Chemistry with Graphene and Graphene Oxide—Challenges for Synthetic Chemists

The chemical production of graphene as well as its controlled wet chemical modification is a challenge for synthetic chemists. Furthermore, the characterization of reaction products requires sophisticated analytical methods. In this Review we first describe the structure of graphene and graphene oxide and then outline the most important synthetic methods that are used for the production of these carbon‐based nanomaterials. We summarize the state‐of‐the‐art for their chemical functionalization by noncovalent and covalent approaches. We put special emphasis on the differentiation of the terms graphite, graphene, graphite oxide, and graphene oxide. An improved fundamental knowledge of the structure and the chemical properties of graphene and graphene oxide is an important prerequisite for the development of practical applications.     

Nanoarchitectured Graphene‐Based Supercapacitors for Next‐Generation Energy‐Storage Applications

Tremendous development in the field of portable electronics and hybrid electric vehicles has led to urgent and increasing demand in the field of high‐energy storage devices. In recent years, many research efforts have been made for the development of more efficient energy‐storage devices such as supercapacitors, batteries, and fuel cells. In particular, supercapacitors have great potential to meet the demands of both high energy density and power density in many advanced technologies. For the last half decade, graphene has attracted intense research interest for electrical double‐layer capacitor (EDLC) applications. The unique electronic, thermal, mechanical, and chemical characteristics of graphene, along with the intrinsic benefits of a carbon material, make it a promising candidate for supercapacitor applications. This Review focuses on recent research developments in graphene‐based supercapacitors, including doped graphene, activated graphene, graphene/metal oxide composites, graphene/polymer composites, and graphene‐based asymmetric supercapacitors. The challenges and prospects of graphene‐based supercapacitors are also discussed.     

Assembly and application advancement of organic‐functionalized graphene‐based materials: A review

Owing to the remarkable physicochemical properties such as hydrophobicity, conductivity, elasticity, and light weight, graphene‐based materials have emerged as one of the most appealing carbon allotropes in materials science and chemical engineering. Unfortunately, pristine graphene materials lack functional groups for further modification, severely hindering their practical applications. To render graphene materials with special characters for different applications, graphene oxide or reduced graphene oxide has been functionalized with different organic agents and assembled together, via covalent binding and various noncovalent forces such as π–π interaction, electrostatic interaction, and hydrogen bonding. In this review, we briefly discuss the state‐of‐the‐art synthetic strategies and properties of organic‐functionalized graphene‐based materials, and then, present the prospective applications of organic‐functionalized graphene‐based materials in sample preparation.     

Effects on the aromatic character of DNA/RNA nucleobases due to its adsorption onto graphene

In the last years, experimental/theoretical studies have shown that graphene has a strong affinity toward nucleobases, serving as a promising nanomaterial for self‐assembly, sensing, and/or sequencing of DNA/RNA constituents. Then, a complete picture of the properties of the nucleobase–graphene systems is required for its use in technological applications. This work describes a detailed quantum chemical analysis of the aromaticity of adsorbed nucleobases on graphene, comparing between aromaticity indexes based on magnetic, geometry, electron density, and electron delocalization properties of graphene–nucleobase systems. Contrary to the stated by magnetic‐based aromaticity criteria (such as nucleus‐independent chemical shifts), it is proved that the aromatic character of nucleobases is not increased/decreased upon binding on graphene. Therefore, magnetic aromaticity criteria are not recommended to analyze aromaticity in related systems, unless a fragmented scheme be adopted. Finally, these results are expected to expand the knowledge about the understanding of biomolecules‐graphene interactions.     

气相助剂辅助绝缘衬底上石墨烯生长：现状与展望

借助化学气相沉积(chemical vapor deposition,CVD)技术在绝缘衬底上直接生长的石墨烯薄膜,在能源存储/转换等领域有着广阔的应用前景。然而,绝缘衬底表面石墨烯的生长呈现成核密度高、畴区尺寸小、生长速率低等特点,获得的石墨烯薄膜往往具有较高的晶界密度和较低的层数均匀度,严重制约着石墨烯基器件性能的发挥。在反应体系中引入气相助剂可有效降低碳源裂解和石墨烯生长的能垒,从而实现石墨烯品质与生长速率的提升。本文综述气相助剂辅助绝缘衬底上石墨烯制备的方法：首先对绝缘衬底上石墨烯的生长行为进行分析;随后着重介绍几类常见的气相助剂辅助石墨烯生长的策略和机理;最后,总结绝缘衬底上制备高品质石墨烯存在的挑战,并对未来的发展方向进行展望。     

