surface via hierarchical Ullmann and dehydrogenative coupling. The selective formation of porous 12-GNRs was achieved through thermodynamic and kinetic reaction control combined with tailored precursor design. The structure and electronic properties of the porous 12-GNR were elucidated by scanning tunneling microscopy/spectroscopy and density functional theory calculations, revealing that the pores induce a 2.17 eV band gap increase compared to the nonporous 12-AGNR on the same surface.

Recent progress in on-surface synthesis of nanoporous graphene materials

Nanoporous graphene (NPG) materials are generated by removing internal degree-3 vertices from graphene and introducing nanopores with specific topological structures, which have been widely explored and exploited for applications in electronic devices, membranes, and energy storage. The inherent properties of NPGs, such as the band structures, field effect mobilities and topological properties, are crucially determined by the geometric structure of nanopores. On-surface synthesis is an emerging strategy to fabricate low-dimensional carbon nanostructures with atomic precision. In this review, we introduce the progress of on-surface synthesis of atomically precise NPGs, and classify NPGs from the aspects of element types, topological structures, pore shapes, and synthesis strategies. We aim to provide a comprehensive overview of the recent advancements, promoting interdisciplinary collaboration to further advance the synthesis and applications of NPGs.

Porous Nanographenes, Graphene Nanoribbons, and Nanoporous Graphene Selectively Synthesized from the Same Molecular Precursor

We demonstrate a family of molecular precursors based on 7,10-dibromo-triphenylenes that can selectively produce different varieties of atomically precise porous graphene nanomaterials through the use of different synthetic environments. Upon Yamamoto polymerization of these molecules in solution, the free rotations of the triphenylene units around the C-C bonds result in the formation of cyclotrimers in high yields. In contrast, in on-surface polymerization of the same molecules on Au(111) these rotations are impeded, and the coupling proceeds toward the formation of long polymer chains. These chains can then be converted to porous graphene nanoribbons (pGNRs) by annealing. Correspondingly, the solution-synthesized cyclotrimers can also be deposited onto Au(111) and converted into porous nanographenes (pNGs) via thermal treatment. Thus, both processes start with the same molecular precursor and end with a porous graphene nanomaterial on Au(111), but the type of product, pNG or pGNR, depends on the specific coupling approach. We also produced extended nanoporous graphenes (NPGs) through the lateral fusion of highly aligned pGNRs on Au(111) that were grown at high coverage. The pNGs can also be synthesized directly in solution by Scholl oxidative cyclodehydrogenation of cyclotrimers. We demonstrate the generality of this approach by synthesizing two varieties of 7,10-dibromo-triphenylenes that selectively produced six nanoporous products with different dimensionalities. The basic 7,10-dibromo-triphenylene monomer is amenable to structural modifications, potentially providing access to many new porous graphene nanomaterials. We show that by constructing different porous structures from the same building blocks, it is possible to tune the energy band gap in a wide range.

Separation of Halogen Atoms by Sodium from Dehalogenative Reactions on a Au(111) Surface

On-surface dehalogenative reactions have been promising in the construction of nanostructures with diverse morphologies and intriguing electronic properties, while halogen (X), as the main byproduct, often impedes the formation of extended nanostructures and property characterization, and the reaction usually requires high C-X activation temperatures, especially on relatively inert Au(111). Enormous efforts in precursor design, halogen-to-halide conversion, and the introduction of extrinsic metal atoms have been devoted to either eliminating dissociated halogens or reducing reaction barriers. However, it is still challenging to separate halogens from molecular systems while facilitating C-X activation under mild conditions. Herein, a versatile halogen separation strategy has been developed based on the introduction of extrinsic sodium (Na) into dehalogenative reactions on Au(111) as model systems that both isolates the dissociated halogens and facilitates the C-Br activation under mild conditions. Moreover, the combination of scanning tunneling microscopy imaging and density functional theory calculations reveals the formation of sodium halides (NaX) from halogens in these separation processes as well as the reduction in reaction temperatures and barriers, demonstrating the versatility of extrinsic sodium as an effective "cleaner" and "dehalogenator" of surface halogens. Our study demonstrates a valuable strategy to facilitate the on-surface dehalogenative reactions, which will assist in the precise fabrication of low-dimensional carbon nanostructures.

Visualizing the Hierarchical Evolution of Aryl-Metal Bonding in Organometallic Nanostructures on Ag(111)

On-surface dehalogenative coupling reactions are promising for constructing nanostructures with diverse properties and functionalities. Extensive efforts have been devoted to single aryl-halogen (C-X) substituents and substitutions at various functionalization sites (typically including meta- and para-substitutions) to generate aryl-aryl single bonds. Moreover, multiple C-X substituents at the ortho-site and the peri- and bay-regions have been applied to create a variety of ring scaffolds. However, for multiple C-X substituents, the hierarchy of aryl-metal bond formation and dissociation remains elusive. Herein, by combining scanning tunneling microscopy imaging and density functional theory calculations, we have visualized and demonstrated the hierarchical evolution of aryl-metal bonding in organometallic intermediates involved in a dehalogenative coupling reaction on Ag(111), using a molecular precursor with both para-substitution and potential bay-region substitution. Our results elucidate how metal atoms are progressively embedded into and removed from organometallic intermediates, enhancing the understanding of on-surface dehalogenative coupling reactions for the controlled construction of the desired nanostructures.