62 research outputs found
Controlling the Formation and Structure of Nanoparticle Superlattices through Surface Ligand Behavior
The
tailoring of nanoparticle superlattices is fundamental to the design
of novel nanostructured materials and devices. To obtain specific
collective properties of these nanoparticle superlattices, reliable
protocols for their self-assembly are required. This study provides
insight into the self-assembly process by using oleate-covered CeO<sub>2</sub> nanoparticles (cubic and polyhedral shapes) through the correlation
of experimental and theoretical investigations. The self-assembly
of CeO<sub>2</sub> nanoparticles is controlled by tuning the colloid
deposition parameters (temperature and evaporation rate), and the
ordered structures so obtained were correlated to the Gibbs free energy
variation of the system. The analysis of the interparticle force contributions
for each structure showed the importance of both the effective ligand
mean size and its Flory–Huggins parameter in determining the
total potential energies. Additionally, the roles of ligand solubility
and effective mean size were used to understand the formation of specific
superlattice phases as a function of temperature and ligand accommodation
in the arrangement. Furthermore, the face-to-face interactions between
nanoparticles were correlated to the type of exposed crystallographic
facet in each particle
Measurement of the Dewetting, Nucleation, and Deactivation Kinetics of Carbon Nanotube Population Growth by Environmental Transmission Electron Microscopy
Understanding
the collective growth of carbon nanotube (CNT) populations
is key to tailoring their properties for many applications. During
the initial stages of CNT growth by chemical vapor deposition, catalyst
nanoparticle formation by thin-film dewetting and the subsequent CNT
nucleation processes dictate the CNT diameter distribution, areal
density, and alignment. Herein, we use <i>in situ</i> environmental
transmission electron microscopy (E-TEM) to observe the catalyst annealing,
growth, and deactivation stages for a population of CNTs grown from
a thin-film catalyst. Complementary <i>in situ</i> electron
diffraction and TEM imaging show that, during the annealing step in
hydrogen, reduction of the iron oxide catalyst is concomitant with
changes in the thin-film morphology; complete dewetting and the formation
of a population of nanoparticles is only achieved upon the introduction
of the carbon source, acetylene. The dewetting kinetics, i.e., the
appearance of distinct nanoparticles, exhibits a sigmoidal (autocatalytic)
curve with 95% of all nanoparticles appearing within 6 s. After nanoparticles
form, they either nucleate CNTs or remain inactive, with incubation
times measured to be as small as 3.5 s. Via E-TEM we also directly
observe the crowding and self-alignment of CNTs after dewetting and
nucleation. In addition, <i>in situ</i> electron energy
loss spectroscopy reveals that the collective rate of carbon accumulation
decays exponentially. We conclude that the kinetics of catalyst formation
and CNT nucleation must both be addressed in order to achieve uniform
and high CNT density, and their transient behavior may be a primary
cause of the well-known nonuniform density of CNT forests
Measurement of the Dewetting, Nucleation, and Deactivation Kinetics of Carbon Nanotube Population Growth by Environmental Transmission Electron Microscopy
Understanding
the collective growth of carbon nanotube (CNT) populations
is key to tailoring their properties for many applications. During
the initial stages of CNT growth by chemical vapor deposition, catalyst
nanoparticle formation by thin-film dewetting and the subsequent CNT
nucleation processes dictate the CNT diameter distribution, areal
density, and alignment. Herein, we use <i>in situ</i> environmental
transmission electron microscopy (E-TEM) to observe the catalyst annealing,
growth, and deactivation stages for a population of CNTs grown from
a thin-film catalyst. Complementary <i>in situ</i> electron
diffraction and TEM imaging show that, during the annealing step in
hydrogen, reduction of the iron oxide catalyst is concomitant with
changes in the thin-film morphology; complete dewetting and the formation
of a population of nanoparticles is only achieved upon the introduction
of the carbon source, acetylene. The dewetting kinetics, i.e., the
appearance of distinct nanoparticles, exhibits a sigmoidal (autocatalytic)
curve with 95% of all nanoparticles appearing within 6 s. After nanoparticles
