Identification of key parameters for the selective growth of single or double wall carbon nanotubes on FeMo/Al2O3 CVD catalysts

The successive organometallic chemical vapor deposition of metallic molybdenum and then iron from their carbonyl precursors at 220 degrees C on gamma-alumina results in the formation of a bi-layered film on the alumina surface. These catalysts might be active for the selective synthesis of single or double wall carbon nanotubes from methane at 900 degrees C provided that (i) a fine tuning of the activation step is performed and (ii) the gas phase composition is carefully controlled. The negative influence of nitrogen on the selectivity towards SWCNT and DWCNT has been evidenced. Addition of hydrogen is harmful to SWCNT growth, whereas a low partial pressure of hydrogen is necessary to grow selectively DWCNT. These results can be rationalized in terms of kinetics of reduction of the active catalytic species. Based on XRD, Raman spectroscopy, XPS, FESEM/EDX and TEM analyses, a general pathway for SWCNT formation on the FeMo/Al2O3 CVD catalysts is proposed that involves the active mixed FeMoO4 oxide phase. (C) 2007 Published by Elsevier B.V.