CCVD Synthesis of Single- And Double-Walled Carbon Nanotubes: Influence of the Addition of Molybdenum to Fe−Al2O3 Self-Supported Foams

Powders of R-Al1.8Fe0.2O3 solid solution are prepared by combustion and are attrition-milled with different amounts of ammonium heptamolybdate before being transformed into self-supported foams by impregnation of a polyurethane foam. Carbon nanotubes, mostly single- and double-walled, are prepared by catalytic chemical vapor deposition using the foams as catalytic materials. Extensive characterization reveals that the addition of a small amount of molybdenum first favors the formation of double-walled nanotubes over that of singlewalled nanotubes and second activates smaller nanoparticles, thus producing smaller-diameter nanotubes. A detailed Mo¨ssbauer spectroscopy study reveals that there is no interaction between iron- and molybdenum species, pointing to a role of molybdenum favoring some phenomenon happening in the gas phase, as opposed to any alloying effect