Karl W. Putz

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%

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).

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.

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.

Characterization of local elastic modulus in confined polymer films via AFM indentation.

The properties of polymers near an interface are altered relative to their bulk value due both to chemical interaction and geometric confinement effects. For the past two decades, the dynamics of polymers in confined geometries (thin polymer film or nanocomposites with high-surface area particles) has been studied in detail, allowing progress to be made toward understanding the origin of the dynamic effects near interfaces. Observations of mechanical property enhancements in polymer nanocomposites have been attributed to similar origins. However, the existing measurement methods of these local mechanical properties have resulted in a variety of conflicting results on the change of mechanical properties of confined polymers. Here, an atomic force microscopy (AFM)-based method is demonstrated that directly measures the mechanical properties of polymers adjacent to a substrate with nanometer resolution. This method allows us to consistently observe the gradient in mechanical properties away from a substrate in various materials systems, and paves the way for a unified understanding of thermodynamic and mechanical response of polymers. This gradient is both longer (up to 170 nm) and of higher magnitude (50% increase) than expected from prior results. Through the use of this technique, we will be better able to understand how to design polymer nanocomposites and polymeric structures at the smallest length scale, which affects the fields of structures, electronics, and healthcare.

Combustion-Assisted Photonic Annealing of Printable Graphene Inks via Exothermic Binders

High-throughput and low-temperature processing of high-performance nanomaterial inks is an important technical challenge for large-area, flexible printed electronics. In this report, we demonstrate nitrocellulose as an exothermic binder for photonic annealing of conductive graphene inks, leveraging the rapid decomposition kinetics and built-in energy of nitrocellulose to enable versatile process integration. This strategy results in superlative electrical properties that are comparable to extended thermal annealing at 350 °C, using a pulsed light process that is compatible with thermally sensitive substrates. The resulting porous microstructure and broad liquid-phase patterning compatibility are exploited for printed graphene microsupercapacitors on paper-based substrates.

Development of Multifunctional, Toughened Carbon Fiber-Reinforced Composites

The substitution of fiber-reinforced composite materials for heavier metallic structures in the design of aircrafts contributes to enhanced fuel economy with reduced emissions. However, the use of these materials is limited by the brittle, insulating polymer adhesive between the load-bearing fibers. Thus, the addition of multifunctional nanoparticles for multiscale reinforcement may hold the key to tailoring the matrix-dominated properties, particularly as the cost of nanoparticles are reduced to industrially relevant levels. In this paper, stacked-cup carbon nanofibers (CNF) and elastomeric triblock copolymers were dispersed in the matrix phase of carbon fiber-reinforced composites based on highperformance epoxy systems. Improvements of ~22-40% in short beam shear strength were observed with the incorporation of 1 part per hundred resin (phr) CNF. The addition of the soft triblock copolymer enhances tensile strain to failure but also yields a slight degradation in modulus and strength. However, these degradations were offset by the addition of CNFs. The interlaminar fracture toughness increases by ~25% over the base composite with the addition of 1 phr CNF, yet the addition of triblock copolymer yields no further improvement despite yielding a 50% enhancement over the base composite. Scanning electron microscopy images of fracture surfaces reveals local energy dissipation processes brought about by the nanostructured phases, as well as significant CNF agglomeration which limits their efficacy and leads to stress concentrations, which is confirmed by the lack of enhancement in transverse electrical conductivity. These results show good promise for CNFs as low-cost reinforcement for composites, but they also highlight the importance of attaining good dispersion in order to realize the full potential of the nanoparticles.

Optothermally Reversible Carbon Nanotube–DNA Supramolecular Hybrid Hydrogels

Supramolecular hydrogels (SMHs) are three-dimensional constructs wherein the majority of the volume is occupied by water. Since the bonding forces between the components of SMHs are noncovalent, SMH properties are often tunable, stimuli-responsive, and reversible, which enables applications including triggered drug release, sensing, and tissue engineering. Meanwhile, single-walled carbon nanotubes (SWCNTs) possess superlative electrical and thermal conductivities, high mechanical strength, and strong optical absorption at near-infrared wavelengths that have the potential to add unique functionality to SMHs. However, SWCNT-based SMHs have thus far not realized the potential of the optical properties of SWCNTs to enable reversible response to near-infrared irradiation. Here, we present a novel SMH architecture comprised solely of DNA and SWCNTs, wherein noncovalent interactions provide structural integrity without compromising the intrinsic properties of SWCNTs. The mechanical properties of these SMHs are readily tuned by varying the relative concentrations of DNA and SWCNTs, which varies the cross-linking density as shown by molecular dynamics simulations. Moreover, the SMH gelation transition is fully reversible and can be triggered by a change in temperature or near-infrared irradiation. This work explores a new regime for SMHs with potential utility for a range of applications including sensors, actuators, responsive substrates, and 3D printing.