Individual Graphene Sheets Research Articles

Defect-Driven Heterogeneous Electron Transfer between an Individual Graphene Sheet and Electrode.

Understanding the heterogeneous electron-transfer (ET) kinetics on graphene is essential for its extensive applications. Here, on the basis of the redox-induced fluorescence variation of monolayer graphene itself, the heterogeneous ET kinetics at the interface between the electrode and the monolayer graphene was studied label-freely at the single-sheet level. By tuning the defect density on graphene, an optimal heterogeneous ET rate was observed at a moderate defect density, indicating defect-driven ET kinetics. The heterogeneities of both the intrasheet and intersheet ET kinetics were revealed at the single-sheet level. With the optimal defective graphene sheets as a sensing material for oxygen gas, a cost-effective electrochemical oxygen sensor was obtained with high sensitivity, fast response/recovery, and remarkable durability. The results obtained here deepen our understanding of the electrochemical properties of graphene and imply that rational defect control can enhance the ET process between the electrode and graphene and then improve the performance of graphene-based functional materials or devices.

Enhancing the interfacial thermal conduction of the graphene sheets via chemical bond–bond connections

Graphene, after its discovery in 2004, was known to possess extremely high thermal conductance. In the practical applications, however, the thermal conductance was unfortunately low compared with its theoretical value since the heat transfer among individual graphene sheets is largely hindered by the boundary phonon dissipation. In this first-principles study, we propose a new strategy to enhance the interfacial thermal transport, that is, chemical bond–bond connections. Organic, metal and metal-oxide groups are adopted to link two graphene nanoribbons, acting as a thermal bridge. In such models, the thermal conductance is significantly enhanced, as compared with the non-linked counterparts in which the van der Waals interactions dominate. In numbers, the highest thermal conductance of 0.577 GW·m–2·K–1 at 300K (which preserves as much as 13.2% compared with the pristine nanoribbon) can be obtained when linked by –Al– groups. The enhancement mechanism as regards different bond–bond connections is also discussed. Our study paves the way to exploring the enhancement of interfacial heat transfer. Even though the work was done based on the graphene models, it can be generally extended to a variety of inorganic and organic families.

Super-elasticity of three-dimensionally cross-linked graphene materials all the way to deep cryogenic temperatures.

Until now, materials with high elasticity at deep cryogenic temperatures have not been observed. Previous reports indicated that graphene and carbon nanotube-based porous materials can exhibit reversible mechano-elastic behavior from liquid nitrogen temperature up to nearly a thousand degrees Celsius. Here, we report wide temperature-invariant large-strain super-elastic behavior in three-dimensionally cross-linked graphene materials that persists even to a liquid helium temperature of 4 K, a property not previously observed for any other material. To understand the mechanical properties of these graphene materials, we show by in situ experiments and modeling results that these remarkable properties are the synergetic results of the unique architecture and intrinsic elastic/flexibility properties of individual graphene sheets and the covalent junctions between the sheets that persist even at harsh temperatures. These results suggest possible applications for such materials at extremely low temperature environments such as those in outer space.

Cylindrical Bending Vibration of Multiple Graphene Sheet Systems Embedded in an Elastic Medium

Based on the Eringen nonlocal elasticity theory and multiple time scale method, an asymptotic nonlocal elasticity theory is developed for cylindrical bending vibration analysis of simply-supported, [Formula: see text]-layered, and uniformly or nonuniformly-spaced, graphene sheet (GS) systems embedded in an elastic medium. Both the interactions between the top and bottom GSs and their surrounding medium and the interactions between each pair of adjacent GSs are modeled as one-parameter Winkler models with different stiffness coefficients. In the formulation, the small length scale effect is introduced to the nonlocal constitutive equations by using a nonlocal parameter. The nondimensionalization, asymptotic expansion, and successive integration mathematical processes are performed for a typical GS. After assembling the motion equations for each individual GS to form those of the multiple GS system, recurrent sets of motion equations can be obtained for various order problems. Nonlocal multiple classical plate theory (CPT) is derived as a first-order approximation of the current nonlocal plane strain problem, and the motion equations for higher-order problems retain the same differential operators as those of nonlocal multiple CPT, although with different nonhomogeneous terms. Some nonlocal plane strain solutions for the natural frequency parameters of the multiple GS system with and without being embedded in the elastic medium and their corresponding mode shapes are presented to demonstrate the performance of the asymptotic nonlocal elasticity theory.

