Owen C. Compton

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.

Electrically Conductive “Alkylated” Graphene Paper via Chemical Reduction of Amine‐Functionalized Graphene Oxide Paper

2010 WILEY-VCH Verlag Gm Two-dimensional graphene nanosheets and graphene-based materials have garnered significant attention in recent years due to their excellent materials properties. Many graphenebased materials can be conveniently synthesized from graphite oxide (GO), which can be prepared in bulk quantities from graphite under strong oxidizing conditions. GO is a layered material featuring a variety of oxygen-containing functionalities with epoxide and hydroxyl groups on the basal plane and carbonyl and carboxyl groups along the edges, which provide a platform for rich chemistry to occur both within the intersheet gallery and along sheet edges. In addition, GO can be easily exfoliated into individual graphene oxide sheets, which can be reassembled into thin films or paper-like materials. For the latter case, flow-directed filtration of an aqueous graphene oxide dispersion produces very large sheets of a free-standing, foil-like material known as graphene oxide paper. This paper retains all the functional groups found in GO, preserving all of its native chemistry. While graphene oxide paper can be chemically modified in a facile fashion and has goodmechanical properties, it was found to be electrically conductive only after thermal annealing, which presumably converts it into graphene paper. Unfortunately, this thermal treatment also degrades its structural integrity. Graphene paper, fabricated via flow-directed filtration of an electrostatically stabilized aqueous graphene dispersion that was pre-prepared via hydrazine reduction of graphene oxide sheets, has excellent electrical conductivity and similar mechanical properties as graphene oxide paper maintained at temperatures below 100 8C. However, the hydrazine reduction of graphene oxide sheets can remove a significant amount of oxygen-containing functionalities and lead to graphene papers with low functional-group content. To produce functionalized graphene paper from graphene oxide sheets, we envisioned two strategies: 1) preparing functionalized graphene sheets before assembling them into ‘‘paper’’ or 2) reducing a pre-assembled, functionalized graphene oxide paper. Here, we present the successful preparation of a conductive, ‘‘alkylated’’ graphene paper via the post-synthetic modification of ‘‘alkylated’’ graphene oxide paper. By treating pre-assembled graphene oxide paper with hexylamine prior to hydrazine reduction, we can convert this insulating paper into conductive ‘‘alkylated’’ graphene paper while maintaining its well-ordered structure and good mechanical properties. Since reduction in the absence of hexylamine affords a less-ordered material with inconsistent conductivity, we attribute the uniform conductivity we observe for the ‘‘alkylated’’ paper to the structure-stabilizing presence of the hexylamine. GO prepared using the Hummers method was sonicated to yield aqueous dispersions of graphene oxide sheets, which were vacuum-filtered through an Anodisc membrane to yield graphene oxide paper (see Supporting Information (SI) for further details). Hexylamine-modified (HA-) graphene oxide paper was prepared by flowing a methanol solution of the amine (100mM) through the as-prepared wet paper, which already has a ‘‘well-stacked’’ structure. In contrast, if graphene oxide sheets aremodified first with hexylamine, they become hydrophobic and quickly precipitate in water, precluding the formation of well-ordered paper (Fig. S1 in SI). HA-graphene paper was then obtained by flowing an aqueous hydrazine monohydrate solution (2 M), a commonly used reducing agent for graphene oxide, through the as-prepared, wet HA-graphene oxide paper at 90 8C under vacuum assistance. Unmodified graphene paper was prepared by a similar reduction of unmodified wet graphene oxide paper. As the structures of the papers were already established during the assembly, our method conveniently omits the use of ammonia andmineral oil stabilizing agents found in an alternative method for preparing graphene paper from aqueous dispersions of graphene sheets. Functionalization prior to reduction is key to the proper preparation of HA-graphene paper (Fig. S2 in SI); performing reduction first removes themajority of reactive oxygen-containing functionalities from graphene oxide and prevents any substantial hexylamine functionalization. Successful hexylamine functionalization and reduction of the graphene oxide paper were confirmed by elemental analysis (EA) and Karl–Fischer titration (Table S2 in SI). As fabricated, graphene oxide paper has a Cgraphene/O ratio of 2.9 with a water content of 17wt%. In contrast, the water content for the HA-graphene oxide paper is significantly decreased to 1.49wt%