Aligned Nanotubes

Since the discovery of carbon nanotubes by Iijima in 1991, various carbon nanotubes with either a single‐ or multilayered graphene cylinder(s) have been produced, along with their noncarbon counterparts (for example, inorganic and polymer nanotubes). These nanostructured materials often possess size‐dependent properties and show new phenomena related to the nanosize confinement of the charge carriers inside, which leads to the possibility of developing new materials with useful properties and advanced devices with desirable features for a wide range of applications. In particular, carbon nanotubes have been shown to exhibit superior properties attractive for various potential applications, ranging from their use as novel electron emitters in flat‐panel displays to electrodes in electrochemical sensors. For many of the applications, it is highly desirable to have aligned/patterned forms of carbon nanotubes so that their structure/property can be easily assessed and so that they can be effectively incorporated into devices. In this Review, we present an overview on the development of aligned and micropatterned nanotubes, with an emphasis on carbon nanotubes.     

Harnessing the Liquid‐Phase Exfoliation of Graphene Using Aliphatic Compounds: A Supramolecular Approach

The technological exploitation of the extraordinary properties of graphene relies on the ability to achieve full control over the production of a high‐quality material and its processing by up‐scalable approaches in order to fabricate large‐area films with single‐layer or a few atomic‐layer thickness, which might be integrated in working devices. A simple method is reported for producing homogenous dispersions of unfunctionalized and non‐oxidized graphene nanosheets in N‐methyl‐2‐pyrrolidone (NMP) by using simple molecular modules, which act as dispersion‐stabilizing compounds during the liquid‐phase exfoliation (LPE) process, leading to an increase in the concentration of graphene in dispersions. The LPE‐processed graphene dispersion was shown to be a conductive ink. This approach opens up new avenues for the technological applications of this graphene ink as low‐cost electrodes and conducting nanocomposite for electronics.     

Graphene‐polymer nanocomposites for biomedical applications

Despite the significant efforts in the synthesis of new polymers, the mechanical properties of polymer matrices can be considered modest in most cases, which limits their application in demanding areas. The isolation of graphene and evaluation of its outstanding properties, such as high thermal conductivity, superior mechanical properties, and high electronic transport, have attracted academic and industrial interest, and opened good perspectives for the integration of graphene as a filler in polymer matrices to form advanced multifunctional composites. Graphene‐based nanomaterials have prompted the development of flexible nanocomposites for emerging applications that require superior mechanical, thermal, electrical, optical, and chemical performance. These multifunctional nanocomposites may be tailored to synergistically combine the characteristics of both components if proper structural and interfacial organization is achieved. The investigations carried out in this aim have combined graphene with different polymers, leading to a variety of graphene‐based nanocomposites. The extensive research on graphene and its functionalization, as well as polymer graphene composites, aiming at applications in the biomedical field, are reviewed in this paper. An overview of the polymer matrices adequate for the biomedical area and the production techniques of graphene composites is presented. Finally, the applications of such nanocomposites in the biomedical field, particularly in drug delivery, wound healing, and biosensing, are discussed.     

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.     

Rational Design of Carboxyl Groups Perpendicularly Attached to a Graphene Sheet: A Platform for Enhanced Biosensing Applications

Graphene oxide (GO)‐based materials offer great potential for biofunctionalization with applications ranging from biosensing to drug delivery. Such biofunctionalization utilizes specific functional groups, typically a carboxyl moiety, as anchoring points for biomolecule. However, due to the fact that the exact chemical structure of GO is still largely unknown and poorly defined (it was postulated to consist of various oxygen‐containing groups, such as epoxy, hydroxyl, carboxyl, carbonyl, and peroxy in varying ratios), it is challenging to fabricate highly biofunctionalized GO surfaces. The predominant anchoring sites (i.e., carboxyl groups) are mainly present as terminal groups on the edges of GO sheets and thus account for only a fraction of the oxygen‐containing groups on GO. Herein, we suggest a direct solution to the long‐standing problem of limited abundance of carboxyl groups on GO; GO was first reduced to graphene and consequently modified with only carboxyl groups grafted perpendicularly to its surface by a rational synthesis using free‐radical addition of isobutyronitrile with subsequent hydrolysis. Such grafted graphene oxide can contain a high amount of carboxyl groups for consequent biofunctionalization, at which the extent of grafting is limited only by the number of carbon atoms in the graphene plane; in contrast, the abundance of carboxyl groups on “classical” GO is limited by the amount of terminal carbon atoms. Such a graphene platform embedded with perpendicularly grafted carboxyl groups was characterized in detail by X‐ray photoelectron spectroscopy, cyclic voltammetry, and electrochemical impedance spectroscopy, and its application was exemplified with single‐nucleotide polymorphism detection. It was found that the removal of oxygen functionalities after the chemical reduction enhanced the electron‐transfer rate of the graphene. More importantly, the introduction of carboxyl groups promoted a more efficient immobilization of DNA probes on the electrode surface and improved the performance of graphene as a biosensor in comparison to GO. The proposed material can be used as a universal platform for biomolecule immobilization to facilitate rapid and sensitive detection of DNA or proteins for point‐of‐care investigations. Such reactive carboxyl groups grafted perpendicularly on GO holds promise for a highly efficient tailored biofunctionalization for applications in biosensing or drug delivery.     

