Functionalization of Hydrogenated Graphene: Transition‐Metal‐Catalyzed Cross‐Coupling Reactions of Allylic C−H Bonds

The chemical functionalization of hydrogenated graphene can modify its physical properties and lead to better processability. Herein, we describe the chemical functionalization of hydrogenated graphene through a dehydrogenative cross‐coupling reaction between an allylic C?H bond and the α‐C?H bond of tetrahydrothiophen‐3‐one using Cu(OTf)₂ as the catalyst and DDQ as the oxidant. The chemical functionalization was confirmed by X‐ray photoelectron spectroscopy and Fourier transform infrared spectroscopy and visualized by scanning electron microscopy. The functionalized hydrogenated graphene material demonstrated improved dispersion stability in water, bringing new quality to the elusive hydrogenated graphene (graphane) materials. Hydrogenated graphene provides broad possibilities for chemical modifications owing to its reactivity.     

Insights from the Adsorption of Halide Ions on Graphene Materials

Graphene has recently found applications in a wide range of fields. Density functional calculations show that halide ions can be adsorbed on pristine graphene, but only F^? has an appreciable binding energy (?97.0 kJ mol^?1). Graphene materials, which are mainly electron donors, can be made strong electron acceptors by edge functionalization with F atoms. The binding strengths of halide ions are greatly enhanced by edge functionalization and show direct proportionality with the degree of functionalization Θ and increased charge transfer. In contrast, the adsorption strengths of metal ions on pristine graphene are clearly superior to those of halide ions but decline substantially with increasing degree of edge functionalization, and for Θ=100 %, the binding energies of ?95.7, ?44.8, and ?25.9 kJ mol^?1 that are calculated for Li⁺, Na⁺, and K⁺, respectively, are obviously inferior to that of F^? (?186.3 kJ mol^?1). Thus, the electronic properties of graphene are fundamentally regulated by edge functionalization, and the preferential adsorption of certain metal ions or anions can be facilely realized by choice of an appropriate degree of functionalization. Adsorbed metal ions and anions behave differently on gradual addition of water molecules, and their binding strengths remain substantial when graphene materials are in the pristine and highly edge functionalized states, respectively.     

On the Addition of Aryl Radicals to Graphene: The Importance of Nonbonded Interactions

Dispersion‐corrected density functional theory is utilized to study the addition of aryl radicals to perfect and defective graphene. Although the perfect sheet shows a low reactivity against aryl diazonium salts, the agglomeration of these groups and the addition onto defect sites improves the feasibility of the reaction by increasing binding energies per aryl group up to 27 kcal mol^?1. It is found that if a single phenyl radical interacts with graphene, the covalent and noncovalent additions have similar binding energies, but in the particular case of the nitrophenyl group, the adsorption is stronger than the chemisorption. The single vacancy shows the largest reactivity, increasing the binding energy per aryl group by about 80 kcal mol^?1. The zigzag edge ranks second, enhancing the reactivity 5.4 times with respect to the perfect sheet. The less reactive defect site is the Stone–Wales type, but even in this case the addition of an isolated aryl radical is exergonic. The arylation process is favored if the groups are attached nearby and on different sublattices. This is particularly true for the ortho and para positions. However, the enhancement of the binding energies decreases quickly if the distance between the two aryl radicals is increased, thereby making the addition on the perfect sheet difficult. A bandgap of 1–2 eV can be opened on functionalization of the graphene sheets with aryl radicals, but for certain configurations the sheet can maintain its semimetallic character even if there is one aryl radical per eight carbon atoms. At the highest level of functionalization achieved, that is, one aryl group per five carbon atoms, the bandgap is 1.9 eV. Regarding the effect of using aryl groups with different substituents, it is found that they all induce the same bandgap and thus the presence of NO₂, H, or Br is not relevant for the alteration of the electronic properties. Finally, it is observed that the presence of tetrafluoroborate can induce metallic character in graphene.     

