| Abstract
| - The reduction in H2/CH4 atmosphere of aluminum−iron oxides produces metal particles small enough tocatalyze the formation of single-walled carbon nanotubes. Several experiments have been made using thesame temperature profile and changing only the maximum temperature (800−1070 °C). Characterizations ofthe catalyst materials are performed using notably 57Fe Mössbauer spectroscopy. Electron microscopy and amacroscopical method are used to characterize the nanotubes. The nature of the iron species (Fe3+, α-Fe,γ-Fe−C, Fe3C) is correlated to their location in the material. The nature of the particles responsible for thehigh-temperature formation of the nanotubes is probably an Fe−C alloy which is, however, found as Fe3C bypostreaction analysis. Increasing the reduction temperature increases the reduction yield and thus favors theformation of surface-metal particles, thus producing more nanotubes. The obtained carbon nanotubes aremostly single-walled and double-walled with an average diameter close to 2.5 nm. Several formationmechanisms are thought to be active. In particular, it is shown that the second wall can grow inside the firstone 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 carbonnanotubes.
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