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_3−δ 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_3−δ 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 CO₂. 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 CO₂ 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 CO₂. The simplest hydrocarbon target is methane, which can utilize existing infrastructure for natural gas storage, distribution, and consumption. Electrochemical methods for methanizing CO₂ 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 CO₂ to form an adsorbed radical, followed by a rate-limiting non-electrochemical step in which the adsorbed CO₂ radical reacts with a second CO₂ 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 CO₂ 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 CO₂

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

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