7 research outputs found
Tunable Localized Surface Plasmon Resonances in Tungsten Oxide Nanocrystals
Transition-metal oxide nanocrystals are interesting candidates
for localized surface plasmon resonance hosts because they exhibit
fascinating properties arising from the unique character of their
outer-d valence electrons. WO<sub>3−δ</sub> nanoparticles
are known to have intense visible and near-IR absorption, but the
origin of the optical absorption has remained unclear. Here we demonstrate
that metallic phases of WO<sub>3−δ</sub> nanoparticles
exhibit a strong and tunable localized surface plasmon resonance,
which opens up the possibility of rationally designing plasmonic tungsten
oxide nanoparticles for light harvesting, bioimaging, and sensing
Dendritic Assembly of Gold Nanoparticles during Fuel-Forming Electrocatalysis
We
observe the dendritic assembly of alkanethiol-capped gold nanoparticles
on a glassy carbon support during electrochemical reduction of protons
and CO<sub>2</sub>. We find that the primary mechanism by which surfactant-ligated
gold nanoparticles lose surface area is by taking a random walk along
the support, colliding with their neighbors, and fusing to form dendrites,
a type of fractal aggregate. A random walk model reproduces the fractal
dimensionality of the dendrites observed experimentally. The rate
at which the dendrites form is strongly dependent on the solubility
of the surfactant in the electrochemical double layer under the conditions
of electrolysis. Since alkanethiolate surfactants reductively desorb
at potentials close to the onset of CO<sub>2</sub> reduction, they
do not poison the catalytic activity of the gold nanoparticles. Although
catalyst mobility is typically thought to be limited for room-temperature
electrochemistry, our results demonstrate that nanoparticle mobility
is significant under conditions at which they electrochemically catalyze
gas evolution, even in the presence of a high surface area carbon
and binder. A careful understanding of the electrolyte- and polarization-dependent
nanoparticle aggregation kinetics informs strategies for maintaining
catalyst dispersion during fuel-forming electrocatalysis
Enhanced Electrochemical Methanation of Carbon Dioxide with a Dispersible Nanoscale Copper Catalyst
Although
the vast majority of hydrocarbon fuels and products are
presently derived from petroleum, there is much interest in the development
of routes for synthesizing these same products by hydrogenating CO<sub>2</sub>. The simplest hydrocarbon target is methane, which can utilize
existing infrastructure for natural gas storage, distribution, and
consumption. Electrochemical methods for methanizing CO<sub>2</sub> currently suffer from a combination of low activities and poor selectivities.
We demonstrate that copper nanoparticles supported on glassy carbon
(n-Cu/C) achieve up to 4 times greater methanation current densities
compared to high-purity copper foil electrodes. The n-Cu/C electrocatalyst
also exhibits an average Faradaic efficiency for methanation of 80%
during extended electrolysis, the highest Faradaic efficiency for
room-temperature methanation reported to date. We find that the level
of copper catalyst loading on the glassy carbon support has an enormous
impact on the morphology of the copper under catalytic conditions
and the resulting Faradaic efficiency for methane. The improved activity
and Faradaic efficiency for methanation involves a mechanism that
is distinct from what is generally thought to occur on copper foils.