Fabrication of GNS/MoS2 composite with different morphology and its tribological performance as a lubricant additive

MoS2 was anchored on the surface of graphene sheets (GNS) with hydrothermal process and chemical vapor deposition to obtain GNS/MoS2 nanoflowers (GNS/MoS2-NFs) and GNS/MoS2 nanoplates (GNS/MoS2 NPs). The GNS/MoS2 composite prepared with different methods was observed with XRD, Raman, SEM, TEM and XPS respectively, and the characterization results confirmed the different morphology of GNS/MoS2-NFs and GNS/MoS2 NPs. For GNS/MoS2-NFs, the MoS2 nanoflowers were dotted on the graphene surface, while the MoS2 nanoplates were uniformly attached on the graphene surface in GNS/MoS2 NPs. The tribological properties of (dibutyl phthalate) DBP containing GNS/MoS2-NFs or GNS/MoS2 NPs was compared with four-ball tribotester at 1200 rpm under 392 N for 30 min. The GNS/MoS2-NFs additive presented better performance than GNS/MoS2 NPs additive even at different concentration. For example, the friction coefficient and wear scar diameter of base oil were reduced by 42.8% and 16.9% with the introduction of 0.02 wt% GNS/MoS2-NFs, while that can only be reduced by 37.6% and 11.9% with the addition of 0.02 wt% GNS/MoS2-NPs. The two additives after friction was further analyzed to explore the lubrication mechanism, and a possible mechanism of GNS/MoS2-NFs was tentatively proposed. In addition, the friction-reducing and anti-wear performance of GNS/MoS2-NFs dispersed DBP can be improved more effectively comparing to that of individual GNS, MoS2 nanoflowers, and the mixture of them. The lubrication mechanism of different additives was investigated with the observation of wear scar, and it indicated that the synergistic effect between GNS and MoS2 nanoflowers played an important role during the friction due to its microstructure.

High-yield scalable graphene nanosheet production from compressed graphite using electrochemical exfoliation

Electrochemical exfoliation is a promising bulk method for producing graphene from graphite; in this method, an applied voltage drives ionic species to intercalate into graphite where they form gaseous species that expand and exfoliate individual graphene sheets. However, a number of obstacles have prevented this approach from becoming a feasible production route; the disintegration of the graphite electrode as the method progresses is the chief difficulty. Here we show that if graphite powders are contained and compressed within a permeable and expandable containment system, the graphite powders can be continuously intercalated, expanded, and exfoliated to produce graphene. Our data indicate both high yield (65%) and extraordinarily large lateral size (>30 μm) in the as-produced graphene. We also show that this process is scalable and that graphene yield efficiency depends solely on reactor geometry, graphite compression, and electrolyte transport.

Experimental Investigation on 3D Graphene-CNT Hybrid Foams with Different Interactions.

Due to the exceptional properties of graphene, numerous possibilities for real applications in various fields have been provided. However, it is a challenge to fabricate bulk graphene materials with properties arising from the nature of individual graphene sheets, and which assemble into monolithic three-dimensional structures. If 3D structured graphene foam were made instead of 2D structured graphene, it is expected that it would be a facile fabrication, with relatively low cost with the possibility of scale-up, and would maintain the intrinsic properties of graphene. To solve the weaknesses of 2D structured graphene, this study aimed to fabricate a 3D graphene-carbon nanotubes (CNT) hybrid foam. In this study, CNT was used to reinforce the graphene foams. In addition, two different surfactants, known as sodium dodecylbenzene sulphonate (SDBS) and cetyltrimethylammonium bromide (CTAB), were applied to help CNT dispersion. The π–π interaction was induced by SDBS/CNT, while ionic interaction was derived from CTAB/CNT. To confirm the charge effect with different surfactants, SEM, Zeta-potential, FT-IR, Raman spectroscopy, and compression tests were performed. When using a cationic surfactant, CTAB, compressive modulus, and strength increased due to the formation of relatively strong ionic bonding.

Modeling the postbuckling behavior of thermal-resistant ultrathin films attached to glass substrate

Abstract This paper investigates the postbuckling behavior of a novel glass-protection film system under thermal environment using the element-free kp-Ritz method. The governing formulation is derived based on the first-order shear deformation plate theory considering small strains and the nonlocal elasticity theory which takes small scale effect into account. The modified Newton-Raphson method incorporated with the arc-length continuation technique is used to trace the nonlinear response of the film-foundation system. The influences of boundary conditions, nonlocal parameters, geometry and elastic foundation on the nonlinear response of glass-protection film are examined. The results show that adhesion force between graphene sheets (GSs) film and the elastic substrate can significantly enhance thermal buckling resistance of GSs film. It is also concluded that van der Waals forces between GSs film lead to consistent post-buckling behavior of individual GSs. Present study can provide suggestions on firefighting design, such as setting alarm temperature based on transition temperature.