form, they either nucleate CNTs or remain inactive, with incubation
times measured to be as small as 3.5 s. Via E-TEM we also directly
observe the crowding and self-alignment of CNTs after dewetting and
nucleation. In addition, <i>in situ</i> electron energy
loss spectroscopy reveals that the collective rate of carbon accumulation
decays exponentially. We conclude that the kinetics of catalyst formation
and CNT nucleation must both be addressed in order to achieve uniform
and high CNT density, and their transient behavior may be a primary
cause of the well-known nonuniform density of CNT forests
Atomically Visualizing Elemental Segregation-Induced Surface Alloying and Restructuring
Using
in situ transmission electron microscopy that spatially and
temporally resolves the evolution of the atomic structure in the surface
and subsurface regions, we find that the surface segregation of Au
atoms in a CuÂ(Au) solid solution results in the nucleation and growth
of a (2 Ă— 1) missing-row reconstructed, half-unit-cell thick
L1<sub>2</sub> Cu<sub>3</sub>AuÂ(110) surface alloy. Our in situ electron
microscopy observations and atomistic simulations demonstrate that
the (2 Ă— 1) reconstruction of the Cu<sub>3</sub>AuÂ(110) surface
alloy remains as a stable surface structure as a result of the favored
Cu–Au diatom configuration
Improved Coking Resistance of “Intelligent” Ni Catalysts Prepared by Atomic Layer Deposition
Conformal
CaTiO<sub>3</sub> films were deposited onto MgAl<sub>2</sub>O<sub>4</sub> by atomic layer deposition (ALD) and then examined
as “intelligent” catalyst supports for Ni in the steam
and CO<sub>2</sub> reforming of methane. CaTiO<sub>3</sub> films (1
nm) were characterized by scanning transmission electron microscopy
and XRD and shown to be stable to at least 1073 K. Catalysts with
1 and 20 wt % Ni were studied, and it was found that, following calcination
at 1073 K, the Ni-CaTiO<sub>3</sub>/MgAl<sub>2</sub>O<sub>4</sub> catalysts
required high-temperature reduction to achieve activities comparable
to that of their Ni/MgAl<sub>2</sub>O<sub>4</sub> counterparts. However,
the Ni-CaTiO<sub>3</sub>/MgAl<sub>2</sub>O<sub>4</sub> catalysts exhibited
dramatically improved tolerance toward carbon-whisker formation. The
carbon content on the 1 wt % Ni catalyst on CaTiO<sub>3</sub>/MgAl<sub>2</sub>O<sub>4</sub> was small even after heating the catalyst in
a dry, 10% CH<sub>4</sub>–90% He mixture at 1073 K for 12 h.
Possible mechanisms for the high carbon tolerance of the perovskite-containing
catalysts are discussed
Compositional Inhomogeneity of Multinary Semiconductor Nanoparticles: A Case Study of Cu<sub>2</sub>ZnSnS<sub>4</sub>
Probe-corrected
scanning transmission electron microscopy and energy-dispersive
X-ray spectroscopy were used to characterize the inter- and intraparticle
compositional inhomogeneity of multinary Cu<sub>2</sub>ZnSnS<sub>4</sub> (CZTS) nanoparticles. CZTS nanoparticles were prepared following
three distinct synthesis protocols described in the literature. Strong
fluctuations in composition were observed for Cu and Zn in individual
nanoparticles, independent of the synthesis method. Certain particles
have regions that have compositions close to that of Cu<sub>2</sub>SnS<sub>3</sub>, as well as, in the extreme case, the presence of
nearly pure ZnS species. This is an observation that has not been
reported in prior studies of these systems and underscores the need
to both more carefully study the polydispersity of multinary semiconductor
nanoparticles (MSNs) and to improve synthetic protocols and characterization
of MSNs. Notablyî—¸despite the observation of compositional fluctuations
in individual nanoparticlesî—¸reactive sintering in Se vapor
was shown to reduce the nanoscale compositional fluctuations in the
resulting sintered grains, facilitating the use of these heterogeneous
particles in optoelectronic devices
Revealing Correlation of Valence State with Nanoporous Structure in Cobalt Catalyst Nanoparticles by <i>In Situ</i> Environmental TEM
