Chemical methods for the production of graphenes

Large, solution-processable graphene quantum dots as light absorbers for photovoltaics

Graphenes have very attractive properties for photovoltaics. Their tunable bandgap and large optical absorptivity are desirable for efficient light harvesting. Their electronic levels and interfacing with other materials for charge transfer processes can both be tuned with well-developed carbon chemistry. Graphenes have also been shown to have very large charge mobilities, which could be useful for charge collection in solar cells. In addition, they consist of elements abundant on Earth and are environmentally friendly. However, these important properties have not been taken advantage of because graphenes that are large enough to be useful for photovoltaics have extremely poor solubility and have a strong tendency to aggregate into graphite. Here we present a novel solubilization strategy for large graphene nanostructures. It has enabled us to synthesize solution-processable, black graphene quantum dots with uniform size through solution chemistry, and we show that they can be used as sensitizers for solar cells.

Nitrene addition to exfoliated graphene: a one-step route to highly functionalized graphene

We demonstrate a high yield method of functionalizing graphene nanosheets through nitrene addition of azido-phenylalanine [Phe(N(3))] to exfoliated micro-crystalline graphite (microG). This method provides a direct route to highly functionalized graphene sheets. TEM analysis of the product shows few layer (n < 5) graphene sheets. The product was determined to have 1 phenylalanine substituent per 13 carbons.

Graphene oxide: surface activity and two-dimensional assembly

Graphene oxide (GO) is a promising precursor for preparing graphene-based composites and electronics applications. Like graphene, GO is essentially one-atom thick but can be as wide as tens of micrometers, resulting in a unique type of material building block, characterized by two very different length scales. Due to this highly anisotropic structure, the collective material properties are highly dependent on how these sheets are assembled. Therefore, understanding and controlling the assembly behavior of GO has become an important subject of research. In this Research News article the surface activity of GO and how it can be employed to create two-dimensional assemblies over large areas is discussed.

Graphene oxide as a matrix for enzyme immobilization

Graphene oxide (GO), having a large specific surface area and abundant functional groups, provides an ideal substrate for study enzyme immobilization. We demonstrated that the enzyme immobilization on the GO sheets could take place readily without using any cross-linking reagents and additional surface modification. The atomically flat surface enabled us to observe the immobilized enzyme in the native state directly using atomic force microscopy (AFM). Combining the AFM imaging results of the immobilized enzyme molecules and their catalytic activity, we illustrated that the conformation of the immobilized enzyme is mainly determined by interactions of enzyme molecules with the functional groups of GO.

Atomic-scale observation of rotational misorientation in suspended few-layer graphene sheets

Single or few-layer graphene (FLG) sheets offer extraordinary electronic, thermal and mechanical properties and are expected to find a variety of applications. Fully exploiting the properties of graphene will require a method for the production of high-quality graphene sheets (almost pristine graphene) in large quantities. In this regard, we report a two-step method for obtaining a homogenous colloidal suspension of single or FLG sheets up to 0.15 mg ml(-1) in N,N-dimethylformamide solution. The graphene nanostructures are directly imaged using a high-resolution transmission electron microscope (HRTEM) operated at 200 kV with a point resolution of 0.16 nm. We observed rotational misorientation within the flake in the HRTEM images of 2, 4 and 6 layers of graphene sheets, giving rise to Moiré patterns. By filtering in the frequency domain using a Fourier transform, we reconstruct the graphene lattice of each sheet and determine the relative rotation between consecutive graphene layers up, to six separate sheets. Direct evidence is obtained for FLG sheets with packing that is different to the standard AB Bernal packing of bulk graphite. Furthermore, we observed periodic ripples in suspended graphene sheets in our TEM measurements. Electrostatic force microscopy was used to characterize the electric potential distribution on the surface of FLG sheets on SiO2/Si substrates in ambient conditions. The FLG sheets were found to exhibit a conducting nature with small potential variations on the surface.

Supercapacitors based on flexible graphene/polyaniline nanofiber composite films

Composite films of chemically converted graphene (CCG) and polyaniline nanofibers (PANI-NFs) were prepared by vacuum filtration the mixed dispersions of both components. The composite film has a layered structure, and PANI-NFs are sandwiched between CCG layers. Furthermore, it is mechanically stable and has a high flexibility; thus, it can be bent into large angles or be shaped into various desired structures. The conductivity of the composite film containing 44% CCG (5.5 x 10(2) S m(-1)) is about 10 times that of a PANI-NF film. Supercapacitor devices based on this conductive flexible composite film showed large electrochemical capacitance (210 F g(-1)) at a discharge rate of 0.3 A g(-1). They also exhibited greatly improved electrochemical stability and rate performances.