Electrochemical data indicate that the early steps of methanation
on n-Cu/C involve a pre-equilibrium one-electron transfer to CO<sub>2</sub> to form an adsorbed radical, followed by a rate-limiting
non-electrochemical step in which the adsorbed CO<sub>2</sub> radical
reacts with a second CO<sub>2</sub> molecule from solution. These
nanoscale copper electrocatalysts represent a first step toward the
preparation of practical methanation catalysts that can be incorporated
into membrane-electrode assemblies in electrolyzers
In Situ Transmission Electron Microscopy of Cadmium Selenide Nanorod Sublimation
In
situ electron microscopy is used to observe the morphological evolution
of cadmium selenide nanorods as they sublime under vacuum at a series
of elevated temperatures. Mass loss occurs anisotropically along the
nanorod’s long axis. At temperatures close to the sublimation
threshold, the phase change occurs from both tips of the nanorods
and proceeds unevenly with periods of rapid mass loss punctuated by
periods of relative stability. At higher temperatures, the nanorods
sublime at a faster, more uniform rate, but mass loss occurs from
only a single end of the rod. We propose a mechanism that accounts
for the observed sublimation behavior based on the terrace–ledge–kink
(TLK) model and how the nanorod surface chemical environment influences
the kinetic barrier of sublimation
Elucidating the Reactivity and Mechanism of CO<sub>2</sub> Electroreduction at Highly Dispersed Cobalt Phthalocyanine
Transforming carbon
dioxide to carbon monoxide with electrochemical
methods allows for small-scale, modular conversion of point sources
of carbon dioxide. In this work, through the preparation of a well-dispersed
cobalt phthalocyanine model catalyst immobilized on carbon paper,
we revealed high turnover frequencies for reducing carbon dioxide
at low catalyst loadings, which are obscured at higher loadings due
to aggregation. The low catalyst loadings have also enabled mechanistic
studies that provide a detailed understanding of the molecular-level
picture of how cobalt phthalocyanine facilitates proton and electron
transfers in the rate-limiting step. We are able to tune the rate-limiting
step from electron transfer to concerted proton–electron transfer,
enabling higher rates of carbon dioxide reduction. Our results highlight
the significance of dispersion for understanding the intrinsic catalytic
performance of metal phthalocyanines for electroreduction of CO<sub>2</sub>
Interaction Potentials of Anisotropic Nanocrystals from the Trajectory Sampling of Particle Motion using <i>in Situ</i> Liquid Phase Transmission Electron Microscopy
We demonstrate a generalizable strategy
to use the relative trajectories
of pairs and groups of nanocrystals, and potentially other nanoscale
objects, moving in solution which can now be obtained by <i>in
situ</i> liquid phase transmission electron microscopy (TEM)
to determine the interaction potentials between nanocrystals. Such
nanoscale interactions are crucial for collective behaviors and applications
of synthetic nanocrystals and natural biomolecules, but have been
very challenging to measure <i>in situ</i> at nanometer
or sub-nanometer resolution. Here we use liquid phase TEM to extract
the mathematical form of interaction potential between nanocrystals
from their sampled trajectories. We show the power of this approach
to reveal unanticipated features of nanocrystal–nanocrystal
interactions by examining the anisotropic interaction potential between
charged rod-shaped Au nanocrystals (Au nanorods); these Au nanorods
assemble, in a tip-to-tip fashion in the liquid phase, in contrast
to the well-known side-by-side arrangements commonly observed for
drying-mediated assembly. These observations can be explained by a
long-range and highly anisotropic electrostatic repulsion that leads
to the tip-selective attachment. As a result, Au nanorods stay unassembled
at a lower ionic strength, as the electrostatic repulsion is even
longer-ranged. Our study not only provides a mechanistic understanding
of the process by which metallic nanocrystals assemble but also demonstrates
a method that can potentially quantify and elucidate a broad range
of nanoscale interactions relevant to nanotechnology and biophysics
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Study of Heat Transfer Dynamics from Gold Nanorods to the Environment <i>via</i> Time-Resolved Infrared Spectroscopy
Studying the local solvent surrounding
nanoparticles is important
to understanding the energy exchange dynamics between the particles
and their environment, and there is a need for spectroscopic methods
that can dynamically probe the solvent region that is in nearby contact
with the nanoparticles. In this work, we demonstrate the use of time-resolved
infrared spectroscopy to track changes in a vibrational mode of local
water on the time scale of hundreds of picoseconds, revealing the
dynamics of heat transfer from gold nanorods to the local water environment.
We applied this probe to a prototypical plasmonic photothermal system
consisting of organic CTAB bilayer capped gold nanorods, as well as
gold nanorods coated with varying thicknesses of inorganic mesoporous-silica.
The heat transfer time constant of CTAB capped gold nanorods is about
350 ps and becomes faster with higher laser excitation power, eventually
generating bubbles due to superheating in the local solvent. Silica
coating of the nanorods slows down the heat transfer and suppresses
the formation of superheated bubbles