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Organometallics supply carbon and metal catalyst needed for carbon nanotube synthesis. It is shown that experiments involving prior milling of iron-phthalocyanine (FePc, FeC32H16N8) before pyrolysis at 900 oC in argon produces carbon nanotubes with diameters ranging from 5 to 15 nm. Under the same conditions, the diameters of nanotubes produced from non-milled FePc range from 20 to more than 50 nm. This appears to be due to changes in molecular packing of the phthalocyanine precursor. Carbon K-edge near edge X-ray absorption fine structure (NEXAFS) and infrared (IR) spectroscopes show that the packing of benzene subunits of FePc molecules is modified upon milling. Differential scanning calorimetry (DSC) with at heating rates of 1, 5, 10, 25 and 50 deg/min also provides evidence for packing change. The technique shows that the enthalpy (ΔH) and activation energy (ΔE) of sublimation decreases by 7 kJ/mol and 20 kJ/mol, respectively. This decreases the sublimation temperature from about 450 to 200 oC. It is suggested that the decrease in nanotube diameter is due to greater homogeneity in the gas phase on pyrolysis after milling, which leads to more systematic capture of carbon species during the catalytic growth of the carbon nanotubes. |