Monolithic 3D Cross-Linked Polymeric Graphene Materials and the Likes: Preparation and Their Redox Catalytic Applications

While there have been tremendous studies about graphene and its applications in the past decade, so far the proposed huge potential of this material has not been materialized. One of the prerequisites to overcome these challenges is maintaining the nature and intrinsic properties of individual graphene sheets while in the state of bulk material. Thus, in this Perspective contribution, the fabrication/synthesis of the monolithic polymeric and three-dimensional (3D) cross-linked bulk materials (3DGraphene) with (doped) 2D graphene sheets as the building block will first be briefly summarized. Then, the second part will cover the redox catalytic application of these bulk materials including doped 3DGraphene, the graphene-like material polymeric C3N4 and their hybrid materials. These will include mainly oxygen reduction reactions (ORR), hydrogen evolution reactions (HER), and CO2 reduction for their latest development. Finally, challenges and outlook related to the design of 3D cross-linked graphene based materials and their catalytic applications will be briefly discussed.

Conductivity Maximum in 3D Graphene Foams.

In conventional foams, electrical properties often play a secondary role. However, this scenario becomes different for 3D graphene foams (GrFs). In fact, one of the motivations for synthesizing 3D GrFs is to inherit the remarkable electrical properties of individual graphene sheets. Despite immense experimental efforts to study and improve the electrical properties of 3D GrFs, lack of theoretical studies and understanding limits further progress. The causes to this embarrassing situation are identified as the multiple freedoms introduced by graphene sheets and multiscale nature of this problem. In this article, combined with transport modeling and coarse-grained molecular dynamic (MD) simulations, a theoretical framework is established to systematically study the electrical conducting properties of 3D GrFs with or without deformation. In particular, through large-scale and massive calculations, a general relation between contact area and conductance for two van der Waals bonded graphene sheets is demonstrated, in terms of which the conductivity maximum phenomenon in GrFs is first theoretically proposed and its competition mechanism is explained. Moreover, the theoretical prediction is consistent with previous experimental observations.

Structural Control of Graphene-Based Materials for Unprecedented Performance.

Recent research on engineering the structure of graphene-based materials has enabled a range of unprecedented properties. In this Perspective, we discuss how these rationally designed graphene-based materials possess exciting features as a result of engineering their structures, including structures of individual graphene sheets, interaction/spacing between sheets, and assembled structures. We also consider the challenges and future opportunities in the fundamental research and practical uses of designed graphene-based materials.

Engineering Graphenes from the Nano- to the Macroscale for Electrochemical Energy Storage

Carbon is a key component in current electrochemical energy storage (EES) devices and plays a crucial role in the improvement in energy and power densities for the future EES devices. As the simplest carbon and the basic unit of all sp2 carbons, graphene is widely used in EES devices because of its fascinating and outstanding physicochemical properties; however, when assembled in the macroscale, graphene-derived materials do not demonstrate their excellence as individual sheets mostly because of unavoidable stacking. This review proposal shows to engineer graphene nanosheets from the nano- to the macroscale in a well-designed and controllable way and discusses how the performance of the graphene-derived carbons depends on the individual graphene sheets, nanostructures, and macrotextures. Graphene-derived carbons in EES applications are comprehensively reviewed with three representative devices, supercapacitors, lithium-ion batteries, and lithium–sulfur batteries. The review concludes with a comment on the opportunities and challenges for graphene-derived carbons in the rapidly growing EES research area.