Computational Studies on the Structures of Nanographenes with Various Edge Functionalities

Computational studies have often been carried out on hydrogen-terminated nanographenes (NGs). These structures are, however, far from those deduced from experimental observations, which have suggested armchair edges with two carboxy groups on the edges as dominant. We conducted computational studies on NGs consisting of C₄₂, C₆₀, C₇₈, C₉₆, C₁₄₂, and C₁₇₄ carbon atoms with hydrogen, carboxy, and N-methyl imide-terminated armchair edges. DFT calculations inform distorted basal planes and similar HOMO-LUMO gaps, indicating that the edge oxidation and functionalization do not very influence the electronic structure. Comparison of observed UV-vis spectra of carboxy- and N-octadecyl chain terminated NGs with calculated spectra of model NGs informs the contribution of π-π* transitions on the basal plane to the absorptions in the visible region. A dimeric structure of NG and octadecyl-installed NG demonstrate that both the distorted basal planes and the steric contacts among the functional groups widen the surface-to-surface distance thereby allowing the invasion of solvent molecules between the surfaces. This picture is consistent with the improved solubility of edge-modified NGs.     

Thermal Disproportionation of Oxo-Functionalized Graphene

Graphene production by wet chemistry is an ongoing scientific challenge. Controlled oxidation of graphite introduces oxo functional groups; this material can be processed and converted back to graphene by reductive defunctionalization. Although thermal processing yields conductive carbon, a ruptured and undefined carbon lattice is produced as a consequence of CO₂ formation. This thermal process is not understood, but it is believed that graphene is not accessible. Here, we thermally process oxo-functionalized graphene (oxo-G) with a low (4–6 %) and high degree of functionalization (50–60 %) and find on the basis of Raman spectroscopy and transmission electron microscopy performed at atomic resolution (HRTEM) that thermal processing leads predominantly to an intact carbon framework with a density of lattice defects as low as 0.8 %. We attribute this finding to reorganization effects of oxo groups. This finding holds out the prospect of thermal graphene formation from oxo-G derivatives.     

Selective Functionalization of Graphene at Defect‐Activated Sites by Arylazocarboxylic tert‐Butyl Esters

The development of versatile functionalization concepts for graphene is currently in the focus of research. Upon oxo‐functionalization of graphite, the full surface of graphene becomes accessible for C?C bond formation to introduce out‐of‐plane functionality. Herein, we present the arylation of graphene with arylazocarboxylic tert‐butyl esters, which generates aryl radicals after activation with an acid. Surprisingly, the degree of functionalization is related to the concentration of lattice vacancy defects in the graphene material. Consequently, graphene materials that are free from lattice defects are not reactive. The reaction can be applied to graphene dispersed in solvents and leads to bitopic functionalization as well as monotopic functionalization when the graphene is deposited on surfaces. As the arylazocarboxylic tert‐butyl ester moiety can be attached to various molecules, the presented method paves the way to functional graphene derivatives, with the density of defects determining the degree of functionalization.     

Improving Catalyst Activity in Hydrocarbon Functionalization by Remote Pyrene–Graphene Stacking

A copper complex bearing an N-heterocyclic carbene ligand with a pyrene “tail” attached to the backbone has been prepared and supported on reduced graphene oxide (rGO). The free and supported copper materials have been employed as homogeneous and heterogeneous catalysts in the functionalization of hydrocarbons such as n-hexane, cyclohexane, and benzene through incorporation of the CHCO₂Et unit from ethyl diazoacetate. The graphene-anchored complex displays higher reaction rates and induces higher yields than its soluble counterpart, features that can be rationalized in terms of a decrease in electron density at the metal center due to a remote net electronic flux from the supported copper complex to the graphene surface.     

Controlled Chemistry Approach to the Oxo‐Functionalization of Graphene

Graphene is the best‐studied 2D material available. However, its production is still challenging and the quality depends on the preparation procedure. Now, more than a decade after the outstanding experiments conducted on graphene, the most successful wet‐chemical approach to graphene and functionalized graphene is based on the oxidation of graphite. Graphene oxide has been known for more than a century; however, the structure bears variable large amounts of lattice defects that render the development of a controlled chemistry impossible. The controlled oxo‐functionalization of graphene avoids the formation of defects within the σ‐framework of carbon atoms, making the synthesis of specific molecular architectures possible. The scope of this review is to introduce the field of oxo‐functionalizing graphene. In particular, the differences between GO and oxo‐functionalized graphene are described in detail. Moreover analytical methods that allow determining lattice defects and functional groups are introduced followed by summarizing the current state of controlled oxo‐functionalization of graphene.     

