Distinguishing Self-Assembled Pyrene Structures from Exfoliated Graphene

Sonication-assisted graphene production from graphite is a popular lab-scale approach in which ultrasound energy breaks down graphite sheets into graphene flakes in aqueous medium. Dispersants (surfactant molecules) are incorporated into the solution to prevent individual graphene flakes from reaggregating. However, in solution these dispersants self-assemble into various structures, which can interfere with the characterization of the graphene produced. In this study, we characterized graphene dispersions stabilized by a family of pyrene-based surfactants that facilitate a high exfoliation yield. These surfactants self-assembled to form flakes and ribbonsshapes very similar to those of graphene structures. The dispersant structures were present both in the graphene dispersion and in the precipitate after the solvent had been evaporated and could therefore have been mistakenly identified as graphene by electron microscopy techniques and other characterization techniques, such as Raman and X-ray photoelectron spectroscopy. Contrary to previous reports, we showedby removing the dispersants by filtration and washingthat the surfactants did not affect the shape of the graphene prepared by sonication

Characterization of Graphene-Nanoplatelets Structure via Thermogravimetry

The rapid increase in graphene-based applications has been accompanied by novel top-down manufacturing methods for graphene and its derivatives (e.g., graphene nanoplatelets (GnPs)). The characterization of the <i>bulk</i> properties of these materials by imaging and surface techniques (e.g., electron microscopy and Raman spectroscopy) is only possible through laborious and time-consuming statistical analysis, which precludes simple and efficient quality control during GnP production. We report that thermogravimetry (TG) may be utilized, beyond its conventional applications (e.g., quantification of impurities or surfactants, or labile functional groups) to characterize <i>bulk</i> GnP properties. We characterize the structural parameters of GnP (i.e., defect density, mean lateral dimension, and polydispersity) by imaging and surface techniques, on one hand, and by a systematic TG, on the other. The combined data demonstrate that the combustion temperature of commercially available and laboratory-prepared GnPs is correlated with their mean lateral dimension and defect density, while the combustion temperature range is proportional to their polydispersity index. Mapping all these parameters allows one to evaluate the GnPs’ structure following a simple thermogravimetric experiment (without necessitating further statistical analysis). Finally, TG is also used to detect and quantify different GnP constituents in powder and to conduct rapid quality-control tests during GnP production

Breaking through the Solid/Liquid Processability Barrier: Thermal Conductivity and Rheology in Hybrid Graphene–Graphite Polymer Composites

Thermal conductivity (TC) enhancement of an insulating polymer matrix at low filler concentration is possible through the loading of a high aspect ratio, thermally conductive single filler. Unfortunately, the dispersion of high-aspect-ratio particles greatly influences the rheological behavior of the polymer host at relatively low volume fractions, which makes further polymer processing or mixing difficult. A possible remedy is using two (hybrid) fillers, differing in their aspect ratios: (1) a plate-like filler, which sharply increases both viscosity and TC, and (2) an isotropic filler, which gradually increases these properties. We examine this hypothesis in a thermosetting silicone rubber by loading it with different ratios, (1)/(2), of graphene nanoplatelets (GNPs) (1) and graphite powder (2). We constructed a “phase diagram” delineating two composite processability regions: solid-like (moldable) or fluid-like (pourable). This diagram may be employed to tailor the mixture’s viscosity to a desired TC value by varying the fillers’ volume fraction. The phase diagram highlights the low volume fraction value, above which the composite is solid-like (low processability) for a single high-aspect-ratio nanofiller. By using hybrid filling, one can overcome this limit and prepare a fluid-like composite at a desired TC, not accessible by the single nanofiller. Thus, it provides an indicative tool for polymer processing, especially in applications such as the encapsulation of electronic devices. This approach was demonstrated for a heat source (resistor) potted by silicon rubber graphene–graphite composites, for which a desired TC was obtained in both solid- and liquid-like regions

Chiroptical Activity in Silver Cholate Nanostructures Induced by the Formation of Nanoparticle Assemblies

We report on an interesting mechanism of inducing chiroptical response at plasmonic silver nanoparticles (NPs) through the formation of plasmonic hot spots in small metal-NP–chiral-surfactant assemblies. Circular dichroism (CD) was measured at the surface plasmon resonance of cholate-coated silver nanostructures (AgCT) in the visible region of the spectrum. Low temperature cryogenic transmission electron microscopy (cryo-TEM) micrographs of the AgCT nanostructures in solution reveal small assemblies of silver NPs. Upon pH increase these assemblies are separated into individual NPs, and the induced plasmonic CD vanishes. This process was monitored via spectroscopy (CD and absorption), cryo-TEM, small-angle X-ray scattering (SAXS), and dynamic light scattering (DLS) measurements. The synthesis of well-separated AgCT NPs, which was performed with a large excess of sodium cholate (NaCT), also did not show any chiroptical effects. We interpret and model the formation of strong CD signals in the visible range in terms of the molecule–plasmon interaction in plasmonic hot spots formed in nanoparticle aggregates. Importantly, this study of the chiral induction, transfer to the visible range, and local field enhancement offers very attractive possibilities for sensing and detection of chirality of small amounts of molecules using visible light

Enhanced Mechanical and Electromechanical Properties of Compositionally Complex Zirconia Zr_1–<i>x</i>(Gd_1/5Pr_1/5Nd_1/5Sm_1/5Y_1/5)<i>_x</i>O_2−δ Ceramics

Compositionally complex oxides (CCOs) or high-entropy oxides (HEOs) are new multielement oxides with unexplored physical and functional properties. In this work, we report fluorite structure-derived compositionally complex zirconia with composition Zr1–x(Gd1/5Pr1/5Nd1/5Sm1/5Y1/5)xO2−δ (x = 0.1 and 0.2) synthesized in solid-state reaction route and sintered via hot pressing at 1350 °C. We explore the evolution of these oxides’ structural, microstructural, mechanical, electrical, and electromechanical properties regarding phase separation and sintering mechanisms. Highly dense ceramics are achieved by bimodal mass diffusion, composing nanometric tetragonal and micrometric cubic grains microstructure. The material exhibits an anomalously large electrostriction response exceeding the M33 value of 10–17 m2/V2 at room temperature and viscoelastic properties of primary creep in nanoindentation measurement under fast loading. These findings are strikingly similar to those reported for doped ceria and bismuth oxide derivates, highlighting the presence of a large concentration of point defects linked to structural distortion and anelastic behavior, which are characteristics of nonclassical ionic electrostrictors