Holey Graphene for Energy Storage

Graphene, owing to its large specific surface area (SSA) and excellent electronic conductivity has been a focus of research as an electrode material in electrochemical capacitors (EC). However, van der Wall’s attraction between the individual graphene sheets results in stacking of sheets while fabricating electrodes. This phenomenon decreases the SSA and restricts electrolyte diffusion pathways thereby deteriorating the performance of EC. Another challenge is that bulk manufacturing of graphene with a low cost process does not exist. These challenges can be resolved by using reduced graphene oxide (RGO) which is structurally similar to graphene with few oxygen functional groups and defects. Solution based process to synthesize RGO are indeed facile. Furthermore, previous studies indicate that modified RGO morphologies can be used as potential electrode material for electrochemical capacitors (EC). By creating nanopores within the RGO sheets, the specific surface area of electrode and diffusion of electroactive species can be improved. Here, we present a simple solution based process to manufacture holey graphene from graphene oxide (GO) via microwave (MW) radiations. GO aqueous dispersion prepared by modified Hummer’s method can be converted to RGO via thermal, chemical or photochemical methods. Then, RGO in presence of hydrogen peroxide is irradiated with microwaves. The application of MW radiation generates OH free radicals, which etch the carbon atoms present at the active defective sites on RGO. This creates a carbon vacancy on RGO sheet, consequently extending to form the nanopores. This holey graphene is characterized using TEM, XPS, BET, Raman & FTIR. Next, holey graphene is utilized to build an electrochemical capacitor and tested to show improved diffusion of electrolyte and higher energy density. The fundamental knowledge gained from this study can also be applied in the fields of batteries, hydrogen storage and catalysis.

Plasma-electric field controlled growth of oriented graphene for energy storage applications

It is well known that graphene grows as flat sheets aligned with the growth substrate. Oriented graphene structures typically normal to the substrate have recently attracted major attention. Most often, the normal orientation is achieved in a plasma-assisted growth and is believed to be due to the plasma-induced in-built electric field, which is usually oriented normal to the substrate. This work focuses on the effect of an in-built electric field on the growth direction, morphology, interconnectedness, structural properties and also the supercapacitor performance of various configurations of graphene structures and reveals the unique dependence of these features on the electric field orientation. It is shown that tilting of growth substrates from parallel to the normal direction with respect to the direction of in-built plasma electric field leads to the morphological transitions from horizontal graphene layers, to oriented individual graphene sheets and then interconnected 3D networks of oriented graphene sheets. The revealed transition of the growth orientation leads to a change in structural properties, wetting nature, types of defect in graphitic structures and also affects their charge storage capacity when used as supercapacitor electrodes. This simple and versatile approach opens new opportunities for the production of potentially large batches of differently oriented and structured graphene sheets in one production run.

Single crystalline electronic structure and growth mechanism of aligned square graphene sheets

Recently, commercially available copper foil has become an efficient and inexpensive catalytic substrate for scalable growth of large-area graphene films for fundamental research and applications. Interestingly, despite its hexagonal honeycomb lattice, graphene can be grown into large aligned square-shaped sheets on copper foils. Here, by applying angle-resolved photoemission spectroscopy with submicron spatial resolution (micro-ARPES) to study the three-dimensional electronic structures of square graphene sheets grown on copper foils, we verified the high quality of individual square graphene sheets as well as their merged regions (with aligned orientation). Furthermore, by simultaneously measuring the graphene sheets and their substrate copper foil, we not only established the (001) copper surface structure but also discovered that the square graphene sheets’ sides align with the ⟨110⟩ copper direction, suggesting an important role of copper substrate in the growth of square graphene sheets—which will help the development of effective methods to synthesize high-quality large-size regularly shaped graphene sheets for future applications. This work also demonstrates the effectiveness of micro-ARPES in exploring low-dimensional materials down to atomic thickness and sub-micron lateral size (e.g., besides graphene, it can also be applied to transition metal dichalcogenides and various van der Waals heterostructures)

Graphene-Graphene Interactions: Friction, Superlubricity, and Exfoliation.

Graphite's lubricating properties due to the "weak" interactions between individual layers have long been known. However, these interactions are not weak enough to allow graphite to readily exfoliate into graphene on a large scale. Separating graphite layers down to a single sheet is an intense area of research as scientists attempt to utilize graphene's superlative properties. The exfoliation and processing of layered materials is governed by the friction between layers. Friction on the macroscale can be intuitively understood, but there is little understanding of the mechanisms involved in nanolayered materials. Using molecular dynamics and a new forcefield, graphene's unusual behavior in a superlubric state is examined, and the energy dissipated between two such surfaces sliding past each other is shown. The dependence of friction on temperature and surface roughness is described, and agreement with experiment is reported. The accuracy of the simulated behavior enables the processes that drive exfoliation of graphite into individual graphene sheets to be described. Taking into account the friction between layers, a peeling mechanism of exfoliation is predicted to be of lower energy cost than shearing.

Three-Dimensionally Hierarchical Graphene Based Aerogel Encapsulated Sulfur as Cathode for Lithium/Sulfur Batteries.