Edge‐Selectively Functionalized Graphene Nanoplatelets

Graphene, as a single layer of graphite, is currently the focal point of research into condensed matter owing to its promising properties, such as exceptional mechanical strength, high thermal conductivity, large specific surface area, and ultrahigh electron‐transport properties. Therefore, various physical and chemical synthetic procedures to prepare graphene and/or graphene nanoplatelets have been rapidly developed. Specifically, the synthesis of edge‐selectively functionalized graphene (EFG) has been recently reported by using simple and scalable approaches, such as “direct” Friedel‐Crafts acylation reactions in a mild acidic medium and a mechanochemical ball‐milling process. In these approaches, chemical functionalization predominantly take place at the edges of the graphitic layers via the covalent attachment of targeted organic “molecular wedges”. In addition, the distortion of the crystalline structures in the basal plane, which is beneficial for preserving the unique properties of the graphitic framework, can be minimized. In addition, the efficient exfoliation of graphene can be achieved, owing to the strong repulsive forces from the covalently linked wedges and strong shear forces during the reaction. Furthermore, EFG shows promising potential in many useful applications, such as highly conductive large‐area films, metal‐free electrocatalysts for the oxygen‐reduction reaction (ORR), and as additives in composite materials with enhanced properties. Herein, we summarize the recent progress and general aspects of EFG, including synthesis, reaction mechanism, properties, and applications.     

Ultrafast Electron Transfer Kinetics of Graphene Grown by Chemical Vapor Deposition

High electrochemical reactivity is required for various energy and sensing applications of graphene grown by chemical vapor deposition (CVD). Herein, we report that heterogeneous electron transfer can be remarkably fast at CVD‐grown graphene electrodes that are fabricated without using the conventional poly(methyl methacrylate) (PMMA) for graphene transfer from a growth substrate. We use nanogap voltammetry based on scanning electrochemical microscopy to obtain very high standard rate constants k⁰≥25 cm s^?1 for ferrocenemethanol oxidation at polystyrene‐supported graphene. The rate constants are at least 2–3 orders of magnitude higher than those at PMMA‐transferred graphene, which demonstrates an anomalously weak dependence of electron‐transfer rates on the potential. Slow kinetics at PMMA‐transferred graphene is attributed to the presence of residual PMMA. This unprecedentedly high reactivity of PMMA‐free CVD‐grown graphene electrodes is fundamentally and practically important.     

Regeneration of a Conjugated sp2 Graphene System through Selective Defunctionalization of Epoxides by Using a Proven Synthetic Chemistry Mechanism

Graphene is a promising material capable of driving technological advancement. It is, however, a challenge to obtain pristine graphene in large quantities given the limitation of current synthetic methods. Among the numerous methods available, the chemical approach provides an optimistic outlook and has garnered much interest within the graphene community as a potential alternative. One of the most crucial steps of the chemical approach is the chemical reduction of graphene oxide as this dictates the final quality of the graphene sheets. In recent years, much of the focus has shifted to the usage of established reducing agents or oxygen removal reagents, frequently applied in organic chemistry, onto a graphene oxide platform. Herein, the selective removal of epoxide groups and subsequent regeneration of disrupted conjugated sp² system is highlighted, based on the synergistic effect of indium and indium(I) chloride. The morphological, structural, and electrical properties of the resulting graphene were fully characterized with X‐ray photoelectron, Fourier transform IR, solid‐state ¹³C NMR, and Raman spectroscopy; thermogravimetric analysis; scanning electron microscopy; and conductivity measurements. The as‐prepared graphene showed a tenfold increase in conductivity against conventional graphene treated with hydrazine reducing agent and demonstrated a high dispersion stability in ethanol. Moreover, the selective defunctionalization of the epoxide groups provides opportunities for potential tailoring of graphene properties for prospective applications.     

Macroscopic single‐domain graphene sheet on Ni(111)

We observed in situ growth of a single graphene sheet on Ni(111) by low‐energy electron microscopy. The sheet was grown epitaxially beyond the steps on the substrate. The crystalline shapes of graphene islands were clearly seen; the straight edges of the island are crossed at either 60 or 120°, and the linear edges shifted perpendicular to the edge keeping the equilibrium shape. Graphene islands were united to form a single sheet without any grain boundaries and any wrinkles. The Ni substrate of several centimeters in size was covered with a single‐domain graphene sheet. Copyright © 2011 John Wiley & Sons, Ltd.     

