Regioselective multistep reconstructions of half‐saturated zigzag carbon nanotubes

The open edge reconstruction of half‐saturated (6,0) zigzag carbon nanotube (CNT) was introduced by density functional calculations. The multistep rearrangement was demonstrated as a regioselective process to generate a defective edge with alternating pentagons and heptagons. Not only the thermal stability was found to be enhanced significantly after reconstruction but also the total spin of CNT was proved to be reduced gradually from high‐spin septet to close‐shell singlet, revealing the critical role of deformed edge on the geometrical and magnetic properties of open‐ended CNTs. Kinetically, the initial transformation was confirmed as the rate‐determining step with relatively the largest reaction barrier and the following steps can take place spontaneously. © 2016 Wiley Periodicals, Inc.     

Lattice mismatch induced nonlinear growth of graphene

As a two-dimensional material, graphene can be obtained via epitaxial growth on a suitable substrate. Recently, an interesting nonlinear behavior of graphene growth has been observed on some metal surfaces, but the underlying mechanism is still elusive. Taking the Ir(111) surface as an example, we perform a mechanistic study on graphene growth using a combined approach of first-principles calculations and kinetic Monte Carlo (kMC) simulations. Small carbon clusters on the terrace or at step sites are studied first. Then, we investigate how these small carbon species are attached to graphene edges. Generally, attachment of carbon atoms is thermodynamically favorable. However, due to substrate effect, there are also some edge sites where graphene growth must proceed via cluster attachment. The overall growth rate is determined by these cluster attachment processes, which have a much lower chance of happening compared to the monomer attachment. On the basis of such an inhomogeneous growth picture, kMC simulations are performed by separating different time scales, and the experimentally found quintic-like behavior is well reproduced. Different nonlinear growth behaviors are predicted for different graphene orientations, which is consistent with previous experiments. Inhomogeneity induced by lattice mismatch revealed in this study is expected to be a universal phenomenon and will play an important role in the growth of many other heteroepitaxial systems.     

Controllable growth of conical and cylindrical TiO2-carbon core-shell nanofiber arrays and morphologically dependent electrochemical properties

Quasi‐aligned cylindrical and conical core–shell nanofibers consisting of carbon shells and TiO₂ nanowire cores are produced in situ on Ti foils without using a foreign metallic catalyst and template. A cylindrical nanofiber has a TiO₂ nanowire core 30–50 nm in diameter and a 5–10 nm‐thick cylindrical carbon shell, while in the conical nanostructure the TiO₂ nanowire core has a diameter of 20–40 nm and the thickness of the carbon shell varies from about 200 nm at the bottom to about 5 nm at the tip. Electrochemical analysis reveals well‐defined redox peaks of the [Fe(CN)₆]^3?/4? redox couple and heterogeneous charge‐transfer rate constants of 0.010 and 0.062 cm s^?1 for the cylindrical and conical nanofibers, respectively. The coverage of exposed edge planes on the cylindrical and conical carbon shells is estimated to be 2.5 and 15.5 % respectively. The more abundant exposed edge planes on the conical nanofiber decrease the overpotential and increase the voltammetric resolution during electrochemical detection of uric acid and ascorbic acid. Our results suggest that the density of edge‐plane sites estimated from Raman scattering is not necessarily equal to the density of exposed edge‐plane sites, and only carbon electrodes with a large density of exposed edge planes or free graphene sheet ends exhibit better electrochemical performance.     

The Structure and Stability of Magic Carbon Clusters Observed in Graphene Chemical Vapor Deposition Growth on Ru(0001) and Rh(111) Surfaces

To improve the atomically controlled growth of graphene by chemical vapor deposition (CVD), understanding the evolution from various carbon species to a graphene nucleus on various catalyst surfaces is essential. Experimentally, an ultrastable carbon cluster on Ru(0001) and Rh(111) surfaces was observed, while its structure and formation process were still under debate. Using ab initio calculations and kinetic analyses, we disclose a specific type of carbon cluster, composed of a C₂₁ core and a few dangling C atoms, which is exceptional stable in a size range from 21 to 27 C atoms. The most stable one of them, an isomer of C₂₄ characterized by three dangling C atoms attached to the C₂₁ core (denoted as C₂₁‐3C), is the most promising candidate of the experimental observation. The ultrastability of C₂₁‐3C originates from both the stable core and the appropriate passivation of the dangling carbon atoms by the catalyst surface.     