Hydrogen bond networks in graphene oxide composite paper: structure and mechanical properties

A multilayered composite structure formed by a random stacking of graphene oxide (GO) platelets is an attractive candidate for novel applications in nanoelectromechanical systems and paper-like composites. We employ molecular dynamics simulations with reactive force fields to elucidate the structural and mechanical properties of GO paper-like materials. We find that the large-scale properties of these composites are controlled by hydrogen bond networks that involve functional groups on individual GO platelets and water molecules within the interlayer cavities. Water content controls both the extent and collective strength of these interlayer hydrogen bond networks, thereby affecting the interlayer spacing and elastic moduli of the composite. Additionally, the chemical composition of the individual GO platelets also plays a critical role in establishing the mechanical properties of the composite--a higher density of functional groups leads to increased hydrogen bonding and a corresponding increase in stiffness. Our studies suggest the possibility of tuning the properties of GO composites by altering the density of functional groups on individual platelets, the water content, and possibly the functional groups participating in hydrogen bonding with interlayer water molecules.

Atomic structure of reduced graphene oxide

Using high resolution transmission electron microscopy, we identify the specific atomic scale features in chemically derived graphene monolayers that originate from the oxidation-reduction treatment of graphene. The layers are found to comprise defect-free graphene areas with sizes of a few nanometers interspersed with defect areas dominated by clustered pentagons and heptagons. Interestingly, all carbon atoms in these defective areas are bonded to three neighbors maintaining a planar sp(2)-configuration, which makes them undetectable by spectroscopic techniques. Furthermore, we observe that they introduce significant in-plane distortions and strain in the surrounding lattice.

Chemical vapor deposition of graphene films

Graphene films were grown on nickel films and foils using chemical vapor deposition. To date, similar growth has been reported at around 1000 degrees C using methane or ethylene as source gases. However, by using acetylene, we have achieved growth of graphene films between 650 and 700 degrees C. The electrical and optical properties, including high resolution transmission electron microscopy of these films, suggest that this technique is both viable and scalable for potential large area optoelectronic applications.

High-concentration solvent exfoliation of graphene

A method is demonstrated to prepare graphene dispersions at high concentrations, up to 1.2 mg mL(-1), with yields of up to 4 wt% monolayers. This process relies on low-power sonication for long times, up to 460 h. Transmission electron microscopy shows the sonication to reduce the flake size, with flake dimensions scaling as t(-1/2). However, the mean flake length remains above 1 microm for all sonication times studied. Raman spectroscopy shows defects are introduced by the sonication process. However, detailed analysis suggests that predominantly edge, rather than basal-plane, defects are introduced. These dispersions are used to prepare high-quality free-standing graphene films. The dispersions can be heavily diluted by water without sedimentation or aggregation. This method facilitates graphene processing for a range of applications.

Preparation of graphene by a low-temperature thermal reduction at atmosphere pressure

It is still a challenge to prepare graphene from graphene oxide (GO) under a mild thermal reduction. Here we report that GO can even be reduced at a low temperature (below 150 degrees C) at atmospheric pressure in a mixture of N,N-dimethylacetamide (DMAc) and water (10 : 3, v/v) under the protection of nitrogen gas. FT-IR, XRD, XPS, Raman, and AFM studies confirmed the thermal reduction of GO and the formation of graphene. The as-prepared graphene can be fully dispersed in pure DMAc to form an organic suspension, and the suspension is stable for at least two months at room temperature.

Graphene oxide, highly reduced graphene oxide, and graphene: versatile building blocks for carbon-based materials

Isolated graphene, a nanometer-thick two-dimensional analog of fullerenes and carbon nanotubes, has recently sparked great excitement in the scientific community given its excellent mechanical and electronic properties. Particularly attractive is the availability of bulk quantities of graphene as both colloidal dispersions and powders, which enables the facile fabrication of many carbon-based materials. The fact that such large amounts of graphene are most easily produced via the reduction of graphene oxide--oxygenated graphene sheets covered with epoxy, hydroxyl, and carboxyl groups--offers tremendous opportunities for access to functionalized graphene-based materials. Both graphene oxide and graphene can be processed into a wide variety of novel materials with distinctly different morphological features, where the carbonaceous nanosheets can serve as either the sole component, as in papers and thin films, or as fillers in polymer and/or inorganic nanocomposites. This Review summarizes techniques for preparing such advanced materials via stable graphene oxide, highly reduced graphene oxide, and graphene dispersions in aqueous and organic media. The excellent mechanical and electronic properties of the resulting materials are highlighted with a forward outlook on their applications.