Friedel–Crafts Acylation on Graphene

There is an excitement around graphene as a promising new material for nanotechnological and nanoarchitectonic applications. Despite the low chemical reactivity of graphene, chemical functionalization remains a prominent and viable solution to tailor its chemical, physical, and electronic properties. Herein, we report the covalent functionalization of reduced graphene oxide through Friedel–Crafts reactions under mild conditions of polyphosphoric-acid/phosphorus-pentoxide and 4-aminobenzoic acid. Successful functionalization was confirmed by X-ray photoelectron spectroscopy (XPS), FTIR, and Raman spectroscopy. The success of the Friedel–Crafts reaction provides an important expansion of the synthetic “toolbox” for future modifications of graphene towards the specific needs of different applications.     

Seamless Rim‐Functionalization of h‐BN with Silica—Experiment and Theoretical Modeling

Boron nitride contains six‐ring layers, which are isostructural to graphene, and it exhibits similar extraordinary mechanical strength. Unlike graphene, hexagonal boron nitride (h‐BN) is an insulator and has some polar features that make it a perfect material for those applications graphene is not suitable for, for example, purely ionic conductors, insulating membranes, transparent coatings, composite ceramics, high oxidation resistance materials. We report here a selective rim‐functionalization of h‐BN with SiO₂ by using the Stöber process. A closed, protruding ring of SiO₂ is formed covering all edges perpendicular to the [001] zones of the h‐BN stacks and thus shield the most reactive centers of BN layers. SEM and HAADF‐STEM images, X‐ray spectroscopy, and atomic force microscopy confirm the rim‐functionalization by SiO₂. XRD demonstrates the absence of any intercalation phenomenon of BN and reveals the glassy nature of the SiO₂ rims. Selected variations of synthesis and theoretical modeling both confirm that rim activation by water prior to the Stöber condensation is crucial. First‐principles calculations also confirm that dangling bonds of clean BN edges merge to give interlayer bonds that make further functionalization much more difficult. The reported reaction pathway should allow for other new functionalizations of pure BN and of the rimmed SiO₂/h‐BN composites.     

An efficient way to functionalize graphene sheets with presynthesized polymer via ATNRC chemistry

Graphene nanosheets offer intriguing electronic, thermal and mechanical properties and are expected to find a variety of applications in high‐performance nanocomposite materials. The great challenge of exfoliating and dispersing pristine graphite or graphene sheets in various solvents or matrices can be achieved by facilely and properly chemical functionalization of the carbon nanosheets. Here we reported an efficient way to functionalize graphene sheets with presynthesized polymer via a combination of atom transfer nitroxide radical coupling chemistry with the grafting‐onto strategy, which enable us to functionalize graphene sheets with well‐defined polymer synthesized via living radical polymerization. A radical scavenger species, 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO), was firstly anchored onto ? COOH groups on graphene oxide (GO) to afford TEMPO‐functionalized graphene sheets (GS‐TEMPO), meanwhile, the GO sheets were thermally reduced. Next, GS‐TEMPO reacted with Br‐terminated well‐defined poly(N‐isopropylacrylamide) (PNIPAM) homopolymer, which was presynthesized by SET‐LRP, in the presence of CuBr/N,N,N′,N′,N″‐pentamethyldiethylenetriamine to form PNIPAM‐graphene sheets (GS‐PNIPAM) nanocomposite in which the polymers were covalently linked onto the graphene via the alkoxyamine conjunction points. The PNIPAM‐modified graphene sheets are easily dispersible in organic solvents and water, and a temperature‐induced phase transition was founded in the water suspension of GS‐PNIPAM. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

The Influence of Dimensional Effects on the Composition and Properties of Polydicarbonfluoride

The influence of dimensional effects on the compositions and properties of polydicarbonfluoride (C₂F)_n prepared from multilayered graphenes was investigated. Multilayered graphenes were produced by destructive thermal decomposition of intercalation compounds of “idealized” (C₂F)_n that were obtained by reaction of gaseous ClF₃ with natural graphite at a room temperature. The precursors of multilayered graphenes have a common formula (C₂F?xR)_n where R is an organic or inorganic component. It was shown that polydicarbonfluoride prepared from multilayered graphene does not form stable intercalation compound with ClF₃, in contrast to polydicarbonfluoride prepared from graphite, that forms its intercalation compound with ClF₃ during fluorination of initial graphite in the ClF₃ excess. Investigations of polydicarbonfluoride prepared from multilayered graphene showed that it cannot form intercalation compounds with different classes of organic and inorganic compounds as polydicarbonfluoride prepared from graphite can do. The absence of such intercalation activity for polydicarbonfluoride prepared from multilayered graphene can be explained by high exfoliation degree of multilayered graphene (3–4 nm) along the c‐axis that results in the presence of two‐dimensional (2D) structure properties in multilayered graphene. Dimensional effects transformed the chemical properties of polydicarbonfluoride prepared from multilayered graphene and lowered its decomposition temperature by 150 K in comparison with polydicarbonfluoride prepared from graphite.     