A simple and effective method was developed to obtain the electrode for lithium/sulfur (Li/S) batteries with high specific capacity and cycling durability via adopting an interconnected sulfur/activated carbon/graphene (reduced graphene oxide) aerogel (S/AC/GA) cathode architecture. The AC/GA composite with a well-defined interconnected conductive network was prepared by a reduction-induced self-assembly process, which allows for obtaining compact and porous structures. During this process, reduced graphene oxide (RGO) was formed, and due to the presence of oxygen-containing functional groups on its surface, it not only improves the electronic conductivity of the cathode but also effectively inhibits the polysulfides dissolution and shuttle. The introduced activated carbon allowed for lateral and vertical connection between individual graphene sheets, completing the formation of a stable three-dimensionally (3D) interconnected graphene framework. Moreover, a high specific surface area and 3D interconnected porous structure efficiently hosts a higher amount of active sulfur material, about 65 wt %. The designed S/AC/GA composite electrodes deliver an initial capacity of 1159 mAh g−1 at 0.1 C and can retain a capacity of 765 mAh g−1 after 100 cycles in potential range from 1 V to 3 V.

Transverse impact analysis of double-layered graphene sheets on an elastic foundation

In using graphene sheets (GSs) to design nanoscale devices or nanostructure elements, it is very important to understand the properties of these elements against various mechanical loading conditions. The aim of this study is to investigate the impact response of double layer graphene sheets (DLGSs) on elastic foundations when they are subjected to low-velocity impact by nanoparticles. The analytical model is based on general continuum mechanics theory, which considers that a layer stack of two individual GSs is bound together by van der Waals (vdW) forces, and the interaction of DLGSs embedded to elastic foundations is modeled based on the Winkler model. The result shows that elastic foundation materials have a significant effect on the impact characteristics of GSs. The impact response of GSs subjected to nanomass has exceedingly short times with picoseconds order. This investigation may be helpful in engineering design of using GSs as nanostructure elements for nanoelectromechanical system (NEMS).

Sugar blowing-assisted reduction and interconnection of graphene oxide into three-dimensional porous graphene

Assembly of individual graphene sheets into macroscopic three-dimensional (3D) graphene is very important for the applications of graphene. In this paper, sugar blowing-assisted thermal reduction and interconnection of graphene oxide is developed to synthesize three-dimensional porous graphene (3D-PG). In the process, NH4Cl added in graphene oxide solutions acts as a blowing agent, while glucose added in the solutions can enhance the bubble burst and the cross link of graphene sheets. We also describe the use of the 3D-PG films as electrodes of supercapacitor. It was demonstrated that the 3D-PG films display a high capacitance about 115 F/g at a scan rate of 10 mV/s. More importantly, because the addition of glucose could strengthen the interconnectivities among graphene sheets, further accelerating the charge transport, the 3D-PG films show better power performance than the self-assembled graphene hydrogel (SGH). Our results demonstrate that sugar blowing-assisted thermal reduction of graphene oxide is a facile and effective route for the preparation of 3D-PG with graphene sheets tightly glued.

Graphene-modified Monolithic Capillary Column for the Microextraction of Trace Benzodiazepines in Biological Samples

ABSTRACTA graphene monolithic column was fabricated in a capillary using π-electron-rich poly(N-vinylcarbazole-divinylbenzene) as the supporter through in situ one-step polymerization for the enrichment of trace benzodiazepines in biological samples. This new three-dimensional monolith showed uniformity and a continuous column bed; more importantly, it retained the unique properties of graphene that are typically associated with individual graphene sheets. Based on the large delocalized π-electron system, graphene forms π–π stacking interactions with benzodiazepines and benzene rings of poly(N-vinylcarbazole-divinylbenzene), which not only enhance the extraction performance for benzodiazepines compared to the neat polymer but also provide chemical stability of the graphene monolith. Moreover, several factors likely to affect the extraction, including ionic strength, sample pH, sample volume, and eluant volume were studied in detail. The optimized method gave a linear range of 0.005–1 ng mL−1, and detection limits of 1.12–2.35 ng L−1. Finally, the graphene monolith was successfully applied to the separation and enrichment of benzodiazepines from urine and hair samples coupled with high-performance liquid chromatography–mass spectrometry. The recoveries were in the range of 78.6–85.6% for urine and 87.2–94.3% for hair with relative standard deviations of 3.4–6.9 and 2.9–8.3%, respectively.