75 research outputs found
A Summary of Methods for Fire Tests of Roof Coverings
AbstractThe testing method about the fire performance of roof covering and materials has not been put into operation in China. This article focuses on two main international testing about fire performance of roof covering and materials, comparing the difference between the two test methods
Three-Dimensional Valency Mapping in Ceria Nanocrystals
Using electron tomography combined with electron energy loss spectroscopy (EELS), we are able to map the valency of the Ce ions in CeO<sub>2â<i>x</i></sub> nanocrystals in three dimensions. Our results show a clear facet-dependent reduction shell at the surface of ceria nanoparticles; {111} surface facets show a low surface reduction, whereas at {001} surface facets, the cerium ions are more likely to be reduced over a larger surface shell. Our generic tomographic technique allows a full 3D data cube to be reconstructed, containing an EELS spectrum in each voxel. This possibility enables a three-dimensional investigation of a plethora of material-specific physical properties such as valency, chemical composition, oxygen coordination, or bond lengths, triggering the synthesis of nanomaterials with improved properties
New Insights into the Early Stages of Nanoparticle Electrodeposition
Electrodeposition is an increasingly important method to synthesize supported nanoparticles, yet the early stages of electrochemical nanoparticle formation are not perfectly understood. In this paper, the early stages of silver nanoparticle electrodeposition on carbon substrates have been studied by aberration-corrected TEM, using carbon-coated TEM grids as electrochemical electrodes. In this manner we have access to as-deposited nanoparticle size distribution and structural characterization at the atomic scale combined with electrochemical measurements, which represents a breakthrough in a full understanding of the nanoparticle electrodeposition mechanisms. Whereas classical models, based upon characterization at the nanoscale, assume that electrochemical growth is only driven by direct attachment, the results reported hereafter indicate that early nanoparticle growth is mostly driven by nanocluster surface movement and aggregation. Hence, we conclude that electrochemical nulceation and growth models should be revised and that an electrochemical aggregative growth mechanism should be considered in the early stages of nanoparticle electrodeposition
Layered Silicate Clays as Templates for Anisotropic Gold Nanoparticle Growth
Clay minerals are
abundant natural materials arising in the presence
of water and are composed of small particles of different sizes and
shapes. The interlamellar space between layered silicate clays can
also be used to host a variety of different organic and inorganic
guest molecules or particles. Recent studies of clayâmetal
hybrids formed by impregnation of nanoparticles into the interlayer
spaces of the clays have not demonstrated the ability for templated
growth following the shape of the particles. Following this line of
interest, a method for the synthesis of gold nanoparticles on the
synthetic layered silicate clay laponite was developed. This approach
can be used to make metalâclay nanoparticles with a variety
of morphologies while retaining the molecular adsorption properties
of the clay. The surface enhanced Raman scattering enhancement of
these particles was also found to be greater than that obtained from
other metal nanoparticles of a similar morphology, likely due to increased
dye adsorption by the presence of the clay. The hybrid particles presented
herein will contribute to further study of plasmonic sensing, catalysis,
dye aggregation, and novel composite materials
Do Binary Supracrystals Enhance the Crystal Stability?
We
study the oxygen thermal stability of two binary systems. The
larger particles are magnetic amorphous Co (7.2 nm) or Fe<sub>3</sub>O<sub>4</sub> (7.5 nm) nanocrystals, whereas the smaller ones (3.7
nm) are Au nanocrystals. The nanocrystal ordering as well as the choice
of the magnetic nanoparticles very much influence the stability of
the binary system. A perfect crystalline structure is obtained with
the Fe<sub>3</sub>O<sub>4</sub>/Au binary supracrystals. For the Co/Au
binary system, oxidation of Co results in the chemical transformation
from Co to CoO, where the size of the amorphous Co nanoparticles increases
from 7.2 to 9.8 nm in diameter. During the volume expansion of the
Co nanoparticles, Au nanoparticles within the binary assemblies coalesce
and are at the origin of the instability of the binary nanoparticle
supracrystals. On the other hand, for the Fe<sub>3</sub>O<sub>4</sub>/Au binary system, the oxidation of Fe<sub>3</sub>O<sub>4</sub> to
Îł-Fe<sub>2</sub>O<sub>3</sub> does not lead to a size change
of the nanoparticles, which maintains the stability of the binary
nanoparticle supracrystals. A similar behavior is observed for an
AlB<sub>2</sub>-type CoâAg binary system: The crystalline structure
is maintained, whereas in disordered assemblies, coalescence of Ag
nanocrystals is observed
Monitoring Galvanic Replacement Through Three-Dimensional Morphological and Chemical Mapping
Galvanic
replacement reactions on metal nanoparticles are often
used for the preparation of hollow nanostructures with tunable porosity
and chemical composition, leading to tailored optical and catalytic
properties. However, the precise interplay between the three-dimensional
(3D) morphology and chemical composition of nanostructures during
galvanic replacement is not always well understood as the 3D chemical
imaging of nanoscale materials is still challenging. It is especially
far from straightforward to obtain detailed information from the inside
of hollow nanostructures using electron microscopy techniques such
as SEM or TEM. We demonstrate here that a combination of state-of-the-art
EDX mapping with electron tomography results in the unambiguous determination
of both morphology transformation and elemental composition of nanostructures
in 3D, during galvanic replacement of Ag nanocubes. This work provides
direct and unambiguous experimental evidence toward understanding
the galvanic replacement reaction. In addition, the powerful approach
presented here can be applied to a wide range of nanoscale transformation
processes, which will undoubtedly guide the development of novel nanostructures
Highly Efficient Hyperbranched CNT Surfactants: Influence of Molar Mass and Functionalization
End-group-functionalized hyperbranched
polymers were synthesized
to act as a carbon nanotube (CNT) surfactant in aqueous solutions.