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.     

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.     

Chemistry and physics of a single atomic layer: strategies and challenges for functionalization of graphene and graphene-based materials

Graphene has attracted great interest for its superior physical, chemical, mechanical, and electrical properties that enable a wide range of applications from electronics to nanoelectromechanical systems. Functionalization is among the significant vectors that drive graphene towards technological applications. While the physical properties of graphene have been at the center of attention, we still lack the knowledge framework for targeted graphene functionalization. In this critical review, we describe some of the important chemical and physical processes for graphene functionalization. We also identify six major challenges in graphene research and give perspectives and practical strategies for both fundamental studies and applications of graphene (315 references).     

A Review on Graphene Oxide Two-dimensional Macromolecules: from Single Molecules to Macro-assembly

Graphene oxide(GO), which consists of two-dimensional(2 D) sp2 carbon hexagonal networks and oxygen-contained functional groups, has laid the foundation of mass production and applications of graphene materials. Made by chemical oxidation of graphite, GO is highly dispersible or even solubilized in water and polar organic solvents, which resolves the hard problem of graphene processing and opens a door to wet-processing of graphene. Despite its defects, GO is easy to functionalize, dope, punch h...     

Characteristic Spectral Patterns in the Carbon‐13 Nuclear Magnetic Resonance Spectra of Hexagonal and Crenellated Graphene Fragments

Nuclear magnetic resonance (NMR) spectroscopy is an important molecular characterisation method that may aid the synthesis and production of graphenes, especially the molecular‐scale graphene nanoislands that have gathered significant attention due to their potential electronic and optical applications. Herein, carbon‐13 NMR chemical shifts were calculated using density functional theory methods for finite, increasing‐size fragments of graphene, hydrogenated graphene (graphane) and fluorinated graphene (fluorographene). Both concentric hexagon‐shaped (zigzag boundary) and crenellated (armchair) fragments were investigated to gain information on the effect of different types of flake boundaries. Convergence trends of the ¹³C chemical shift with respect to increasing fragment size and the boundary effects were found and rationalised in terms of low‐lying electronically excited states. The results predict characteristic behaviour in the ¹³C NMR spectra. Particular attention was paid to the features of the signals arising from the central carbon atoms of the fragments, for graphene and crenellated graphene on the one hand and graphane and fluorographene on the other hand, to aid the interpretation of the overall spectral characteristics. In graphene, the central nuclei become more shielded as the system size increases whereas the opposite behaviour is observed for graphane and fluorographene. The ¹³C signals from some of the perimeter nuclei of the crenellated fragments obtain smaller and larger chemical shift values than central nuclei for graphene and graphane/fluorographene, respectively. The diameter of the graphenic quantum dots with zigzag boundary correlates well with the predicted carbon‐13 chemical shift range, thus enabling estimation of the size of the system by NMR spectroscopy. The results provide data of predictive quality for future NMR analysis of the graphene nanoflake materials.     

Strongly Veined Carbon Nanoleaves as a Highly Efficient Metal‐Free Electrocatalyst

Effective integration of one‐dimensional carbon nanofibers (CNF) and two‐dimensional carbon sheets into three‐dimensional (3D) conductive frameworks is essential for their practical applications as electrode materials. Herein, a novel “vein‐leaf”‐type 3D complex of carbon nanofibers with nitrogen‐doped graphene (NG) was prepared through a simple thermal condensation of urea and bacterial cellulose. During the formation of the 3D complex CNF@NG, the graphene species was tethered to CNF via carbon–carbon bonds. Such an interconnected 3D network facilitates both the electron transfer and mass diffusion for electrochemical reactions.     

