Lattice fringe signatures of epitaxy on nanotubes

Carbon nanotubes are of potential interest as heterogeneous catalysis supports, in part because they offer a high surface area hexagonal array of carbon atoms for columnar or epitaxial attachment. Fringe visibility modeling of electron microscope lattice images allows one to investigate the relationship between individual nanoparticles and such nanotube supports. We show specifically how (111) columnar or epitaxial growth of FCC metal lattices, on carbon nanotubes viewed side-on, results in well-defined patterns of (111)-fringe orientations with respect to the tube axis. In the epitaxial case, the observations also provide information on chirality of the nanotube's outermost graphene sheet.Comment: 4 pages, 5 figures, 9 refs, cf. http://newton.umsl.edu/~run/nano/epitaxy.htm

High Performance Cathodes for Li-Air Batteries

The overall objective of this project was to develop and fabricate a multifunctional cathode with high activities in acidic electrolytes for the oxygen reduction and evolution reactions for Li-air batteries. It should enable the development of Li-air batteries that operate on hybrid electrolytes, with acidic catholytes in particular. The use of hybrid electrolytes eliminates the problems of lithium reaction with water and of lithium oxide deposition in the cathode with sole organic electrolytes. The use of acid electrolytes can eliminate carbonate formation inside the cathode, making air breathing Li-air batteries viable. The tasks of the project were focused on developing hierarchical cathode structures and bifunctional catalysts. Development and testing of a prototype hybrid Li-air battery were also conducted. We succeeded in developing a hierarchical cathode structure and an effective bifunctional catalyst. We accomplished integrating the cathode with existing anode technologies and made a pouch prototype Li-air battery using sulfuric acid as catholyte. The battery cathodes contain a nanoscale multilayer structure made with carbon nanotubes and nanofibers. The structure was demonstrated to improve battery performance substantially. The bifunctional catalyst developed contains a conductive oxide support with ultra-low loading of platinum and iridium oxides. The work performed in this project has been documented in seven peer reviewed journal publications, five conference presentations, and filing of two U.S. patents. Technical details have been documented in the quarterly reports to DOE during the course of the project

Synthesis and Composition Evolution of Bimetallic Pd-Pt Alloy Nanoparticles

This paper reports a study on the synthesis of Pd-Pt alloy nanoparticles and composition evolution of the alloys. The synthesis involves Pd and Pt acetylacetonate as the metal precursors and trioctylphosphine (TOP) as the solvent. Thermal decomposition of the Pd-TOP complex resulted in Pd nanoparticles, while substitution of Pt in the Pt-TOP complex by Pd allowed formation of the Pd-Pt alloys. It was observed that the Pd-Pt nanoparticles formed at the very beginning in the synthesis process are Pd rich with various nanoparticle sizes ranging from 1.5 to 25 nm in diameter. These nanoparticles averaged out through a digestive ripening process and reached a final size of 3.5 nm in about 10 min. The alloy compositions evolved throughout the synthesis process and only reached the preset Pd to Pt ratio of the precursors in 120 min. It was found that Pt acetylacetonate alone in TOP cannot produce Pt nanoparticles, which was attributed to the formation of a Pt-TOP complex and a strong coordination of Pt to the phosphine. This observation led us to propose an atomic exchange process between the Pt-TOP complex and the Pd atoms at the nanoparticle surface. As a result, the alloy formation process is limited by a substitution and diffusion rate of the Pt atoms at the surface of the alloy nanoparticles

Methanol Electro-Oxidation on Pt-Ru Alloy Nanoparticles Supported on Carbon Nanotubes

Carbon nanotubes (CNTs) have been investigated in recent years as a catalyst support for proton exchange membrane fuel cells. Improved catalyst activities were observed and attributed to metal-support interactions. We report a study on the kinetics of methanol electro-oxidation on CNT supported Pt-Ru alloy nanoparticles. Alloy catalysts with different compositions, Pt53Ru47/CNT, Pt69Ru31/CNT and Pt77Ru23/CNT, were prepared and investigated in detail. Experiments were conducted at various temperatures, electrode potentials, and methanol concentrations. It was found that the reaction order of methanol electro-oxidation on the PtRu/CNT catalysts was consistent with what has been reported for PtRu alloys with a value of 0.5 in methanol concentrations. However, the electro-oxidation reaction on the PtRu/CNT catalysts displayed much lower activation energies than that on the Pt-Ru alloy catalysts unsupported or supported on carbon black (PtRu/CB). This study provides an overall kinetic evaluation of the PtRu/CNT catalysts and further demonstrates the beneficial role of CNTs

Synthesis and Restructuring of Inorganic Nano-Particles in Counterflow Diffusion Flames

