Density-functional calculations of HCN adsorption on the pristine and Si-doped graphynes

Graphyne, a lattice of benzene rings connected by acetylene bonds, is one-atom-thick planar sheet of sp- and sp²-bonded carbons differing from the hybridization of graphene (considered as pure sp²). Here, HCN adsorption on the pristine and Si-doped graphynes was studied using density-functional calculations in terms of geometric, energetic, and electronic properties. It was found that HCN molecule is weakly adsorbed on the pristine graphyne and slightly affects its electronic properties. While, Si-doped graphyne shows high reactivity toward HCN, and, in the most favorable state, the calculated adsorption energy is about ?10.1 kcal/mol. The graphyne, in which sp-carbon was substituted by Si atom, is more favorable for HCN adsorption in comparison with sp²-carbon. It was shown that the electronic properties of Si-doped graphyne are strongly sensitive to the presence of HCN molecule and therefore it may be used in sensor devices.     

Direct Formation of C−C Triple‐Bonded Structural Motifs by On‐Surface Dehalogenative Homocouplings of Tribromomethyl‐Substituted Arenes

On‐surface synthesis shows significant potential in constructing novel nanostructures/nanomaterials, which has been intensely studied in recent years. The formation of acetylenic scaffolds provides an important route to the fabrication of emerging carbon nanostructures, including carbyne, graphyne, and graphdiyne, which feature chemically vulnerable sp‐hybridized carbon atoms. Herein, we designed and synthesized a tribromomethyl‐substituted compound. By using a combination of high‐resolution scanning tunneling microscopy, non‐contact atomic force microscopy, and density functional theory calculations, we demonstrated that it is feasible to convert these compounds directly into C?C triple‐bonded structural motifs by on‐surface dehalogenative homocoupling reactions. Concurrently, sp³‐hybridized carbon atoms are converted into sp‐hybridized ones, that is, an alkyl group is transformed into an alkynyl moiety. Moreover, we achieved the formation of dimer structures, one‐dimensional molecular wires, and two‐dimensional molecular networks on Au(111) surfaces, which should inspire further studies towards two‐dimensional graphyne structures.     

Promoting electrochemical reduction of CO2 to ethanol by B/N-doped sp3/sp2 nanocarbon electrode

Electrochemical reduction of CO₂ to value-added chemicals holds promise for carbon utilization and renewable electricity storage. However, selective CO₂ reduction to multi-carbon fuels remains a significant challenge. Here, we report that B/N-doped sp³/sp² hybridized nanocarbon (BNHC), consisting of ultra-small nanoparticles with a sp³ carbon core covered by a sp² carbon shell, is an efficient electrocatalyst for electrochemical reduction of CO₂ to ethanol at relatively low overpotentials. CO₂ reduction occurs with a Faradaic efficiency of 58.8%-69.1% for ethanol and acetate production at ?0.5 ～ ?0.6 V (vs. RHE), among which 51.6%-56.0% is for ethanol. The high selectivity for ethanol is due to the integrated effect of sp³/sp² carbon and B/N doping. Both sp³ carbon and B/N doping contribute to enhanced ethanol production with sp² carbon reducing the overpotential for CO₂ reduction to ethanol.     

Classical atomistic simulations of protein adsorption on carbon nanomaterials

Carbon nanomaterials are receiving an increasingly large interest in a variety of fields, including also nanomedicine. In this area, much attention is devoted to investigating and modeling the behavior of these nanomaterials when they interact with biological fluids and with biological macromolecules, in particular proteins and oligopeptides. The interaction with these molecules is in fact crucial to understand and predict the efficacy of nanomaterials as drug carriers or therapeutic agents as well as their potential toxicity when they occupy the active site of a protein or severely affect the secondary and tertiary structure, or even the local dynamics, thus inhibiting their biological function. In this review, therefore, we describe the most recent work carried out in the last few years to model the interaction between carbon nanomaterials, either pristine or functionalized, and proteins or oligopeptides using classical atomistic methods, mainly molecular dynamics simulations. The attention is focused on 0-dimensional fullerenes, mainly C₆₀, on 1-dimensional carbon nanotubes, mostly the single-walled armchair and some chiral ones, and on 2-dimensional graphene and graphyne, the latter containing also sp hybridized atoms in addition to the sp² ones common to the other carbon nanomaterials.     