Measurement of multicomponent solubility parameters for graphene facilitates solvent discovery

We have measured the dispersibility of graphene in 40 solvents, with 28 of them previously unreported. We have shown that good solvents for graphene are characterized by a Hildebrand solubility parameter of delta(T) approximately 23 MPa(1/2) and Hansen solubility parameters of delta(D) approximately 18 MPa(1/2), delta(P) approximately 9.3 MPa(1/2), and delta(H) approximately 7.7 MPa(1/2). The dispersibility is smaller for solvents with Hansen parameters further from these values. We have used transmission electron microscopy (TEM) analysis to show that the graphene is well exfoliated in all cases. Even in relatively poor solvents, >63% of observed flakes have <5 layers.

Graphene nanomesh

Graphene has significant potential for application in electronics, but cannot be used for effective field-effect transistors operating at room temperature because it is a semimetal with a zero bandgap. Processing graphene sheets into nanoribbons with widths of less than 10 nm can open up a bandgap that is large enough for room-temperature transistor operation, but nanoribbon devices often have low driving currents or transconductances. Moreover, practical devices and circuits will require the production of dense arrays of ordered nanoribbons, which remains a significant challenge. Here, we report the production of a new graphene nanostructure--which we call a graphene nanomesh--that can open up a bandgap in a large sheet of graphene to create a semiconducting thin film. The nanomeshes are prepared using block copolymer lithography and can have variable periodicities and neck widths as low as 5 nm. Graphene nanomesh field-effect transistors can support currents nearly 100 times greater than individual graphene nanoribbon devices, and the on-off ratio, which is comparable with the values achieved in individual nanoribbon devices, can be tuned by varying the neck width. The block copolymer lithography approach used to make the nanomesh devices is intrinsically scalable and could allow for the rational design and fabrication of graphene-based devices and circuits with standard semiconductor processing.

Visualizing graphene based sheets by fluorescence quenching microscopy

Graphene based sheets have stimulated great interest due to their superior mechanical, electrical, and thermal properties. A general visualization method that allows quick observation of these single atomic layers would be highly desirable as it can greatly facilitate sample evaluation and manipulation, and provide immediate feedback to improve synthesis and processing strategies. Here we report that graphene based sheets can be made highly visible under a fluorescence microscope by quenching the emission from a dye coating, which can be conveniently removed afterward by rinsing without disrupting the sheets. Current imaging techniques for graphene based sheets rely on the use of special substrates. In contrast, the fluorescence quenching mechanism is no longer limited by the type of substrate. Graphene, reduced graphene oxide, or even graphene oxide sheets deposited on arbitrary substrates can now be readily visualized with good contrast for layer counting. Direct observation of suspended sheets in solution was also demonstrated. The fluorescence quenching microscopy offers unprecedented imaging flexibility and could become a general tool for characterizing graphene based materials.

Exfoliation of graphite oxide in propylene carbonate and thermal reduction of the resulting graphene oxide platelets

Graphite oxide was exfoliated and dispersed in propylene carbonate (PC) by bath sonication. Heating the graphene oxide suspensions at 150 degrees C significantly reduced the graphene oxide platelets; paper samples comprising such reduced graphene oxide platelets had an electrical conductivity of 5230 S/m. By adding tetraethylammonium tetrafluoroborate (TEA BF(4)) to the reduced graphene oxide/PC slurry and making a two-cell ultracapacitor, specific capacitance values of about 120 F/g were obtained.

Understanding the SERS Effects of Single Silver Nanoparticles and Their Dimers, One at a Time

This perspective article highlights recent developments in a class of surface-enhanced Raman scattering (SERS) experiments that aim to correlate SERS enhancement factors with the physical parameters of metal nanostructures. In a typical study, the SERS substrate is fabricated by depositing colloidal nanoparticles on a silicon wafer to obtain individual particles isolated from each other, or small aggregates such as dimeric units. With the help of registration marks, the same nanoparticle, or dimer of nanoparticles, can be quickly located under a Raman microscope (for SERS spectra) and a scanning electron microscope (for structural characterization). The nanoscale characterization achieved by these studies has resulted in unparalleled investigations into the nature of polarization dependency for SERS, the hot spot nature of single nanoparticles and dimers, and the manipulation of hot spots through shape-controlled synthesis and self-assembly. We discuss the new insights these studies have offered, and the future progress they can deliver to the advancement of SERS.

Toward ubiquitous environmental gas sensors-capitalizing on the promise of graphene

Atomically thin sheets of carbon known as "graphene" have captured the imagination of much of the scientific world during the past few years. Although these single sheets of graphite were under our noses for years-within technologies ranging from the humble pencil, which has been around since at least 1565 (Petroski, H. The Pencil: A History of Design and Circumstance; Alfred A. Knopf: New York, 1993), to modern nuclear reactors-graphene was merely considered as part of graphite's crystal structure until 2004, when Novoselov, Geim, and colleagues (Science 2004, 306, 666-669) first presented some of the surprising electrical properties of graphene layers they had isolated by mechanically peeling sheets off graphite crystals. Today, graphene's unique electronic structures and properties, bolstered by other intriguing properties discovered in the intervening years, threaten the dominance of carbon nanotubes, a more mature allotrope of carbon, in potential applications from electronics to sensors. In this review, we will consider the promise of graphene for producing small-scale gas sensors for environmental monitoring.