石墨烯和铂/石墨烯的合成及其表征

以天然鳞状石墨为原料,采用化学氧化法合成氧化石墨,在此基础上采用低温热解膨胀结合微波加热乙二醇还原法合成石墨烯(Gr)以及铂/石墨烯(Pt/Gr)复合材料。SEM和TEM显示所制备的石墨烯为层状结构的半透明薄膜。采用X射线光电子能谱(XPS)和傅立叶转换红外光谱(FTIR)分别确定氧化石墨、膨胀石墨及石墨烯表面含氧官能团的数量和性质。以所制备的碳氧原子比5.94的石墨烯作为载体制备出可用于质子交换膜燃料电池的高负载量的Pt/Gr催化剂,在铂载量高达60%时,表面铂粒子依然具有高分散性,平均粒径为3.8 nm。     

Effects of Edge Functionalization of Nanographenes with Small Aromatic Systems

Regulation of the physical properties of nanographenes (NGs) by edge functionalization is an active research area. We conducted a computational study of the effects of edge functionalization on the physical properties of NGs. The computed NGs were models of experimentally obtained NGs and composed of a C₁₇₄ carbon framework with one to four 3,5-dimethylnaphthalene units on the edge. The effects were assessed structurally, magnetically, and electronically by the least square planarity index, harmonic oscillator model of aromaticity, nucleus-independent chemical shift, and HOMO–LUMO (H–L) gaps. Density functional theory calculations indicate that although the structures of the model NGs are not very sensitive to edge functionalization, but the magnetic and electronic properties are. The installed substituents narrowed the H−L gap and induced a redshift of the photoluminescence (PL) band by the π conjugation between NG and the substituent. These results are consistent with the extension of the absorption band and the redshift of the PL bands of the experimentally modified NGs. Furthermore, the calculations confirmed the contribution of the charge transfer character to the absorption spectra.     

Functionalized graphene/thermoplastic polyester elastomer nanocomposites by reactive extrusion‐based masterbatch: preparation and properties reinforcement

A reactive extrusion process was developed to fabricate polymer/graphene nanocomposites with good dispersion of graphene sheets in the polymer matrix. The functionalized graphene nanosheet (f‐GNS) activated by diphenylmethane diisocyanate was incorporated in thermoplastic polyester elastomer (TPEE) by reactive extrusion process to produce the TPEE/f‐GNS masterbatch. And then, the TPEE/f‐GNS nanocomposites in different ratios were prepared by masterbatch‐based melt blending. The structure and morphology of functionalized graphene were characterized by Fourier transform infrared, X‐ray photoelectron spectroscopy, X‐ray diffraction and transmission electron microscopy (TEM). The incorporation of f‐GNS significantly improved the mechanical, thermal and crystallization properties of TPEE. With the incorporation of only 0.1 wt% f‐GNS, the tensile strength and elongation at break of nanocomposites were increased by 47.6% and 30.8%, respectively, compared with those of pristine TPEE. Moreover, the degradation temperature for 10 wt% mass loss, storage modulus at ?70°C and crystallization peak temperature (T_cp) of TPEE nanocomposites were consistently improved by 17°C, 7.5% and 36°C. The remarkable reinforcements in mechanical and thermal properties were attributed to the homogeneous dispersion and strong interfacial adhesion of f‐GNS in the TPEE matrix. The functionalization of graphene was beneficial to the improvement of mechanical properties because of the relatively well dispersion of graphene sheets in TPEE matrix, as suggested in the TEM images. This simple and effective approach consisting of chemical functionalization of graphene, reactive extrusion and masterbatch‐based melt blending process is believed to offer possibilities for broadening the graphene applications in the field of polymer processing. Copyright © 2014 John Wiley & Sons, Ltd.