Graphene nucleation on transition metal surface: structure transformation and role of the metal step edge

The nucleation of graphene on a transition metal surface, either on a terrace or near a step edge, is systematically explored using density functional theory calculations and applying the two-dimensional (2D) crystal nucleation theory. Careful optimization of the supported carbon clusters, C(N) (with size N ranging from 1 to 24), on the Ni(111) surface indicates a ground state structure transformation from a one-dimensional C chain to a 2D sp(2) C network at N ≈ 10-12. Furthermore, the crucial parameters controlling graphene growth on the metal surface, nucleation barrier, nucleus size, and nucleation rate on a terrace or near a step edge are calculated. In agreement with numerous experimental observations, our analysis shows that graphene nucleation near a metal step edge is superior to that on a terrace. On the basis of our analysis, we propose the use of graphene seeds to synthesize high-quality graphene in large area.     

金属衬底在石墨烯化学气相沉积生长中的作用

以过渡金属为催化衬底的化学气相沉积法(Chemical Vapor Deposition，CVD)已经可以制备与机械剥离样品相媲美的石墨烯，是实现石墨烯工业应用的关键技术之一。原子尺度理论研究能够帮助我们深刻理解石墨烯生长机理，为实验现象提供合理的解释，并有可能成为将来实验设计的理论指导。本文从理论计算的角度，总结了各种金属衬底在石墨烯CVD生长过程中的各种作用与相应的机理，包括在催化碳源裂解、降低石墨烯成核密度等，催化加快石墨烯快速生长，修复石墨烯生长过程中产生的缺陷，控制外延生长石墨烯的晶格取向，以及在降温过程中石墨烯褶皱与金属表面台阶束的形成过程等。在本文最后，我们对当前石墨烯生长领域中亟需解决的理论问题进行了深入探讨与展望。     

On‐Surface Synthesis of Graphene Nanoribbons Catalyzed by Ni Atoms

Nanometer‐wide graphene nanoribbons can be synthesized from halogen aromatics through multistep on‐surface reactions, but the catalytic role of extrinsic transition‐metal atoms in these reactions are still to be explored. Here by low‐temperature scanning tunneling microscopy, we investigated the on‐surface synthesis of graphene nanoribbons from 10,10′‐dibromo‐9,9′‐bianthryl precursors in the presence of Ni atoms. Ni atoms not only act as catalysts in debromination and lead to the formation of an organometallic intermediate at 300 K, but also prompt the fusion reaction between graphene nanoribbons at 673 K. Our work demonstrates a more efficient way to fabricate fused graphene nanoribbons.     

Computational Studies on Non‐covalent Interactions of Carbon and Boron Fullerenes with Graphene

First‐principles DFT calculations are carried out to study the changes in structures and electronic properties of two‐dimensional single‐layer graphene in the presence of non‐covalent interactions induced by carbon and boron fullerenes (C₆₀, C₇₀, C₈₀ and B₈₀). Our study shows that larger carbon fullerene interacts more strongly than the smaller fullerene, and boron fullerene interacts more strongly than that of its carbon analogue with the same nuclearity. We find that van der Waals interactions play a major role in governing non‐covalent interactions between the adsorbed fullerenes and graphene. Moreover, a greater extent of van der Waals interactions found for the larger fullerenes, C₈₀ and B₈₀, relative to smaller C₆₀, and consequently, results in higher stabilisation. We find a small amount of electron transfer from graphene to fullerene, which gives rise to a hole‐doped material. We also find changes in the graphene electronic band structures in the presence of these surface‐decorated fullerenes. The Dirac cone picture, such as that found in pristine graphene, is significantly modified due to the re‐hybridisation of graphene carbon orbitals with fullerenes orbitals near the Fermi energy. However, all of the composites exhibit perfect conducting behaviour. The simulated absorption spectra for all of the graphene–fullerene hybrids do not exhibit a significant change in the absorption peak positions with respect to the pristine graphene absorption spectrum. Additionally, we find that the hole‐transfer integral between graphene and C₆₀ is larger than the electron‐transfer integrals and the extent of these transfer integrals can be significantly tuned by graphene edge functionalisation with carboxylic acid groups. Our understanding of the non‐covalent functionalisation of graphene with various fullerenes would promote experimentalists to explore these systems, for their possible applications in electronic and opto‐electronic devices.     