Crumpled Graphene Nanosheets as Highly Effective Barrier Property Enhancers

In modern society, polymer packaging plays a critical role in the preservation and distribution of perishable goods such as food and prescription medicines. [ 1 , 2 ] Since the effectiveness of poly mer packaging materials in preventing product degradation is directly dependent upon their impermeability to degradative gases [ 3 ] and their opacity to high-energy light, [ 4 ] signifi cant efforts have been devoted to improving these properties. [ 5 ]

Tuning the Mechanical Properties of Graphene Oxide Paper and Its Associated Polymer Nanocomposites by Controlling Cooperative Intersheet Hydrogen Bonding

The mechanical properties of pristine graphene oxide paper and paper-like films of polyvinyl alcohol (PVA)-graphene oxide nanocomposite are investigated in a joint experimental-theoretical and computational study. In combination, these studies reveal a delicate relationship between the stiffness of these papers and the water content in their lamellar structures. ReaxFF-based molecular dynamics (MD) simulations elucidate the role of water molecules in modifying the mechanical properties of both pristine and nanocomposite graphene oxide papers, as bridge-forming water molecules between adjacent layers in the paper structure enhance stress transfer by means of a cooperative hydrogen-bonding network. For graphene oxide paper at an optimal concentration of ~5 wt % water, the degree of cooperative hydrogen bonding within the network comprising adjacent nanosheets and water molecules was found to optimally enhance the modulus of the paper without saturating the gallery space. Introducing PVA chains into the gallery space further enhances the cooperativity of this hydrogen-bonding network, in a manner similar to that found in natural biomaterials, resulting in increased stiffness of the composite. No optimal water concentration could be found for the PVA-graphene oxide nanocomposite papers, as dehydration of these structures continually enhances stiffness until a final water content of ~7 wt % (additional water cannot be removed from the system even after 12 h of annealing).

Chemically active reduced graphene oxide with tunable C/O ratios

Organic dispersions of graphene oxide can be thermally reduced in polar organic solvents under reflux conditions to afford electrically conductive, chemically active reduced graphene oxide (CARGO) with tunable C/O ratios, dependent on the boiling point of the solvent. The reductions are achieved after only 1 h of reflux, and the corresponding C/O ratios do not change upon further thermal treatment. Hydroxyl and carboxyl groups can be removed when the reflux is carried out above 155 °C, while epoxides are removable only when the temperature is higher than 200 °C. The increasing hydrophobic nature of CARGO, as its C/O ratio increases, improves the dispersibility of the nanosheets in a polystyrene matrix, in contrast to the aggregates formed with CARGO having lower C/O ratios. The excellent processability of the obtained CARGO dispersions is demonstrated via free-standing CARGO papers that exhibit tunable electrical conductivity/chemical activity and can be used as lithium-ion battery anodes with enhanced Coulombic efficiency.

Bio‐Inspired Borate Cross‐Linking in Ultra‐Stiff Graphene Oxide Thin Films

Adjacent graphene oxide nanosheets in a thin-film structure have been covalently cross-linked in a fashion similar to the cell walls of higher-order plants. The resulting ultra-stiff structure exhibits a maximum storage modulus of 127 GPa that can be tuned by varying borate concentration.