Fabrication of Boron Nitride Nanosheets by Exfoliation

Nanomaterials with layered structures, with their intriguing properties, are of great research interest nowadays. As one of the primary two‐dimensional nanomaterials, the hexagonal boron nitride nanosheet (BNNS, also called white graphene), which is an analogue of graphene, possesses various attractive properties, such as high intrinsic thermal conductivity, excellent chemical and thermal stability, and electrical insulation properties. After being discovered, it has been one of the most intensively studied two‐dimensional non‐carbon nanomaterials and has been applied in a wide range of applications. To support the exploration of applications of BNNSs, exfoliation, as one of the most promising approaches to realize large‐scale production of BNNSs, has been intensively investigated. In this review, methods to yield BNNSs by exfoliation will be summarized and compared with other potential fabrication methods of BNNSs. In addition, the future prospects of the exfoliation of h‐BN will also be discussed.     

Synthesis of polydopamine‐coated graphene–polymer nanocomposites via RAFT polymerization

Graphene‐polymer nanocomposites have significant potential in many applications such as photovoltaic devices, fuel cells, and sensors. Functionalization of graphene is an essential step in the synthesis of uniformly distributed graphene‐polymer nanocomposites, but often results in structural defects in the graphitic sp² carbon framework. To address this issue, we synthesized graphene oxide (GO) by oxidative exfoliation of graphite and then reduced it into graphene via self‐polymerization of dopamine (DA). The simultaneous reduction of GO into graphene, and polymerization and coating of polydopamine (PDA) on the reduced graphene oxide (RGO) surface were confirmed with XRD, UV–Vis, XPS, Raman, TGA, and FTIR. The degree of reduction of GO increased with increasing DA/GO ratio from 1/4 to 4/1 and/or with increasing temperature from room temperature to 60 °C. A RAFT agent, 2‐(dodecylthiocarbonothioylthio)?2‐methylpropionic acid, was linked onto the surface of the PDA/RGO, with a higher equivalence of RAFT agent in the reaction leading to a higher concentration of RAFT sites on the surface. Graphene‐poly(methyl methacrylate), graphene‐poly(tert‐butyl acrylate), and graphene‐poly(N‐isopropylacrylamide) nanocomposites were synthesized via RAFT polymerization, showing their characteristic solubility in several different solvents. This novel synthetic route was found facile and can be readily used for the rational design of graphene‐polymer nanocomposites, promoting their applications. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3941–3949     

Noble‐Metal‐Free Materials for Surface‐Enhanced Raman Spectroscopy Detection

Surface‐enhanced Raman spectroscopy (SERS) is an attractive tool for the sensing of molecules in the fields of chemical and biochemical analysis as it enables the sensitive detection of molecular fingerprint information even at the single‐molecule level. In addition to traditional coinage metals in SERS analysis, recent research on noble‐metal‐free materials has also yielded highly sensitive SERS activity. This Minireview presents the recent development of noble‐metal‐free materials as SERS substrates and their potential applications, especially semiconductors and emerging graphene‐based nanostructures. Rather than providing an exhaustive review of this field, possible contributions from semiconductor substrates, characteristics of graphene enhanced Raman scattering, as well as effect factors such as surface plasmon resonance, structure and defects of the nanostructures that are considered essential for SERS activity are emphasized. The intention is to illustrate, through these examples, that the promise of noble‐metal‐free materials for enhancing detection sensitivity can further fuel the development of SERS‐related applications.     

Enhanced Oxygen Reduction Reactions in Fuel Cells on H‐Decorated and B‐Substituted Graphene

In the light of recent experimental research on the oxygen reduction reaction (ORR) with carbon materials doped with foreign atoms, we study the performance of graphene with different defects on this catalytic reaction. In addition to the reported N‐graphene, it is found that H‐decorated and B‐substituted graphene can also spontaneously promote this chemical reaction. The local high spin density plays the key role, facilitating the adsorption of oxygen and OOH, which is the start of ORR. The source of the high spin density for all of the doped graphene is attributed to unpaired single π electrons. Meanwhile, the newly formed C? H covalent bond introduces a higher barrier to the p electron flow, leading to more localized and higher spin density for H‐decorated graphene. At the same time, larger structural distortion should be avoided, which could impair the induced spin density, such as for P‐substituted graphene.