Atomic Layer Deposition of Hafnium(IV) Oxide on Graphene Oxide: Probing Interfacial Chemistry and Nucleation by using X‐ray Absorption and Photoelectron Spectroscopies

Interfacing graphene with metal oxides is of considerable technological importance for modulating carrier density through electrostatic gating as well as for the design of earth‐abundant electrocatalysts. Herein, we probe the early stages of the atomic layer deposition (ALD) of HfO₂ on graphene oxide using a combination of C and O K‐edge near‐edge X‐ray absorption fine structure spectroscopies and X‐ray photoelectron spectroscopy. Dosing with water is observed to promote defunctionalization of graphene oxide as a result of the reaction between water and hydroxyl/epoxide species, which yields carbonyl groups that further react with migratory epoxide species to release CO₂. The carboxylates formed by the reaction of carbonyl and epoxide species facilitate binding of Hf precursors to graphene oxide surfaces. The ALD process is accompanied by recovery of the π‐conjugated framework of graphene. The delineation of binding modes provides a means to rationally assemble 2D heterostructures.     

Effects of Carbide Formation in Graphene Growth

Besides carbon solubility, the carbide formation possibility is another important factor to differentiate various substrate materials in graphene growth. A recent experiment indicates that the formation of transition metal carbides (TMCs) can suppress carbon precipitation. In this study, Mo₂C, a representative of TMCs, is used to study the effects of carbide formation in graphene growth from first principles. Carbon diffusion in Mo₂C bulk turns out to be very difficult and it becomes much easier on the Mo₂C(001) surface. Therefore, carbon precipitation suppression and graphene growth can be realized simultaneously. A direction depended diffusion behavior is observed on the Mo₂C(101) surface, which makes it less favorable for graphene growth compared to the (001) surface.     

Three‐Dimensional Graphene Networks with Abundant Sharp Edge Sites for Efficient Electrocatalytic Hydrogen Evolution

To achieve sustainable production of hydrogen (H₂) through water splitting, establishing efficient and earth‐abundant electrocatalysts is of great necessity. Morphology engineering of graphene is now shown to modulate the electronic structure of carbon skeleton and in turn endow it with excellent ability of proton reduction. Three‐dimensional (3D) graphene networks with a high density of sharp edge sites are synthesized. Electrocatalytic measurements indicate that the obtained 3D graphene networks can electrocatalyze H₂ evolution with an extremely low onset potential of about 18 mV in 0.5 m H₂SO₄ solution, together with good stability. A combination of control experiments and density functional theory (DFT) investigations indicates that the exceptional H₂ evolution performance is attributed to the abundant sharp edge sites of the advanced frameworks, which are responsible for promoting the adsorption and reduction of protons.     

A Molecular Pillar Approach To Grow Vertical Covalent Organic Framework Nanosheets on Graphene: Hybrid Materials for Energy Storage

Hybrid 2D–2D materials composed of perpendicularly oriented covalent organic frameworks (COFs) and graphene were prepared and tested for energy storage applications. Diboronic acid molecules covalently attached to graphene oxide (GO) were used as nucleation sites for directing vertical growth of COF‐1 nanosheets (v‐COF‐GO). The hybrid material has a forest of COF‐1 nanosheets with a thickness of 3 to 15 nm in edge‐on orientation relative to GO. The reaction performed without molecular pillars resulted in uncontrollable growth of thick COF‐1 platelets parallel to the surface of GO. The v‐COF‐GO was converted into a conductive carbon material preserving the nanostructure of precursor with ultrathin porous carbon nanosheets grafted to graphene in edge‐on orientation. It was demonstrated as a high‐performance electrode material for supercapacitors. The molecular pillar approach can be used for preparation of many other 2D‐2D materials with control of their relative orientation.     

