Polymorphic transformation of iron-phthalocyanine and the effect on carbon nanotube synthesis

Organometallic compounds such as phthalocyanine are useful precursors for carbon nanotube formation by pyrolysis because they can supply both carbon and metal catalysts needed for synthesis. Prior milling of iron-phthalocyanine (FePc, FeC32H16N8) is investigated and shown to affect the sublimation temperature of the precursor and the nanotube diameter. Without prior milling, a sublimation temperature of 600-650 °C is necessary to produce a sufficient amount of vapors prior to pyrolysis. At that temperature, there is also some decomposition. Milled FePc sublimates at the highest rate at 400-450 °C, where no decomposition occurs. The lower temperature shift of the maximum of the sublimation rate appears to be due to changes in polymorphs upon milling. Carbon K-edge near-edge X-ray absorption fine structure, infrared spectroscopies, and X-ray diffraction analysis show that packing of the phenyl subunits of FePc is modified upon milling and an α-like polymorph is produced. Upon heating, the milled material undergoes polymorphic transformation to a mixture of a and β forms and a third unidentified phase. Above 550 °C, this mixture transforms entirely to the β polymorph. During pyrolysis of the FePc vapors at 900 °C, multiwalled carbon nanotubes (MWCNT) with different diameters are produced between milled and non-milled samples. Transmission electron microscopy shows the average diameter of the MWCNTs produced from the non-milled and milled FePc precursor is about 40-100 nm and 15-50 nm, respectively. It is suggested that the decrease in nanotube diameter caused by the milling of the precursor is due to presence of higher concentrations of un-decomposed FePc molecules with fixed C/Fe atomic ratio in the gas-phase prior to pyrolysis. These results show the importance on the choice of materials for CNT synthesis since small changes in the structure of precursors affect nanotube formation kinetics.