Construction of a sp3/sp2 Carbon Interface in 3D N‐Doped Nanocarbons for the Oxygen Reduction Reaction

The development of highly efficient metal‐free carbon electrocatalysts for the oxygen reduction reaction (ORR) is one very promising strategy for the exploitation and commercialization of renewable and clean energy, but this still remains a significant challenge. Herein, we demonstrate a facile approach to prepare three‐dimensional (3D) N‐doped carbon with a sp³/sp² carbon interface derived from ionic liquids via a simple pyrolysis process. The tunable hybrid sp³ and sp² carbon composition and pore structures stem from the transformation of ionic liquids to polymerized organics and introduction of a Co metal salt. Through tuning both composition and pores, the 3D N‐doped nanocarbon with a high sp³/sp² carbon ratio on the surface exhibits a superior electrocatalytic performance for the ORR compared to that of the commercial Pt/C in Zn–air batteries. Density functional theory calculations suggest that the improved ORR performance can be ascribed to the existence of N dopants at the sp³/sp² carbon interface, which can lower the theoretical overpotential of the ORR.     

Comparative study on structure and properties of titanium/silicon mono‐ and co‐doped amorphous carbon films deposited by mid‐frequency magnetron sputtering

The titanium/silicon mono‐ and co‐doped amorphous carbon films were deposited by mid‐frequency magnetron sputtering Ti target, Si target, and Ti80S20 alloy target, respectively. The effects of doped elements on the composition, surface morphology, microstructure, and mechanical and tribological properties of the films were investigated. The results reveal that the ratio of sp³ and sp² carbon bonds of the films is regulated between 0.28 and 0.62 by a combination of Ti and Si dopant. The addition of small amounts of silicon leads to an increase in sp³ bonds and disorder degree of the sp² carbon. The co‐doped film exhibits significantly superior friction performance than the mono‐doped films. The ultra‐low friction (μ < 0.01) was achieved under a load of 2 N in ambient air with 40% RH. By comparing to the mono‐and co‐doped films, it is thought that the sp³/sp² ratio of the films may play a key role for the superlow friction. Copyright © 2013 John Wiley & Sons, Ltd.     

Hybrid Nanocarbon as a Catalyst for Direct Dehydrogenation of Propane: Formation of an Active and Selective Core–Shell sp2/sp3 Nanocomposite Structure

Hybrid nanocarbon, comprised of a diamond core and a graphitic shell with a variable sp²‐/sp³‐carbon ratio, is controllably obtained through sequential annealing treatment (550–1300 °C) of nanodiamond. The formation of sp² carbon increases with annealing temperature and the nanodiamond surface is reconstructed from amorphous into a well‐ordered, onion‐like carbon structure via an intermediate composite structure—a diamond core covered by a defective, curved graphene outer shell. Direct dehydrogenation of propane shows that the sp²‐/sp³‐nanocomposite exhibits superior catalytic performance to that of individual nanodiamond and graphitic nanocarbon. The optimum catalytic activity of the diamond/graphene composite depends on the maximum structural defectiveness and high chemical reactivity of the ketone groups. Ketone‐type functional groups anchored on the defects/vacancies are active for propene formation; nevertheless, once the oxygen functional groups are desorbed, the defects/vacancies alone might be active sites responsible for the C?H bond activation of propane.     