The formation/growth/coagulation/sintering of flame-generated inorganic aggregates at low particle volume fractions (O(10ˉ¹ppm)) was investigated. Al₂O₃ particles synthesized in a Al(CH₃)₃ (TMA)-seeded atmospheric pressure counterflow diffusion flame (CDF) fueled with CH₄/O₂/N₂ were used as the model material/combustion system. Experimental techniques included thermocouple, laser light scattering (LLS) and thermophoretic sampling/Transmission Electron Microscopy (TEM). Local aggregate morphology evolution was characterized in terms of primary particle size, aggregate size, and fractal structure. Additionally, the effects of temperature and TMA concentrations on morphology and size were also investigated systematically in the CDF. Light scattering signals as well as TEM analysis clearly illustrated particle/aggregate size and morphology evolution as a result of two competing processes, with coagulation increasing aggregate sizes, and sintering reducing aggregate surface areas. Mean primary particle diameters were found to be in the range of 13-47 nm, increasing with TMA concentration and sampling position (increasing residence time). On the other hand, mean aggregate sizes reached a maximum at about 4 mm above the bottom fuel duct (corresponding to a local temperature of only 1250 K) and increased with TMA seed level. Fractal dimension and fractal prefactor of alumina aggregates with negligible sintering rates were found to be 1.52 and 2.4, respectively. The final products were larger spherical particles with up to 60 nm diameter, resulting from complete collapse of the aggregates. These observations were shown to be compatible with our independent evaluation of the characteristic times associated with the participating rate processes in this class of two-phase CDFs. Systematic modification of these characteristic times can be used to control the size and morphology of flame-synthesized particles

Fractal Morphology Analysis of Combustion-Generated Aggregates Using Angular Light Scattering and Electron Microscope Images

Experimental studies of the fractal morphology of flame-generated aggregates are described here, considering not only the fractal dimension, Df, but also the fractal prefactor (lacunarity), k,, both of which are shown to be needed to fully characterize aggregates. Measurements were made using angular light scattering (ALS) and thermophoretic sampling followed by transmission electron microscopy (TEM) for soot aerosols found in laminar and turbulent flame environments. Df and the prefactor k, were simultaneously inferred from ALS measurements using the optical properties of aggregates composed of small primary particles. TEM-based inferences of these fractal properties involve analysis of aggregate- projected images from which the actual morphologies are obtained by correlating the radius of gyration to the outer radius (half of the maximum length) of an aggregate. Both of our procedures for determining the detailed morphology of aggregates yield Df = 1.7 f 0.15 and k, = 2.4 f 0.4 for carbonaceous soot, in good agreement with earlier TEM measurements involving multiple angle images. Furthermore, we show that these values also seem to be valid for other materials such as alumina aggregates, suggesting that not only the fractal dimension but also the fractal prefactor are universal properties of aggregates found in combustion environments due to similar mechanisms of aggregation. This universality for Df and k, is observed at various positions in four different flame types with nine various gaseous and liquid fuels for aggregates with mean primary particle radii and number of primary particles in aggregates in the range 12-26 nm and 2-104, respectivel

In Situ Light-Scattering Measurements of Morphologically Evolving Flame-Synthesized Oxide Nanoaggregates

Nonspherical Al₂Ol₃ aggregates produced in a laminar counterflow nonpremixed methane flame were investigated with an in situ laser light-scattering (LLS) technique in combination with a thermophoretic sampling-transmission electron microscope (TS-TEM) method. These flame-synthesized nanoparticles clearly underwent morphological changes following their formation (from precursor trimethylaluminum hydrolysis), mainly as a result of aggregation and sintering processes in the ~3.3 x 10â´ K/s heating environment. To characterize this particulate morphological evolution conveniently we made multiangular absolute LLS measurements and interpreted them based on the Rayleigh-Debye-Gans scattering theory for fractal aggregates. Optically determined fractal dimension D[subscript f], mean radius of gyration, aggregate size distribution, and local particle volume fraction [phi subscript p] were found to be consistent with our independent ex situ TS-TEM experiments. D[subscript f] (optically inferred) increased from 1.60 to 1.84 with axial position, confirming the morphological evolution of alumina aggregates owing to finite-rate, spatially resolved high-temperature sintering. An extension of our TS-TEM method was successfully applied, for the first time to our knowledge, to inorganic particles. [phi subscript p] inferred by means of this ex situ technique generally agreed with that from the in situ LLS technique, supporting our interpretation of both measurements. Moreover, an optically inferred net sintering rate of alumina aggregates approaching the flame was estimated to be consistent with the available TEM data. The LLS methods and results presented here are expected to permit more comprehensive mechanistic analyses of nanoaggregate sintering and coagulation kinetics in such flame environments, ultimately improving the modeling of more-complex (e.g., turbulent, high-pressure) combustion systems involving nanoparticle formation and evolution