The reduction in H 2 /CH 4 atmosphere of aluminum-iron oxides produces metal particles small enough to catalyze the formation of single-walled carbon nanotubes. Several experiments have been made using the same temperature profile and changing only the maximum temperature (800-1070°C). Characterizations of the catalyst materials are performed using notably 57 Fe Mössbauer spectroscopy. Electron microscopy and a macroscopical method are used to characterize the nanotubes. The nature of the iron species (Fe 3+ , R-Fe, γ-Fe-C, Fe 3 C) is correlated to their location in the material. The nature of the particles responsible for the high-temperature formation of the nanotubes is probably an Fe-C alloy which is, however, found as Fe 3 C by postreaction analysis. Increasing the reduction temperature increases the reduction yield and thus favors the formation of surface-metal particles, thus producing more nanotubes. The obtained carbon nanotubes are mostly single-walled and double-walled with an average diameter close to 2.5 nm. Several formation mechanisms are thought to be active. In particular, it is shown that the second wall can grow inside the first one but that subsequent ones are formed outside. It is also possible that under given experimental conditions, the smallest (<2 nm) catalyst particles preferentially produce double-walled rather than single-walled carbon nanotubes.

ÐMaterials involved in the catalytic formation of carbon nanotubes are for the ®rst time systematically studied by MoÈ ssbauer spectroscopy between 11 K and room temperature. Mg 1Àx Fe x Al 2 O 4 x 0:1, 0.2, 0.3, 0.4) solid solutions are transformed into carbon nanotubes±Fe/Fe 3 C±MgAl 2 O 4 composite powders by reduction in a H 2 ±CH 4 gas mixture. The oxides are defective spinels of general formulae Mg 2 1Àx Fe 2 xÀ3a Fe 3 2a q a Al 3 2 O 2À 4: Ferromagnetic a-Fe, ferromagnetic Fe 3 C and a g-Fe form, the latter possibly corresponding to a g-Fe±C alloy, are detected in the composite powders. An attempt is made to correlate these results with the microstructure of the powder. It seems that the nanoparticles, which catalyze the formation of the carbon nanotubes, are detected as Fe 3 C in the post-reaction MoÈ ssbauer spectroscopy analysis. Re sumeÂÐDes mate riaux implique s dans la formation catalytique de nanotubes de carbone sont pour la premieÁ re fois syste matiquement e tudie s par spectroscopie MoÈ ssbauer entre 11 K et la tempe rature ambiante. Des solutions solides Mg 1Àx Fe x Al 2 O 4 x 0:1, 0.2, 0.3, 0.4) sont transforme es en poudres composites nanotubes de carbone±Fe/Fe 3 C±MgAl 2 O 4 par re duction dans un me lange gazeux de H 2 et de CH 4 . Les oxydes sont des spinelles lacunaires de formule ge ne rale Mg 2 1Àx Fe 2 xÀ3a Fe 3 2a q a Al 3 2 O 2À 4 : Du Fe-a ferro-magne tique, du Fe 3 C ferromagne tique et une forme de Fe-g, cette dernieÁ re correspondant probablement aÁ un alliage Fe±C-g, sont de tecte s dans les poudres composites. Des corre lations sont faites entre ces re sultats et la microstructure de la poudre. Il semble que les nanoparticules qui catalysent la formation des nanotubes de carbone sont de tecte es comme du Fe 3 C dans l'analyse par spectroscopie MoÈ ssbauer des produits obtenus apreÁ s la re action.