Abstract Submitted to the    NANOTUBE'04 Conference:

We have optimized a catalytic process to obtain high-quality SWNT over silica-supported Co-Mo catalysts in the temperature range 700-950^oC. The excellent performance of this catalyst is based on the formation of a surface cobalt molybdate, in which, the Co-Mo interaction inhibits the Co metal agglomeration that typically occurs at high temperatures. When large Co particles are present, less desirable forms of carbon are produced. By contrast, when the Co clusters are very small, selective growth of SWNT occurs. We propose that the SWNT synthesis requires a nucleation step that precedes the growth of nanotubes. This step involves the disruption of Co atoms from its interaction with Mo oxide when the latter becomes carbidic, and it is followed by formation of metallic Co clusters that are responsible for the nanotube growth. The diameter of the tube is determined by the size of the metal cluster, therefore, we can control the nanotube diameter by controlling the growth rate of the Co clusters (i.e., temperature, pressure and composition). For example, spectroscopic characterization (Raman, TEM, fluorescence) indicate that the nanotubes produced at 750C have diameter in the range 0.75-0.80 nm, in which the (6,5) and (7,5) structures constitute most of the semiconducting nanotubes in the sample. In addition to the selective growth, purification and dispersion methods specific for this type of samples have been optimized.