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Single-walled carbon nanotubes can be classified as either metallic or semiconducting, depending on their conductivity, which is determined by their chirality. Existing synthesis methods cannot controllably grow nanotubes with a specific type of conductivity. By varying the noble gas ambient during thermal annealing of the catalyst, and in combination with oxidative and reductive species, we altered the fraction of tubes with metallic conductivity from one-third of the population to a maximum of 91%. In situ transmission electron microscopy studies reveal that this variation leads to differences in both morphology and coarsening behavior of the nanoparticles that we used to nucleate nanotubes. These catalyst rearrangements demonstrate that there are correlations between catalyst morphology and resulting nanotube electronic structure and indicate that chiral-selective growth may be possible.

The Ostwald ripening behavior of Fe catalyst films deposited on thin alumina supporting layers is demonstrated as a function of thermal annealing in H2 and H2/H2O. The addition of H2O in super growth of single-walled carbon nanotube carpets is observed to inhibit Ostwald ripening due to the ability of oxygen and hydroxyl species to reduce diffusion rates of catalyst atoms. This work shows the impact of typical carpet growth environments on catalyst film evolution and the role Ostwald ripening may play in the termination of carpet growth.

We use transmission electron microscopy (TEM) to investigate the evolution of the surface structure of Li x Ni0.8Co0.15Al0.05O2 cathode materials (NCA) as a function of the extent of first charge at room temperature using a combination of high-resolution electron microscopy (HREM) imaging, selected area electron diffraction (SAED), and electron energy loss spectroscopy (EELS). It was found that the surface changes from the layered structure (space group R3̅m) to the disordered spinel structure (Fd3̅m), and eventually to the rock-salt structure (Fm3̅m), and that these changes are more substantial as the extent of charge increases. EELS indicates that these crystal structure changes are also accompanied by significant changes in the electronic structure, which are consistent with delithiation leading to both a reduction of the Ni and an increase in the effective electron density of oxygen. This leads to a charge imbalance, which results in the formation of oxygen vacancies and the development of surface porosity. The degree of local surface structure change differs among particles, likely due to kinetic factors that are manifested with changes in particle size. These results demonstrate that TEM, when coupled with EELS, can provide detailed information about the crystallographic and electronic structure changes that occur at the surface of these materials during delithiation. This information is of critical importance for obtaining a complete understanding of the mechanisms by which both degradation and thermal runaway initiate in these electrode materials.

The water-gas shift (WGS) reaction rate per total mole of Au under 7% CO, 8.5% CO(2), 22% H(2)O, and 37% H(2) at 1 atm for Au/Al(2)O(3) catalysts at 180 °C and Au/TiO(2) catalysts at 120 °C varies with the number average Au particle size (d) as d(-2.2±0.2) and d(-2.7±0.1), respectively. The use of nonporous and crystalline, model Al(2)O(3) and TiO(2) supports allowed the imaging of the active catalyst and enabled a precise determination of the Au particle size distribution and particle shape using transmission electron microscopy (TEM). Further, the apparent reaction orders and the stretching frequency of CO adsorbed on Au(0) (near 2100 cm(-1)) determined by diffuse reflectance infrared spectroscopy (DRIFTS) depend on d. Because of the changes in reaction rates, kinetics, and the CO stretching frequency with number average Au particle size, it is determined that the dominant active sites are the low coordinated corner Au sites, which are 3 and 7 times more active than the perimeter Au sites for Au/Al(2)O(3) and Au/TiO(2) catalysts, respectively, and 10 times more active for Au on TiO(2) versus Al(2)O(3). From operando Fourier transform infrared spectroscopy (FTIR) experiments, it is determined that the active Au sites are metallic in nature. In addition, Au/Al(2)O(3) catalysts have a higher apparent H(2)O order (0.63) and lower apparent activation energy (9 kJ mol(-1)) than Au/TiO(2) catalysts with apparent H(2)O order of -0.42 to -0.21 and activation energy of 45-60 kJ mol(-1) at near 120 °C. From these data, we conclude that the support directly participates by activating H(2)O molecules.

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Au/TiO(2) catalysts used in the water-gas shift (WGS) reaction at 120 °C, 7% CO, 22% H(2)O, 9% CO(2), and 37% H(2) had rates up to 0.1 moles of CO converted per mole of Au per second. However, the rate per mole of Au depends strongly on the Au particle size. The use of a nonporous, model support allowed for imaging of the active catalyst and a precise determination of the gold size distribution using transmission electron microscopy (TEM) because all the gold is exposed on the surface. A physical model of Au/TiO(2) is used to show that corner atoms with fewer than seven neighboring gold atoms are the dominant active sites. The number of corner sites does not vary as particle size increases above 1 nm, giving the surprising result that the rate per gold cluster is independent of size.

We have studied the lifetime, activity, and evolution of Fe catalysts supported on different types of alumina: (a) sputter deposited alumina films (sputtered/Fe), (b) electron-beam deposited alumina films (e-beam/Fe), (c) annealed e-beam deposited alumina films (annealed e-beam/Fe), (d) alumina films deposited by atomic layer deposition (ALD/Fe), and (e) c-cut sapphire (sapphire/Fe). We show that the catalytic behavior, Ostwald ripening, and subsurface diffusion rates of Fe catalyst supported on alumina during water-assisted growth or "supergrowth" of single-walled carbon nanotube (SWNT) carpets are strongly influenced by the porosity of the alumina support. The catalytic activity increases in the following order: sapphire/Fe < annealed e-beam/Fe < ALD/Fe < e-beam/Fe < sputtered/Fe. With a combination of microscopic and spectroscopic characterization, we further show that the Ostwald ripening rates of the catalysts and the porosity of the alumina support correlate with the lifetime and activity of the catalysts. Specifically, our results reveal that SWNT carpet growth is maximized by very low Ostwald ripening rates, mild subsurface diffusion rates, and high porosity, which is best achieved in the sputtered/Fe catalyst. These results not only emphasize the connection between catalytic activity and particle stability during growth, but guide current efforts aimed at rational design of catalysts for enhanced and controlled SWNT carpet growth.