5 research outputs found
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 ribbonsî¸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 showedî¸by removing the dispersants
by filtration and washingî¸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 GnPsâ 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 GrapheneâGraphite 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 âphase diagramâ
delineating two composite processability regions: solid-like (moldable)
or fluid-like (pourable). This diagram may be employed to tailor the
mixtureâs viscosity to a desired TC value by varying the fillersâ
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
grapheneâgraphite 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-NPâchiral-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 moleculeâplasmon 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<sub>1â<i>x</i></sub>(Gd<sub>1/5</sub>Pr<sub>1/5</sub>Nd<sub>1/5</sub>Sm<sub>1/5</sub>Y<sub>1/5</sub>)<i><sub>x</sub></i>O<sub>2âδ</sub> 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 Zr1âx(Gd1/5Pr1/5Nd1/5Sm1/5Y1/5)xO2âδ (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 oxidesâ 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 10â17 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