SonBinh T. Nguyen

Graphene-based composite materials

Graphene sheets—one-atom-thick two-dimensional layers of sp2-bonded carbon—are predicted to have a range of unusual properties. Their thermal conductivity and mechanical stiffness may rival the remarkable in-plane values for graphite (∼3,000 W m-1 K-1 and 1,060 GPa, respectively); their fracture strength should be comparable to that of carbon nanotubes for similar types of defects; and recent studies have shown that individual graphene sheets have extraordinary electronic transport properties. One possible route to harnessing these properties for applications would be to incorporate graphene sheets in a composite material. The manufacturing of such composites requires not only that graphene sheets be produced on a sufficient scale but that they also be incorporated, and homogeneously distributed, into various matrices. Graphite, inexpensive and available in large quantity, unfortunately does not readily exfoliate to yield individual graphene sheets. Here we present a general approach for the preparation of graphene-polymer composites via complete exfoliation of graphite and molecular-level dispersion of individual, chemically modified graphene sheets within polymer hosts. A polystyrene–graphene composite formed by this route exhibits a percolation threshold of ∼0.1 volume per cent for room-temperature electrical conductivity, the lowest reported value for any carbon-based composite except for those involving carbon nanotubes; at only 1 volume per cent, this composite has a conductivity of ∼0.1 S m-1, sufficient for many electrical applications. Our bottom-up chemical approach of tuning the graphene sheet properties provides a path to a broad new class of graphene-based materials and their use in a variety of applications.

Metal–organic framework materials as catalysts

A critical review of the emerging field of MOF-based catalysis is presented. Discussed are examples of: (a) opportunistic catalysis with metal nodes, (b) designed catalysis with framework nodes, (c) catalysis by homogeneous catalysts incorporated as framework struts, (d) catalysis by MOF-encapsulated molecular species, (e) catalysis by metal-free organic struts or cavity modifiers, and (f) catalysis by MOF-encapsulated clusters (66 references).

Preparation and characterization of graphene oxide paper.

Free-standing paper-like or foil-like materials are an integral part of our technological society. Their uses include protective layers, chemical filters, components of electrical batteries or supercapacitors, adhesive layers, electronic or optoelectronic components, and molecular storage. Inorganic ‘paper-like’ materials based on nanoscale components such as exfoliated vermiculite or mica platelets have been intensively studied and commercialized as protective coatings, high-temperature binders, dielectric barriers and gas-impermeable membranes4,5. Carbon-based flexible graphite foils composed of stacked platelets of expanded graphite have long been used in packing and gasketing applications because of their chemical resistivity against most media, superior sealability over a wide temperature range, and impermeability to fluids. The discovery of carbon nanotubes brought about bucky paper, which displays excellent mechanical and electrical properties that make it potentially suitable for fuel cell and structural composite applications. Here we report the preparation and characterization of graphene oxide paper, a free-standing carbon-based membrane material made by flow-directed assembly of individual graphene oxide sheets. This new material outperforms many other paper-like materials in stiffness and strength. Its combination of macroscopic flexibility and stiffness is a result of a unique interlocking-tile arrangement of the nanoscale graphene oxide sheets.

Functionalized graphene sheets for polymer nanocomposites

Polymer-based composites were heralded in the 1960s as a new paradigm for materials. By dispersing strong, highly stiff fibres in a polymer matrix, high-performance lightweight composites could be developed and tailored to individual applications. Today we stand at a similar threshold in the realm of polymer nanocomposites with the promise of strong, durable, multifunctional materials with low nanofiller content. However, the cost of nanoparticles, their availability and the challenges that remain to achieve good dispersion pose significant obstacles to these goals. Here, we report the creation of polymer nanocomposites with functionalized graphene sheets, which overcome these obstacles and provide superb polymer-particle interactions. An unprecedented shift in glass transition temperature of over 40 degrees C is obtained for poly(acrylonitrile) at 1 wt% functionalized graphene sheet, and with only 0.05 wt% functionalized graphene sheet in poly(methyl methacrylate) there is an improvement of nearly 30 degrees C. Modulus, ultimate strength and thermal stability follow a similar trend, with values for functionalized graphene sheet- poly(methyl methacrylate) rivaling those for single-walled carbon nanotube-poly(methyl methacrylate) composites.

Stable aqueous dispersions of graphitic nanoplatelets via the reduction of exfoliated graphite oxide in the presence of poly(sodium 4-styrenesulfonate)

For the first time, stable aqueous dispersions of polymer-coated graphitic nanoplatelets can be prepared via an exfoliation/in-situ reduction of graphite oxide in the presence of poly(sodium 4-styrenesulfonate).

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.

Graphene oxide papers modified by divalent ions-enhancing mechanical properties via chemical cross-linking.

Significant enhancement in mechanical stiffness (10-200%) and fracture strength (approximately 50%) of graphene oxide paper, a novel paperlike material made from individual graphene oxide sheets, can be achieved upon modification with a small amount (less than 1 wt %) of Mg(2+) and Ca(2+). These results can be readily rationalized in terms of the chemical interactions between the functional groups of the graphene oxide sheets and the divalent metals ions. While oxygen functional groups on the basal planes of the sheets and the carboxylate groups on the edges can both bond to Mg(2+) and Ca(2+), the main contribution to mechanical enhancement of the paper comes from the latter.

De novo synthesis of a metal–organic framework material featuring ultrahigh surface area and gas storage capacities

Metal-organic frameworks--a class of porous hybrid materials built from metal ions and organic bridges--have recently shown great promise for a wide variety of applications. The large choice of building blocks means that the structures and pore characteristics of the metal-organic frameworks can be tuned relatively easily. However, despite much research, it remains challenging to prepare frameworks specifically tailored for particular applications. Here, we have used computational modelling to design and predictively characterize a metal-organic framework (NU-100) with a particularly high surface area. Subsequent experimental synthesis yielded a material, matching the calculated structure, with a high BET surface area (6,143 m(2) g(-1)). Furthermore, sorption measurements revealed that the material had high storage capacities for hydrogen (164 mg g(-1)) and carbon dioxide (2,315 mg g(-1))--gases of high importance in the contexts of clean energy and climate alteration, respectively--in excellent agreement with predictions from modelling.

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%

A Catalytically Active, Permanently Microporous MOF with Metalloporphyrin Struts

Through the use of the tetratopic ligand 1,2,4,5-tetrakis(4-carboxyphenyl)benzene as a key building block, a permanently microporous metal-organic framework with Lewis acidic (porphyrin)Zn struts, ZnPO-MOF, can be made in high yields. ZnPO-MOF can efficiently catalyze acyl-transfer reactions primarily by preconcentrating the substrates within its pores, in stark contrast to analogous supramolecular systems.