Site-selective CO disproportionation mediated by localized surface plasmon resonance excited by electron beam.

Recent reports of hot-electron-induced dissociation of small molecules, such as hydrogen, demonstrate the potential application of plasmonic nanostructures for harvesting light to initiate catalytic reactions. Theories have assumed that plasmonic catalysis is mediated by the energy transfer from nanoparticles to adsorbed molecules during the dephasing of localized surface plasmon (LSP) modes optically excited on plasmonic nanoparticles. However, LSP-induced chemical processes have not been resolved at a sub-nanoparticle scale to identify the active sites responsible for the energy transfer. Here, we exploit the LSP resonance excited by electron beam on gold nanoparticles to drive CO disproportionation at room temperature in an environmental scanning transmission electron microscope. Using in situ electron energy-loss spectroscopy with a combination of density functional theory and electromagnetic boundary element method calculations, we show at the subparticle level that the active sites on gold nanoparticles are where preferred gas adsorption sites and the locations of maximum LSP electric field amplitude (resonance antinodes) superimpose. Our findings provide insight into plasmonic catalysis and will be valuable in designing plasmonic antennas for low-temperature catalytic processes.

Temperature Calibration for In Situ Environmental Transmission Electron Microscopy Experiments.

In situ environmental transmission electron microscopy (ETEM) experiments require specimen heating holders to study material behavior in gaseous environments at elevated temperatures. In order to extract meaningful kinetic parameters, such as activation energies, it is essential to have a direct and accurate measurement of local sample temperature. This is particularly important if the sample temperature might fluctuate, for example when room temperature gases are introduced to the sample area. Using selected-area diffraction (SAD) in an ETEM, the lattice parameter of Ag nanoparticles was measured as a function of the temperature and pressure of hydrogen gas to provide a calibration of the local sample temperature. SAD permits measurement of temperature to an accuracy of ±30°C using Ag lattice expansion. Gas introduction can cause sample cooling of several hundred degrees celsius for gas pressures achievable in the ETEM.

Nucleation of graphene and its conversion to single-walled carbon nanotubes.

We use an environmental transmission electron microscope to record atomic-scale movies showing how carbon atoms assemble together on a catalyst nanoparticle to form a graphene sheet that progressively lifts-off to convert into a nanotube. Time-resolved observations combined with theoretical calculations confirm that some nanoparticle facets act like a vice-grip for graphene, offering anchoring sites, while other facets allow the graphene to lift-off, which is the essential step to convert into a nanotube.

Temperature-programmed electrospray-differential mobility analysis for characterization of ligated nanoparticles in complex media.

An electrospray-differential mobility analyzer (ES-DMA) was operated with an aerosol flow-mode, temperature-programmed approach to enhance its ability to characterize the particle size distributions (PSDs) of nanoscale particles (NPs) in the presence of adsorbed and free ligands. Titanium dioxide NPs (TiO2-NPs) stabilized by citric acid (CA) or bovine serum albumin (BSA) were utilized as representative systems. Transmission electron microscopy (TEM) and inductively coupled plasma mass spectrometry were used to provide visual information and elemental-based PSDs, respectively. Results show that the interference resulting from electrospray-dried nonvolatile salt residual nanoscale particles (S-NPs) could be effectively reduced using the thermal treatment process: PSDs were accurately measured at temperatures above 200 °C for CA-stabilized TiO2-NPs and above 400 °C for BSA-stabilized TiO2-NPs. Moreover, TEM confirmed the volumetric shrinkage of S-NPs due to thermal treatment and also showed that the primary structure of TiO2-NPs was relatively stable over the temperature range studied (i.e., below 700 °C). Conversely, the shape factor for TiO2-NPs decreased after treatment above 500 °C, possibly due to a change in the secondary (aggregate) structure. S-NPs from BSA-stabilized TiO2-NPs exhibited higher global activation energies toward induced volumetric shrinkage than those of CA-stabilized TiO2-NPs, suggesting that activation energy is dependent on ligand size. This prototype study demonstrates the efficacy of using ES-DMA coupled with thermal treatment for characterizing the physical state of NPs, even in a complex medium (e.g., containing plasma proteins) and in the presence of particle agglomerates induced by interaction with binding ligands.

Shape-controlled Au particles for InAs nanowire growth.

We present a study of InAs nanowire (NW) growth with shape-controlled Au seed particles. In comparison to more conventional spherical particles, the highly faceted, shaped Au particles are found to enhance the initial growth kinetics of InAs NWs at identical growth conditions. Analysis of the NWs after growth by transmission electron microscopy and energy-dispersive spectroscopy suggests that while In diffuses into the bulk of the shaped Au particles, in accordance with the vapor-liquid-solid (VLS) growth mechanism, the surface faceting is preserved. A key difference is that the shaped Au particles are characterized by a thicker In shell on their surfaces than the spherical Au particles, indicating that increased adsorption of In leads to the observed growth rate enhancement. On the basis of these results, we propose that our picture of VLS growth in regards to liquefaction and droplet formation is incomplete and that the initial particle morphology can be used to tailor NW growth.

Determination of atomic positions from time resolved high resolution transmission electron microscopy images.

