Ian A. Kinloch

Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

The Real Graphene Oxide Revealed: Stripping the Oxidative Debris from the Graphene-like Sheets†

It′ll come out in the wash! Graphene oxide has been shown to be a stable complex of oxidative debris (red ellipses in the picture) strongly adhered to functionalized graphene-like sheets. Under basic conditions the oxidative debris is stripped from the graphene-like sheets, and the resulting graphene oxide is conducting and cannot easily be resuspended in water.

Optimizing the Reinforcement of Polymer-Based Nanocomposites by Graphene

The stress transfer between the internal layers of multilayer graphene within polymer-based nanocomposites has been investigated from the stress-induced shifts of the 2D Raman band. This has been undertaken through the study of the deformation of an ideal composite system where the graphene flakes were placed upon the surface of a polymer beam and then coated with an epoxy polymer. It is found that the rate of band shift per unit strain for a monolayer graphene flake is virtually independent of whether it has one or two polymer interfaces (i.e., with or without an epoxy top coating). In contrast, the rate of band shift is lower for an uncoated bilayer specimen than a coated one, indicating relatively poor stress transfer between the graphene layers. Mapping of the strain in the coated bilayer regions has shown that there is strain continuity between adjacent monolayer and bilayer regions, indicating that they give rise to similar levels of reinforcement. Strain-induced Raman band shifts have also been evaluated for separate flakes of graphene with different numbers of layers, and it is found that the band shift rate tends to decrease with an increase in the number of layers, indicating poor stress transfer between the inner graphene layers. This behavior has been modeled in terms of the efficiency of stress transfer between the inner graphene layers. Taking into account the packing geometry of polymer-based graphene nanocomposites and the need to accommodate the polymer coils, these findings enable the optimum number of graphene layers for the best reinforcement to be determined. It is demonstrated that, in general, multilayer graphene will give rise to higher levels of reinforcement than monolayer material, with the optimum number of layers depending upon the separation of the graphene flakes in the nanocomposite.

Characterization of MoS2–Graphene Composites for High-Performance Coin Cell Supercapacitors

Two-dimensional materials, such as graphene and molybdenum disulfide (MoS2), can greatly increase the performance of electrochemical energy storage devices because of the combination of high surface area and electrical conductivity. Here, we have investigated the performance of solution exfoliated MoS2 thin flexible membranes as supercapacitor electrodes in a symmetrical coin cell arrangement using an aqueous electrolyte (Na2SO4). By adding highly conductive graphene to form nanocomposite membranes, it was possible to increase the specific capacitance by reducing the resistivity of the electrode and altering the morphology of the membrane. With continued charge/discharge cycles the performance of the membranes was found to increase significantly (up to 800%), because of partial re-exfoliation of the layered material with continued ion intercalation, as well as increasing the specific capacitance through intercalation pseudocapacitance. These results demonstrate a simple and scalable application of layered 2D materials toward electrochemical energy storage.

An investigation of the conductivity of peptide nanotube networks prepared by enzyme-triggered self-assembly

We demonstrate that nanotubular networks formed by enzyme-triggered self-assembly of Fmoc-L3 (9-fluorenylmethoxycarbonyl-tri-leucine) show significant charge transport. FT-IR, fluorescence spectroscopy and wide angle X-ray scattering (WAXS) data confirm formation of beta-sheets that are locked together viapi-stacking interactions. Molecular dynamics simulations confirmed the pi-pi stacking distance between fluorenyl groups to be 3.6-3.8 A. Impedance spectroscopy demonstrated that the nanotubular xerogel networks possess minimum sheet resistances of 0.1 MOmega/sq in air and 500 MOmega/sq in vacuum (pressure: 1.03 mbar) at room temperature, with the conductivity scaling linearly with the mass of peptide in the network. These materials may provide a platform to interface biological components with electronics.

Electrochemical Behavior of Monolayer and Bilayer Graphene

Results of a study on the electrochemical properties of exfoliated single and multilayer graphene flakes are presented. Graphene flakes were deposited on silicon/silicon oxide wafers to enable fast and accurate characterization by optical microscopy and Raman spectroscopy. Conductive silver paint and silver wires were used to fabricate contacts; epoxy resin was employed as a masking coating in order to expose a stable, well-defined area of graphene. Both multilayer and monolayer graphene microelectrodes showed quasi-reversible behavior during voltammetric measurements in potassium ferricyanide. However, the standard heterogeneous charge transfer rate constant, k°, was estimated to be higher for monolayer graphene flakes.