1H n.m.r. spectra of 6-acetoxy-2,4-cyclohexadienones

¹H n.m.r. spectra of 36 derivatives of 6-acetoxy-2,4-cyclohexadienones were analysed. All available evidence indicates that all members of the series have similar conformations which do not depart significantly from planarity. Substituent-induced chemical shifts and interproton coupling constants correlate well with those in the analogously substituted ethylene and benzene derivatives. In particular, it appears that a substituent on the central carbon atom exerts a similar influence on ⁴J(HH) across a ‘W’ path when the three intervening carbon atoms are a part of a benzene ring, an allylic system (i.e. sp³–sp²–sp²), a localized sp²–sp²–sp² system or a saturated (i.e. sp³–sp³–sp³) system.     

Enantioconvergent Reductive C(sp)−C(sp3) Cross-Coupling to Access Chiral α-Alkynyl Phosphonates Under Dual Nickel/Photoredox Catalysis

Transition-metal-catalyzed asymmetric carbon−carbon bond formation to forge phosphonates with an α-chiral carbon center through C(sp³)−C(sp³) and C(sp²)−C(sp³) couplings has been successful. However, the enantioselective C(sp)−C(sp³) coupling has not yet been disclosed. Reported herein is an unprecedented enantioconvergent cross-coupling of alkynyl bromides and α-bromo phosphonates to deliver chiral α-alkynyl phosphonates.     

Effect of the Content and Ordering of the sp2 Free Carbon Phase on the Charge Carrier Transport in Polymer-Derived Silicon Oxycarbides

The present work elaborates on the correlation between the amount and ordering of the free carbon phase in silicon oxycarbides and their charge carrier transport behavior. Thus, silicon oxycarbides possessing free carbon contents from 0 to ca. 58 vol.% (SiOC/C) were synthesized and exposed to temperatures from 1100 to 1800 °C. The prepared samples were extensively analyzed concerning the thermal evolution of the sp² carbon phase by means of Raman spectroscopy. Additionally, electrical conductivity and Hall measurements were performed and correlated with the structural information obtained from the Raman spectroscopic investigation. It is shown that the percolation threshold in SiOC/C samples depends on the temperature of their thermal treatment, varying from ca. 20 vol.% in the samples prepared at 1100 °C to ca. 6 vol.% for the samples annealed at 1600 °C. Moreover, three different conduction regimes are identified in SiOC/C, depending on its sp² carbon content: (i) at low carbon contents (i.e., <1 vol.%), the silicon oxycarbide glassy matrix dominates the charge carrier transport, which exhibits an activation energy of ca. 1 eV and occurs within localized states, presumably dangling bonds; (ii) near the percolation threshold, tunneling or hopping of charge carriers between spatially separated sp² carbon precipitates appear to be responsible for the electrical conductivity; (iii) whereas above the percolation threshold, the charge carrier transport is only weakly activated (E_a = 0.03 eV) and is realized through the (continuous) carbon phase. Hall measurements on SiOC/C samples above the percolation threshold indicate p-type carriers mainly contributing to conduction. Their density is shown to vary with the sp² carbon content in the range from 10¹⁴ to 10¹⁹ cm⁻³; whereas their mobility (ca. 3 cm²/V) seems to not depend on the sp² carbon content.     

CNDO/2 calculations of relaxation and reconstruction of diamond and silicon [111] surfaces

CNDO /2 calculations have been performed on the clusters X₄H₉ and X₄Y₉ modeling the [111] diamond and silicon surfaces. The X is either carbon or silicon atom and the Y is a pseudoatom containing one sp³ hybrid orbital. It is shown that in the CNDO /2 approximation in the foregoing pseudoatom models, the charge distribution of the cluster is better than the hydrogen atom, because the electronegativity of the hydrogen differs significantly from the electronegativity of the sp² orbital of the silicon atom. Using the CNDO /2 parametrization, the electronegativity of the hydrogen is very near to the electronegativity of the sp³ orbital of the carbon atom, thus the hydrogen can be used for the saturation of the carbon clusters.     