Assembly of colloidal semiconductor nanorods in solution by depletion attraction

Arranging anisotropic nanoparticles into ordered assemblies remains a challenging quest requiring innovative and ingenuous approaches. The variety of interactions present in colloidal solutions of nonspherical inorganic nanocrystals can be exploited for this purpose. By tuning depletion attraction forces between hydrophobic colloidal nanorods of semiconductors, dispersed in an organic solvent, these could be assembled into 2D monolayers of close-packed hexagonally ordered arrays directly in solution. Once formed, these layers could be fished onto a substrate, and sheets of vertically standing rods were fabricated, with no additional external bias applied. Alternatively, the assemblies could be isolated and redispersed in polar solvents, yielding suspensions of micrometer-sized sheets which could be chemically treated directly in solution. Depletion attraction forces were also effective in the shape-selective separation of nanorods from binary mixtures of rods and spheres. The reported procedures have the potential to enable powerful and cost-effective fabrication approaches to materials and devices based on self-organized anisotropic nanoparticles.

Flexible, transparent, conducting films of randomly stacked graphene from surfactant-stabilized, oxide-free graphene dispersions

Graphite is exfoliated in water to give dispersions of mono- and few-layer graphene stabilized by surfactant. These dispersions can be used to form thin, disordered films of randomly stacked, oxide-free, few-layer graphenes. These films are transparent with a direct current conductivity of up to 1.5 x 10(4) S m(-1). The conductivity is stable under flexing for at least 2000 cycles. The electrical properties are limited by disorder and aggregation suggesting future routes for improvement.

Graphene oxide-facilitated electron transfer of metalloproteins at electrode surfaces

Graphene is a particularly useful nanomaterial that has shown great promise in nanoelectronics. Because of the ultrahigh electron mobility of graphene and its unique surface properties such as one-atom thickness and irreversible protein adsorption at surfaces, graphene-based materials might serve as an ideal platform for accommodating proteins and facilitating protein electron transfer. In this work, we demonstrate that graphene oxide (GO) supports the efficient electrical wiring the redox centers of several heme-containing metalloproteins (cytochrome c, myoglobin, and horseradish peroxidase) to the electrode. Importantly, proteins retain their structural intactness and biological activity upon forming mixtures with GO. These important features imply the promising applications of GO/protein complexes in the development of biosensors and biofuel cells.

A scalable, solution-phase processing route to graphene oxide and graphene ultralarge sheets

High yield production of graphene oxide and graphene sheets with an ultralarge size (up to approximately 200 microm) was realized using a modified solution-phase method.

Three-dimensional Pt-on-Pd bimetallic nanodendrites supported on graphene nanosheet: facile synthesis and used as an advanced nanoelectrocatalyst for methanol oxidation

Graphene nanosheet, the hottest material in physics and materials science, has been studied extensively because of its unique electronic, thermal, mechanical, and chemical properties arising from its strictly 2D structure and because of its potential technical applications. Particularly, these remarkable characteristics enable it to be a promising candidate as a new 2D support to load metal nanoparticles (NPs) for application in fuel cells. However, constructing high-quality graphene/bimetallic NP hybrids with high electrochemical surface area (ECSA) remains a great challenge to date. In this paper, we demonstrate for the first time a wet-chemical approach for the synthesis of high-quality three-dimensional (3D) Pt-on-Pd bimetallic nanodendrites supported on graphene nanosheets (TP-BNGN), which represents a new type of graphene/metal heterostructure. The resulting hybrids were characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), Raman spectroscopy, and electrochemical technique. It is found that small single-crystal Pt nanobranches supported on Pd NCs with porous structure and good dispersion were directly grown onto the surface of graphene nanosheets, which exhibits high electrochemical active area. Furthermore, the number of nanobranches for Pt-on-Pd bimetallic nanodendrites on the surface of graphene nanosheets could be easily controlled via simply changing the synthetic parameters, thus resulting in the tunable catalytic properties. Most importantly, the electrochemical data indicate that the as-prepared graphene/bimetallic nanodendrite hybrids exhibited much higher electrocatalytic activity toward methanol oxidation reaction than the platinum black (PB) and commercial E-TEK Pt/C catalysts.

Organic functionalisation of graphenes

Graphene sheets derived from dispersion of graphite in pyridine were functionalised by the 1,3 dipolar cycloaddition of azomethine ylide. The organically modified graphene sheets are easily dispersible in polar organic solvents and water, and they are extensively characterised using several spectroscopic and microscopy techniques.