A New Member of the Graphene Family: Graphene Acid

A new member of the family of graphene derivatives, namely, graphene acid with a composition close to C₁(COOH)₁, was prepared by oxidation of graphene oxide. The synthetic procedure is based on repeated oxidation of graphite with potassium permanganate in an acidic environment. The oxidation process was studied in detail after each step. The multiple oxidations led to oxidative removal of other oxygen functional groups formed in the first oxidation step. Detailed chemical analysis showed only a minor amount of other oxygen‐containing functional groups such as hydroxyl and the dominant presence of carboxyl groups in a concentration of about 30 wt %. Further oxidation led to complete decomposition of graphene acid. The obtained material exhibits unique sorption capacity towards metal ions and carbon dioxide. The highly hydrophilic nature of graphene acid allowed the assembly of ultrathin free‐standing membranes with high transparency.     

A Comparative Study of CO Oxidation on Nitrogen‐ and Phosphorus‐Doped Graphene

The geometry, electronic structure, and catalytic properties of nitrogen‐ and phosphorus‐doped graphene (N‐/P‐graphene) are investigated by density functional theory calculations. The reaction between adsorbed O₂ and CO molecules on N‐ and P‐graphene is comparably studied via Langmuir–Hinshelwood (LH) and Eley–Rideal (ER) mechanisms. The results indicate that a two‐step process can occur, namely, CO+O₂→CO₂+O_ads and CO+O_ads→CO₂. The calculated energy barriers of the first step are 15.8 and 12.4 kcal mol^?1 for N‐ and P‐graphene, respectively. The second step of the oxidation reaction on N‐graphene proceeds with an energy barrier of about 4 kcal mol^?1. It is noteworthy that this reaction step was not observed on P‐graphene because of the strong binding of O_ads species on the P atoms. Thus, it can be concluded that low‐cost N‐graphene can be used as a promising green catalyst for low‐temperature CO oxidation.     

Preparation of antistatic high‐density polyethylene composites based on synergistic effect of graphene nanoplatelets and multi‐walled carbon nanotubes

Graphene nanoplatelets are promising candidates for enhancing the electrical conductivity of composites. However, because of their poor dispersion, graphene nanoplatelets must be added in large amounts to achieve the desired electrical properties, but such large amounts limit the industrial application of graphene nanoplatelets. Multi‐walled carbon nanotubes also possess high electrical conductivity accompanied by poor dispersion. Therefore, a synergistic effect was generated between graphene nanoplatelets and multi‐walled carbon nanotubes and used for the first time to prepare antistatic materials with high‐density polyethylene via 1‐step melt blending. The synergistic effect makes it possible to significantly improve the electrical properties by adding a small amount of untreated graphene nanoplatelets and multi‐walled carbon nanotubes and increases the possibility of using graphene nanoplatelets in industrial applications. When only 1 wt% graphene nanoplatelets and 0.5 wt% multi‐walled carbon nanotubes were added, the surface and volume resistivity values of the composites were much lower than those of the composites that were only added 3 wt% graphene nanoplatelets. Additionally, as a result of the synergistic effect of graphene nanoplatelets and multi‐walled carbon nanotubes, the composites met the requirements for antistatic materials.     

Large‐Area Synthesis of Superclean Graphene via Selective Etching of Amorphous Carbon with Carbon Dioxide

Contamination commonly observed on the graphene surface is detrimental to its excellent properties and strongly hinders its application. It is still a great challenge to produce large‐area clean graphene film in a low‐cost manner. Herein, we demonstrate a facile and scalable chemical vapor deposition approach to synthesize meter‐sized samples of superclean graphene with an average cleanness of 99 %, relying on the weak oxidizing ability of CO₂ to etch away the intrinsic contamination, i.e., amorphous carbon. Remarkably, the elimination of amorphous carbon enables a significant reduction of polymer residues in the transfer of graphene films and the fabrication of graphene‐based devices and promises strongly enhanced electrical and optical properties of graphene. The facile synthesis of large‐area superclean graphene would open the pathway for both fundamental research and industrial applications of graphene, where a clean surface is highly needed.     