Simultaneously probing the electronic structure and morphology of materials at the nanometer or atomic scale while a chemical reaction proceeds is significant for understanding the underlying reaction mechanisms and optimizing a materials design. This is especially important in the study of nanoparticle catalysts, yet such experiments have rarely been achieved. Utilizing an environmental transmission electron microscope equipped with a differentially pumped gas cell, we are able to conduct nanoscopic imaging and electron energy loss spectroscopy <i>in situ</i> for cobalt catalysts under reaction conditions. Studies reveal quantitative correlation of the cobalt valence states with the particles’ nanoporous structures. The <i>in situ</i> experiments were performed on nanoporous cobalt particles coated with silica, while a 15 mTorr hydrogen environment was maintained at various temperatures (300–600 °C). When the nanoporous particles were reduced, the valence state changed from cobalt oxide to metallic cobalt and concurrent structural coarsening was observed. <i>In situ</i> mapping of the valence state and the corresponding nanoporous structures allows quantitative analysis necessary for understanding and improving the mass activity and lifetime of cobalt-based catalysts, for example, for Fischer–Tropsch synthesis that converts carbon monoxide and hydrogen into fuels, and uncovering the catalyst optimization mechanisms
Atomic Insight into the Layered/Spinel Phase Transformation in Charged LiNi<sub>0.80</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> Cathode Particles
Layered
LiNi<sub>0.80</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> (NCA) holds great promise as a potential cathode material
for high energy density lithium ion batteries. However, its high capacity
is heavily dependent on the stability of its layered structure, which
suffers from a severe structure degradation resulting from a not fully
understood layered → spinel phase transformation. Using high-resolution
transmission electron microscopy and electron diffraction, we probe
the atomic structure evolution induced by the layered → spinel
phase transformation in the NCA cathode. We show that the phase transformation
results in the development of a particle structure with the formation
of complete spinel, spinel domains, and intermediate spinel from the
surface to the subsurface region. The lattice planes of the complete
and intermediate spinel phases are highly interwoven in the subsurface
region. The layered → spinel transformation occurs via the
migration of transition metal (TM) atoms from the TM layer into the
lithium layer. Incomplete migration leads to the formation of the
intermediate spinel phase, which is featured by tetrahedral occupancy
of TM cations in the lithium layer. The crystallographic structure
of the intermediate spinel is discussed and verified by the simulation
of electron diffraction patterns
Mechanism and Enhanced Yield of Carbon Nanotube Growth on Stainless Steel by Oxygen-Induced Surface Reconstruction
It is well-known that carbon nanotubes
(CNTs) can be grown directly
on the surface of stainless steel (SS) alloys, because the native
composition of SS contains elements that seed CNT growth upon hydrocarbon
exposure at elevated temperature. Often such methods use acid immersion
or oxidation in air to treat the surface prior to hydrocarbon exposure
for CNT growth. However, there lacks a general understanding of how
the surface chemistry and morphology influences the nucleation and
growth of CNTs. Using environmental transmission electron microscopy,
we observe that CNT growth is enabled by surface reconstruction of
SS upon oxygen exposure at elevated temperature, followed by further
breakup of the surface upon reduction, and subsequent CNT nucleation
and growth upon hydrocarbon exposure. Using electron energy loss spectroscopy,
we find that catalyst particles consist of both pure iron as well
as iron alloys such as Fe–Cr and Fe–Ni. We use these
insights to study the synthesis of CNTs on bulk net-shaped porous
SS materials and show that annealing of the SS at 1000 °C in
air prior to CVD using an ethylene feedstock mixture produces a 70-fold
increase in CNT yield. Our findings demonstrate how process conditions
can be designed for efficient manufacturing of CNT-enhanced stainless
steel materials, and guide improved understanding of CNT growth on
other industrially relevant metal substrates
Structural Modification of Graphene Sheets to Create a Dense Network of Defect Sites
Pt/graphene
composites were synthesized by loading platinum nanoparticles
onto graphene and etched at 1000 °C in a hydrogen atmosphere.
This results in the formation of a dense array of nanostructured defect
sites in the graphene, including trenches, nanoribbons, islands, and
holes. These defect sites result in an increase in the number of unsaturated
carbon atoms and, consequently, enhance the interaction of the CO<sub>2</sub> molecules with the etched graphene. This leads to a high
capacity for storing CO<sub>2</sub>; 1 g of the etched samples can
store up to 76.3 cm<sup>3</sup> of CO<sub>2</sub> at 273 K under ambient
pressure
- …