Variation of the percentage of triphenylmethyl (trityl) functionalization
and of the molar mass of the hyperbranched polyglycerol (PG) core
resulted in the highest measured surfactant efficiency for a 5000
g/mol
PG with 5.6% of the available hydroxyl end-groups replaced by trityl
functions, as shown by UVâvis measurements. Semiempirical model
calculations suggest an even higher efficiency for PG5000 with 2.5%
functionalization and maximal molecule specific efficiency in general
at low degrees of functionalization. Addition of trityl groups increases
the surfactantânanotube interactions in comparison to unfunctionalized
PG because of ÏâÏ stacking interactions. However,
at higher functionalization degrees mutual interactions between trityl
groups come into play, decreasing the surfactant efficiency, while
lack of water solubility becomes an issue at very high functionalization
degrees. Low molar mass surfactants are less efficient compared to
higher molar mass species most likely because the higher bulkiness
of the latter allows for a better CNT separation and stabilization.
The most efficient surfactant studied allowed dispersing 2.85 mg of
CNT in 20 mL with as little as 1 mg of surfactant. These dispersions,
remaining stable for at least 2 months, were mainly composed of individual
CNTs as revealed by electron microscopy
Supracrystalline Colloidal Eggs: Epitaxial Growth and Freestanding Three-Dimensional Supracrystals in Nanoscaled Colloidosomes
The
concept of template-confined chemical reactions allows the synthesis
of complex molecules that would hardly be producible through conventional
method. This idea was developed to produce high quality nanocrystals
more than 20 years ago. However, template-mediated assembly of colloidal
nanocrystals is still at an elementary level, not only because of
the limited templates suitable for colloidal assemblies, but also
because of the poor control over the assembly of nanocrystals within
a confined space. Here, we report the design of a new system called âsupracrystalline
colloidal eggsâ formed by controlled assembly of nanocrystals
into complex colloidal supracrystals through superlattice-matched
epitaxial overgrowth along the existing colloidosomes. Then, with
this concept, we extend the supracrystalline growth to lattice-mismatched
binary nanocrystal superlattices, in order to reach anisotropic superlattice
growths, yielding freestanding binary nanocrystal supracrystals that
could not be produced previously
The Role of Nanocluster Aggregation, Coalescence, and Recrystallization in the Electrochemical Deposition of Platinum Nanostructures
By
using an optimized characterization approach that combines aberration-corrected
transmission electron microscopy, electron tomography, and in situ
ultrasmall angle X-ray scattering (USAXS), we show that the early
stages of Pt electrochemical growth on carbon substrates may be affected
by the aggregation, self-alignment, and partial coalescence of nanoclusters
of <i>d</i> â 2 nm. The morphology of the resulting
nanostructures depends on the degree of coalescence and recrystallization
of nanocluster aggregates, which in turn depends on the electrodeposition
potential. At low overpotentials, a self-limiting growth mechanism
may block the epitaxial growth of primary nanoclusters and results
in loose dendritic aggregates. At more negative potentials, the extent
of nanocluster coalescence and recrystallization is larger and further
growth by atomic incorporation may be allowed. On one hand, this suggests
a revision of the VolmerâWeber island growth mechanism. Whereas
this theory has traditionally assumed direct attachment as the only
growth mechanism, it is suggested that nanocluster self-limiting growth,
aggregation, and coalescence should also be taken into account during
the early stages of nanoscale electrodeposition. On the other hand,
depending on the deposition potential, ultrahigh porosities can be
achieved, turning electrodeposition in an ideal process for highly
active electrocatalyst production without the need of using high surface
area carbon supports
Multiple Dot-in-Rod PbS/CdS Heterostructures with High Photoluminescence Quantum Yield in the Near-Infrared
Pb cations in PbS quantum rods made from CdS quantum
rods by successive
complete cationic exchange reactions are partially re-exchanged for
Cd cations. Using STEM-HAADF, we show that this leads to the formation
of unique multiple dot-in-rod PbS/CdS heteronanostructures, with a
photoluminescence quantum yield of 45â55%. We argue that the
formation of multiple dot-in-rods is related to the initial polycrystallinity
of the PbS quantum rods, where each PbS crystallite transforms in
a separate PbS/CdS dot-in-dot. Effective mass modeling indicates that
electronic coupling between the different PbS conduction band states
is feasible for the multiple dot-in-rod geometries obtained, while
the hole states remain largely uncoupled
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