Role of Graphitic Nitrogen and π‐Conjugated Functional Groups in Selective Oxidation of Alcohols: A DFT based Mechanistic Elucidation

Selective oxidation of alcohols to aldehydes is challenging reaction due to its accessibility to overoxidation. In this study, we have made an attempt to unravel the mechanistic aspects of selective oxidation of allyl alcohols that contain multiple functional groups catalyzed by N‐doped graphene. The role of graphitic nitrogen and the presence of π‐conjugated functional groups are demonstrated using the state‐of‐the‐art density functional theory calculations. The detailed reaction mechanism for aerobic oxidation of allyl alcohol (AA) and cinnamyl alcohol (CA) are investigated. The formation of activated oxygen species (AOS) over N‐doped graphene (NG) has been adopted from our previous report. The results revealed that ketonic AOS oxidizes allyl alcohols into aldehydes selectively with a relatively lower activation barrier of 20.1 kcal mol^?1. The oxidation of alcohols with the AOS formed at the edge results in high activation barriers owing to its high thermodynamic stability. Similarly, AOS formed at the center leads to the formation of H₂O₂ along with high activation barriers. As a consequence, AOS formed at the center is less active when compared to ketonic AOS. The overoxidation of aldehyde is only possible due to the formation of H₂O₂. However, it is unlikely to happen due to unfavorable ambient conditions. The presence of multiple π‐conjugated functional groups is responsible for the significant reduction in the activation barriers of the second hydrogen transfer step due to the stabilization of intermediate by increasing the acidic nature of the intermediates. On the basis of the results, a generalized reaction mechanism has been proposed. These results would definitely shed light on the effective fabrication of catalysts for oxidation of alcohol and sustainable energy.     

Investigation of the Thermal Stability of the Carbon Framework of Graphene Oxide

In this study, we use our recently prepared graphene oxide (GO) with an almost intact σ‐framework of carbon atoms (ai‐GO) to probe the thermal stability of the carbon framework for the first time. Ai‐GO exhibits few defects because CO₂ formation is prevented during synthesis. Ai‐GO was thermally treated before chemical reduction and the resulting defect density in graphene was subsequently determined by statistical Raman microscopy. Surprisingly, the carbon framework of ai‐GO is stable in thin films up to 100 °C. Furthermore, we find evidence for an increase in the quality of ai‐GO upon annealing at 50 °C before reduction. The carbon framework of GO prepared according to the popular Hummers’ method (GO‐c) appears to be less stable and decomposition starts at 50 °C, which is qualitatively indicated by CO₂‐trapping experiments in μm‐thin films. Information about the stability of GO is important for storing, processing, and using GO in many applications.     

Template effect in the competition between Haeckelite and graphene growth on Ni(111): quantum chemical molecular dynamics simulations

Quantum chemical molecular dynamics (QM/MD) simulations of ensembles of C(2) molecules on the Ni(111) terrace show that, in the absence of a hexagonal template or step edge, Haeckelite is preferentially nucleated over graphene as a metastable intermediate. The nucleation process is dominated by the swift transition of long carbon chains toward a fully connected sp(2) carbon network. Starting from a pentagon as nucleus, pentagons and heptagons condense during ring collapse reactions, which results in zero overall curvature. To the contrary, in the presence of a coronene-like C(24) template, hexagonal ring formation is clearly promoted, in agreement with recent suggestions from experiments. In the absence of step edges or molecular templates, graphene nucleation follows Ostwald's "rule of stages" cascade of metastable states, from linear carbon chains, via Haeckelite islands that finally anneal to graphene.     

Functional Graphene by Thiol‐ene Click Chemistry

Thiol‐ene click reaction was successfully employed for chemical modification of graphene oxide (GO) by one‐step synthesis. Herein, 2,2‐azobis(2‐methylpropionitrile) (AIBN) was used as thermal catalyst and cysteamine hydrochloride (HS?(CH₂)₂?NH₂HCl) was used as thiol‐containing compound, which is incorporated to GO surface upon reaction with the C=C bonds. The hydrochloride acts as protecting group for the amine, which is finally eliminated by adding sodium hydroxide. The modified GO contains both S‐ and N‐containing groups (NS‐GO). We found that NS‐GO sheets form good dispersion in water, ethanol, and ethylene glycol. These graphene dispersions can be processed into functionalized graphene film. Besides, it was demonstrated that NS‐GO was proved to be an excellent host matrix for platinum nanoparticles. The developed method paves a new way for graphene modification and its functional nanocomposites.