Cristina Vallés

Alkali reduction of graphene oxide in molten halide salts: Production of corrugated graphene derivatives for high-performance supercapacitors

Herein we present a green and facile approach to the successful reduction of graphene oxide (GO) materials using molten halide flux at 370 °C. GO materials have been synthesized using a modified Hummers method and subsequently reduced for periods of up to 8 h. Reduced GO (rGO) flakes have been characterized using X-ray-diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR), all indicating a significantly reduced amount of oxygen-containing functionalities on the rGO materials. Furthermore, impressive electrical conductivities and electrochemical capacitances have been measured for the rGO flakes, which, along with the morphology determined from scanning electron microscopy, highlight the role of surface corrugation in these rGO materials.

The rheological behaviour of concentrated dispersions of graphene oxide

The rheological behaviour of concentrated aqueous dispersions of graphene oxide (GO) was studied as a model system and then compared to those of GO in poly(methyl methacrylate) (PMMA). Dynamic and steady shear tests were conducted using a parallel plate rheometer. The aqueous system behaved as a reversibly flocculated dispersion with linear viscoelastic regions (LVR) extending up to strains of 10xa0%. Dynamic frequency sweeps conducted within the LVR showed a classic strong-gel spectrum for high concentrations. Under steady shear, the dispersions shear-thinned up to a Peclet number (Pe) <1, followed by a power law at higher Pe. The dispersions were thixotropic and recovered their structure after 60xa0min rest. The change in rheological properties of the PMMA upon the addition of the GO was less pronounced possibly due to the absence of hydrogen bonding; a relatively small increase in viscosity was found, which is encouraging for the melt processing of graphene composites.

Few layer graphene–polypropylene nanocomposites: the role of flake diameter

Graphene shows excellent potential as a structural reinforcement in polymer nanocomposites due to its exceptional mechanical properties. We have shown previously that graphene composites can be analysed using conventional composite theory with the graphene flakes acting as short fillers which have a critical length of ∼3 μm which is required for good reinforcement. Herein, polypropylene (PP) nanocomposites were prepared using electrochemically-exfoliated few layer graphene (FLG) with two different flake diameters (5 μm and 20 μm). The crystallization temperature and degree of crystallinity of the PP were found to increase with the loading of FLG, which suggests that the flakes acted as crystallisation nucleation sites. Mechanical testing showed that the 5 μm flakes behaved as short fillers and reinforced the PP matrix poorly. The modulus of the 20 μm flake composites, however, increased linearly with loading up to 20 wt%, without any of the detrimental aggregation effects seen in other graphene systems. The mechanical data were compared with our previous work on other graphene composite systems and the apparent need to balance the degree of functionalization to improve matrix compatibility whilst not encouraging aggregation is discussed.

Dispersal of pristine graphene for biological studies

Contradictions in the reported biocompatibility of graphene-related materials have been attributed to differences in their preparation. Herein, we address the conflicting behavior of different pristine graphene dispersions through their careful preparation and characterization in aqueous media. Although exfoliated in different media, all graphene dispersions were physically similar and comprised few-layer graphene flakes of 100 to 400 nm mean length with relatively defect-free basal planes. The dispersions were colloidally stable, including in physiological saline and when organic solvents were exchanged with water by dialysis, due to their negative zeta potentials (−28 mV to −60 mV) from interaction with water and the different dispersants. Thus, we have been able to establish the influence of pre-association with different dispersants, including those likely to be encountered during the transport and excretion of graphene in vivo. Shear-forced association with neutral phospholipids was transient, as the lipids desorbed to form liposomes, and left a hemolytic dispersion, whereas other dispersions were not hemolytic. High boiling point solvents widely used to exfoliate graphene are toxic and viewed difficult to remove, but were readily removed by simple dialysis. Human serum albumin readily and stably adsorbed in predominantly monomeric form to pristine graphene in physiological saline, which may be expected in vivo. This work shows the large influence that different adsorbates can have on the behaviour of otherwise physically-similar graphene.