For many reaction processes, such as catalysis, phase transformations, nanomaterial synthesis etc., nanoscale observations at high spatial (sub-nanometer) and temporal (millisecond) resolution are required to characterize and comprehend the underlying factors that favor one reaction over another. The combination of such spatial and temporal resolution (up to 600 µs), while rich in information, produces a large number of snapshots, each of which must be analyzed to obtain the structural (and thereby chemical) information. Here we present a methodology for automated quantitative measurement of real-time atomic position fluctuations in a nanoparticle. We leverage a combination of several image processing algorithms to precisely identify the positions of the atomic columns in each image. A geometric model is then used to measure the time-evolution of distances and angles between neighboring atomic columns to identify different phases and quantify local structural fluctuations. We apply this technique to determine the atomic-level fluctuations in the relative fractions of metal and metal-carbide phases in a cobalt catalyst nanoparticle during single-walled carbon nanotube (SWCNT) growth. These measurements provided a means to obtain the number of carbon atoms incorporated into and released from the catalyst particle, thereby helping resolve carbon reaction pathways during SWCNT growth. Further we demonstrate the use of this technique to measure the reaction kinetics of iron oxide reduction. Apart from reducing the data analysis time, the statistical approach allows us to measure atomic distances with sub-pixel resolution. We show that this method can be applied universally to measure atomic positions with a precision of 0.01 nm from any set of atomic-resolution video images. With the advent of high time-resolution direct detection cameras, we anticipate such methods will be essential in addressing the metrology problem of quantifying large datasets of time-resolved images in future.

Direct evidence of atomic-scale structural fluctuations in catalyst nanoparticles.

Rational catalyst design requires an atomic scale mechanistic understanding of the chemical pathways involved in the catalytic process. A heterogeneous catalyst typically works by adsorbing reactants onto its surface, where the energies for specific bonds to dissociate and/or combine with other species (to form desired intermediate or final products) are lower. Here, using the catalytic growth of single-walled carbon nanotubes (SWCNTs) as a prototype reaction, we show that the chemical pathway may in-fact involve the entire catalyst particle, and can proceed via the fluctuations in the formation and decomposition of metastable phases in the particle interior. We record in situ and at atomic resolution, the dynamic phase transformations occurring in a Cobalt catalyst nanoparticle during SWCNT growth, using a state-of-the-art environmental transmission electron microscope (ETEM). The fluctuations in catalyst carbon content are quantified by the automated, atomic-scale structural analysis of the time-resolved ETEM images and correlated with the SWCNT growth rate. We find the fluctuations in the carbon concentration in the catalyst nanoparticle and the fluctuations in nanotube growth rates to be of complementary character. These findings are successfully explained by reactive molecular dynamics (RMD) simulations that track the spatial and temporal evolution of the distribution of carbon atoms within and on the surface of the catalyst particle. We anticipate that our approach combining real-time, atomic-resolution image analysis and molecular dynamics simulations will facilitate catalyst design, improving reaction efficiencies and selectivity towards the growth of desired structure.

Nanocatalyst shape and composition during nucleation of single-walled carbon nanotubes.

The dynamic evolution of nanocatalyst particle shape and carbon composition during the initial stages of single-walled carbon nanotube growth by chemical vapor deposition synthesis is investigated. Classical reactive and ab initio molecular dynamics simulations are used, along with environmental transmission electron microscope video imaging analyses. A clear migration of carbon is detected from the nanocatalyst/substrate interface, leading to a carbon gradient showing enrichment of the nanocatalyst layers in the immediate vicinity of the contact layer. However, as the metal nanocatalyst particle becomes saturated with carbon, a dynamic equilibrium is established, with carbon precipitating on the surface and nucleating a carbon cap that is the precursor of nanotube growth. A carbon composition profile decreasing towards the nanoparticle top is clearly revealed by the computational and experimental results that show a negligible amount of carbon in the nanoparticle region in contact with the nucleating cap. The carbon composition profile inside the nanoparticle is accompanied by a well-defined shape evolution of the nanocatalyst driven by the various opposing forces acting upon it both from the substrate and from the nascent carbon nanostructure. This new understanding suggests that tuning the nanoparticle/substrate interaction would provide unique ways of controlling the nanotube synthesis.

Centrifugal Shape Sorting of Faceted Gold Nanoparticles Using an Atomic Plane-Selective Surfactant.

Highly refined shape populations of gold nanoparticles (AuNPs) are important for emerging applications in catalysis, plasmonics, and nanomaterials growth. To date, research efforts have focused on achieving monodisperse shape by synthetic control or postsynthetic processing that relies on centrifugal sedimentation-based sorting schemes where differences in the particle mass and aspect ratios (e.g., rods and spheres) provide a driving force for separation. Here, we present a technique to reversibly modify the sedimentation coefficients of AuNPs possessing different shapes that would otherwise be virtually indistinguishable during centrifugal sedimentation due to their similar densities, masses, and aspect ratios by exploiting the preferential affinity of the surfactant cetyltrimethylammonium bromide (CTAB) for the Au(100) facet. The resulting tailored sedimentation coefficients enable AuNP shape sorting via density gradient centrifugation (DGC). DGC-refined populations of faceted AuNPs are shown to significantly enhance the growth rate of InAs nanowires when used as seed particles, emphasizing the importance of shape control for nanomaterials growth applications.