Strain mapping in a graphene monolayer nanocomposite

Model composite specimens have been prepared consisting of a graphene monolayer sandwiched between two thin layers of polymer on the surface of a poly(methyl methacrylate) beam. It has been found that well-defined Raman spectra can be obtained from the single graphene atomic layer and that stress-induced Raman band shifts enable the strain distribution in the monolayer to be mapped with a high degree of precision. It has been demonstrated that the distribution of strain across the graphene monolayer is relatively uniform at levels of applied strain up to 0.6% but that it becomes highly nonuniform above this strain. The change in the strain distributions has been shown to be due to a fragmentation process due to the development of cracks, most likely in the polymer coating layers, with the graphene remaining intact. The strain distributions in the graphene between the cracks are approximately triangular in shape, and the interfacial shear stress in the fragments is only about 0.25 MPa, which is an order of magnitude lower than the interfacial shear stress before fragmentation. This relatively poor level of adhesion between the graphene and polymer layers has important implications for the use of graphene in nanocomposites, and methods of strengthening the graphene-polymer interface are discussed.

Supercapacitance from Cellulose and Carbon Nanotube Nanocomposite Fibers

Multiwalled carbon nanotube (MWNT)/cellulose composite nanofibers have been prepared by electrospinning a MWNT/cellulose acetate blend solution followed by deacetylation. These composite nanofibers were then used as precursors for carbon nanofibers (CNFs). The effect of nanotubes on the stabilization of the precursor and microstructure of the resultant CNFs were investigated using thermogravimetric analysis, transmission electron microscopy and Raman spectroscopy. It is demonstrated that the incorporated MWNTs reduce the activation energy of the oxidative stabilization of cellulose nanofibers from ∼230 to ∼180 kJ mol–1. They also increase the crystallite size, structural order, and electrical conductivity of the activated CNFs (ACNFs). The surface area of the ACNFs increased upon addition of nanotubes which protrude from the fiber leading to a rougher surface. The ACNFs were used as the electrodes of a supercapacitor. The electrochemical capacitance of the ACNF derived from pure cellulose nanofibers is demonstrated to be 105 F g–1 at a current density of 10 A g–1, which increases to 145 F g–1 upon the addition of 6% of MWNTs.

Electron Transfer Kinetics on Mono- and Multilayer Graphene

Understanding of the electrochemical properties of graphene, especially the electron transfer kinetics of a redox reaction between the graphene surface and a molecule, in comparison to graphite or other carbon-based materials, is essential for its potential in energy conversion and storage to be realized. Here we use voltammetric determination of the electron transfer rate for three redox mediators, ferricyanide, hexaammineruthenium, and hexachloroiridate (Fe(CN)(6)(3-), Ru(NH3)(6)(3+), and IrCl(6)(2-), respectively), to measure the reactivity of graphene samples prepared by mechanical exfoliation of natural graphite. Electron transfer rates are measured for varied number of graphene layers (1 to ca. 1000 layers) using microscopic droplets. The basal planes of mono- and multilayer graphene, supported on an insulating Si/SiO(2) substrate, exhibit significant electron transfer activity and changes in kinetics are observed for all three mediators. No significant trend in kinetics with flake thickness is discernible for each mediator; however, a large variation in kinetics is observed across the basal plane of the same flakes, indicating that local surface conditions affect the electrochemical performance. This is confirmed by in situ graphite exfoliation, which reveals significant deterioration of initially, near-reversible kinetics for Ru(NH3)(6)(3+) when comparing the atmosphere-aged and freshly exfoliated graphite surfaces.

Strong Dependence of Mechanical Properties on Fiber Diameter for Polymer−Nanotube Composite Fibers: Differentiating Defect from Orientation Effects

We have prepared polyvinylalcohol-SWNT fibers with diameters from ∼1 to 15 μm by coagulation spinning. When normalized to nanotube volume fraction, V(f), both fiber modulus, Y, and strength, σ(B), scale strongly with fiber diameter, D: Y/V(f) ∝ D(-1.55) and σ(B)/V(f) ∝ D(-1.75). We show that much of this dependence is attributable to correlation between V(f) and D due to details of the spinning process: V(f) ∝ D(0.93). However, by carrying out Weibull failure analysis and measuring the orientation distribution of the nanotubes, we show that the rest of the diameter dependence is due to a combination of defect and orientation effects. For a given nanotube volume fraction, the fiber strength scales as σ(B) ∝ D(-0.29)D(-0.64), with the first and second terms representing the defect and orientation contributions, respectively. The orientation term is present and dominates for fibers of diameter between 4 and 50 μm. By preparing fibers with low diameter (1-2 μm), we have obtained mean mechanical properties as high as Y = 244 GPa and σ(B) = 2.9 GPa.