No abstract

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.