Fragmentation at sp2 carbon atoms in fragment molecular orbital method

In the fragment molecular orbital (FMO) method, a given molecular system is usually fragmented at sp³ carbon atoms. However, fragmentation at different sites sometimes becomes necessary. Hence, we propose fragmentation at sp² carbon atoms in the FMO method. Projection operators are constructed using sp² local orbitals. To maintain practical accuracy, it is essential to consider the three-body effect. In order to suppress the corresponding increase of computational cost, we propose approximate models considering local trimers. Numerical verification shows that the present models are as accurate as or better than the standard FMO2 method in total energy with fragmentation at sp³ carbon atoms.     

Quinone electrochemistry for the comparative assessment of sp2 surface content of boron doped diamond electrodes

Surface coverage measurements of electroactive quinone groups present on sp² carbon sites, are used to inform on the sp² surface content of boron doped diamond (BDD) electrodes. Laser micromachining of an electrode surface is used to systematically increase the amount of sp² carbon present by increasing the area machined. A linear relationship between quinone surface coverage and surface area lasered is determined (R² = 0.9999). This approach can also be used for comparative assessment of electrodes containing different amounts of surface sp² carbon.     

Palladium‐Catalyzed Carbon–Fluorine and Carbon–Hydrogen Bond Alumination of Fluoroarenes and Heteroarenes

Through serendipitous discovery, a palladium bis(phosphine) complex was identified as a catalyst for the selective transformation of sp²C−F and sp²C−H bonds of fluoroarenes and heteroarenes to sp²C−Al bonds (19 examples, 1 mol % Pd loading). The carbon–fluorine bond functionalization reaction is highly selective for the formation of organoaluminium products in preference to hydrodefluorination products (selectivity=4.4:1 to 27:1). Evidence is presented for a tandem catalytic process in which hydrodefluorination is followed by sp²C−H alumination.     

Designed Synthesis of a 2D Porphyrin‐Based sp2 Carbon‐Conjugated Covalent Organic Framework for Heterogeneous Photocatalysis

The construction of stable covalent organic frameworks (COFs) for various applications is highly desirable. Herein, we report the synthesis of a novel two‐dimensional (2D) porphyrin‐based sp² carbon‐conjugated COF (Por‐sp²c‐COF), which adopts an eclipsed AA stacking structure with a Brunauer—Emmett—Teller surface area of 689 m² g^?1. Owing to the C=C linkages, Por‐sp²c‐COF shows a high chemical stability under various conditions, even under harsh conditions such as 9 m HCl and 9 m NaOH solutions. Interestingly, Por‐sp²c‐COF can be used as a metal‐free heterogeneous photocatalyst for the visible‐light‐induced aerobic oxidation of amines to imines. More importantly, in comparison to imine‐linked Por‐COF, the inherent structure of Por‐sp²c‐COF equips it with several advantages as a photocatalyst, including reusability and high photocatalytic performance. This clearly demonstrates that sp² carbon‐linked 2D COFs can provide an interesting platform for heterogeneous photocatalysis.     

Topology‐Templated Synthesis of Crystalline Porous Covalent Organic Frameworks

A strategy is presented for the synthesis of crystalline porous covalent organic frameworks via topology‐templated polymerization. The template is based on imine‐linked frameworks and their (001) facets seed the C=C bond formation reaction to constitute 2D sp² carbon‐conjugated frameworks. This strategy is applicable to templates with different topologies, enables designed synthesis of frameworks that cannot be prepared via direct polymerization, and creates a series of sp² carbon frameworks with tetragonal, hexagonal, and kagome topologies. The sp² carbon frameworks are highly luminescent even in the solid state and exhibit topology‐dependent π transmission and exciton migration; these key fundamental π functions are unique to sp² carbon‐conjugated frameworks and cannot be accessible by imine‐linked frameworks, amorphous analogues, and 1D conjugated polymers. These results demonstrate an unprecedented strategy for structural and functional designs of covalent organic frameworks.     