Successful stabilization of graphene oxide in electrolyte solutions: Enhancement of biofunctionalization and cellular uptake

Aqueous dispersions of graphene oxide are inherently unstable in the presence of electrolytes, which screen the electrostatic surface charge on these nanosheets and induce irreversible aggregation. Two complementary strategies, utilizing either electrostatic or steric stabilization, have been developed to enhance the stability of graphene oxide in electrolyte solutions, allowing it to stay dispersed in cell culture media and serum. The electrostatic stabilization approach entails further oxidation of graphene oxide to low C/O ratio (~1.1) and increases ionic tolerance of these nanosheets. The steric stabilization technique employs an amphiphilic block copolymer that serves as a noncovalently bound surfactant to minimize the aggregate-inducing nanosheet-nanosheet interactions. Both strategies can stabilize graphene oxide nanosheets with large dimensions (>300 nm) in biological media, allowing for an enhancement of >250% in the bioconjugation efficiency of streptavidin in comparison to untreated nanosheets. Notably, both strategies allow the stabilized nanosheets to be readily taken up by cells, demonstrating their excellent performance as potential drug-delivery vehicles.

Evolution of order during vacuum-assisted self-assembly of graphene oxide paper and associated polymer nanocomposites

Three mechanisms are proposed for the assembly of ordered, layered structures of graphene oxide, formed via the vacuum-assisted self-assembly of a dispersion of the two-dimensional nanosheets. These possible mechanisms for ordering at the filter-solution interface range from regular brick-and-mortar-like growth to complete disordered aggregation and compression. Through a series of experiments (thermal gravimetric analysis, UV-vis spectroscopy, and X-ray diffraction) a semi-ordered accumulation mechanism is identified as being dominant during paper fabrication. Additionally, a higher length-scale ordered structure (lamellae) is identified through the examination of water-swelled samples, indicating that further refinements are required to capture the complete formation mechanism. Identification of this mechanism and the resulting higher-order structure it produces provide insight into possibilities for creation of ordered graphene oxide-polymer nanocomposites, as well as the postfabrication modification of single-component graphene oxide papers.

Experimental-Computational Study of Shear Interactions within Double-Walled Carbon Nanotube Bundles

The mechanical behavior of carbon nanotube (CNT)-based fibers and nanocomposites depends intimately on the shear interactions between adjacent tubes. We have applied an experimental-computational approach to investigate the shear interactions between adjacent CNTs within individual double-walled nanotube (DWNT) bundles. The force required to pull out an inner bundle of DWNTs from an outer shell of DWNTs was measured using in situ scanning electron microscopy methods. The normalized force per CNT-CNT interaction (1.7 ± 1.0 nN) was found to be considerably higher than molecular mechanics (MM)-based predictions for bare CNTs (0.3 nN). This MM result is similar to the force that results from exposure of newly formed CNT surfaces, indicating that the observed pullout force arises from factors beyond what arise from potential energy effects associated with bare CNTs. Through further theoretical considerations we show that the experimentally measured pullout force may include small contributions from carbonyl functional groups terminating the free ends of the CNTs, corrugation of the CNT-CNT interactions, and polygonization of the nanotubes due to their mutual interactions. In addition, surface functional groups, such as hydroxyl groups, that may exist between the nanotubes are found to play an unimportant role. All of these potential energy effects account for less than half of the ~1.7 nN force. However, partially pulled-out inner bundles are found not to pull back into the outer shell after the outer shell is broken, suggesting that dissipation is responsible for more than half of the pullout force. The sum of force contributions from potential energy and dissipation effects are found to agree with the experimental pullout force within the experimental error.

Exfoliation and reassembly of cobalt oxide nanosheets into a reversible lithium-ion battery cathode

An exfoliation-reassembly-activation (ERA) approach to lithium-ion battery cathode fabrication is introduced, demonstrating that inactive HCoO(2) powder can be converted into a reversible Li(1-x) H(x) CoO(2) thin-film cathode. This strategy circumvents the inherent difficulties often associated with the powder processing of the layered solids typically employed as cathode materials. The delamination of HCoO(2) via a combination of chemical and mechanical exfoliation generates a highly processable aqueous dispersion of [CoO(2) ](-) nanosheets that is critical to the ERA approach. Following vacuum-assisted self-assembly to yield a thin-film cathode and ion exchange to activate this material, the generated cathodes exhibit excellent cyclability and discharge capacities approaching that of low-temperature-prepared LiCoO(2) (~83 mAh g(-1) ), with this good electrochemical performance attributable to the high degree of order in the reassembled cathode.