Surface oxidation process of a diamond‐like carbon film analyzed by difference X‐ray photoelectron spectroscopy

Difference X‐ray photoelectron spectroscopy (D‐XPS) revealed the surface oxidation process of a diamond‐like carbon (DLC) film. Evaluation of surface functional groups on DLC solely by the C 1s spectrum is difficult because the spectrum is broad and has a secondary asymmetric lineshape. D‐XPS clarified the subtle but critical changes at the DLC surface caused by wet oxidation. The hydroxyl (C―OH) group was dominant at the oxidized surface. Further oxidized carbonyl (C?O) and carboxyl (including carboxylate) (COO) groups were also obtained; however, the oxidation of C?O to COO was suppressed to some extent because the reaction required C―C bond cleavage. Wet oxidation cleaved the aliphatic hydrogenated and non‐hydrogenated sp² carbon bonds (C―H sp² and C―C sp²) to create a pair of C―OH and hydrogenated sp³ carbon (C―H sp³) bonds. The reaction yield for C―H sp² was superior at the surface, suggesting that the DLC film was hydrogen rich at the surface. Oxidation of aromatic sp² rings or polycyclic aromatic hydrocarbons such as nanographite to phenols did not occur because of their resonance stabilization with electron delocalization. Non‐hydrogenated sp³ carbon (C―C sp³) bonds were not affected by oxidation, suggesting that these bonds are chemically inert. Copyright © 2014 John Wiley & Sons, Ltd.     

Scalable Photoelectrochemical Dehydrogenative Cross‐Coupling of Heteroarenes with Aliphatic C−H Bonds

Heteroarenes are structural motifs found in many bioactive compounds and functional materials. Dehydrogenative cross‐coupling of heteroarenes with aliphatic C?H bonds provides straightforward access to functionalized heteroarenes from readily available materials. Established methods employ stoichiometric chemical oxidants under conditions of heating or light irradiation. By merging electrochemistry and photochemistry, we have achieved efficient photoelectrochemical dehydrogenative cross‐coupling of heteroarenes and C(sp³)?H donors through H₂ evolution, without the addition of metal catalysts or chemical oxidants. Mechanistically, the C(sp³)?H donor is converted to a nucleophilic carbon radical through H‐atom transfer with chlorine atom, which is produced by light irradiation of anodically generated Cl₂ from Cl^?. The carbon radical then undergoes radical substitution to the heteroarene to afford alkylated heteroarene products.     

Copper‐Catalyzed Site‐Selective Intramolecular Amidation of Unactivated C(sp3)?H Bonds

The intramolecular dehydrogenative amidation of aliphatic amides, directed by a bidentate ligand, was developed using a copper‐catalyzed sp³ C H bond functionalization process. The reaction favors predominantly the C H bonds of β‐methyl groups over the unactivated methylene C H bonds. Moreover, a preference for activating sp³ C H bonds of β‐methyl groups, via a five‐membered ring intermediate, over the aromatic sp² C H bonds was also observed in the cyclometalation step. Additionally, sp³ C H bonds of unactivated secondary sp³ C H bonds could be functionalized by favoring the ring carbon atoms over the linear carbon atoms.     

What are the practical limits for the specific surface area and capacitance of bulk sp~2 carbon materials?

The possible practical limits for the specific surface area and capacitance performance of bulk sp~2 carbon materials were investigated experimentally and theoretically using a variety of carbon materials. We find the limit for the specific surface area to be 3500–3700 m~2 g~(-1), and based on this, the corresponding best capacitance was predicted for various electrolyte systems. A model using an effective ionic diameter for the electrolyte ions was proposed and used to calculate the theoretical capacitance. A linear dependence of experimental capacitance versus effective specific surface area of various sp~2 carbon materials was obtained for all studied ionic liquid, organic and aqueous electrolyte systems. Furthermore, excellent agreement between the theoretical and experimental capacitance was observed for all the tested sp~2 carbon materials in these electrolyte systems, indicating that this model can be applied widely in the evaluation of various carbon materials for supercapacitors.