A detailed elaboration of the transformations of iron species, present in natural and Fe(NO_3)_3 loaded montmorillonite, during carbon deposition and carbon nanotube growth is described. According to transmission electron microscopy results, deposited carbon atoms form fibres in the case of pristine montmorillonite and multiwalled carbon nanotubes in the case of Fe(III) loaded montmorillonite. Mossbauer and x-ray diffraction analysis results point to an extensive reduction of structural and intercalated Fe(III) cations to Fe(II) with the latter migrating from the interlayer space to the vacant octahedral sites of the mineral's lattice. Such migration of the non-structural iron catalyst prohibits extensive contamination of the final composite with various metal catalyst impurities. The crucial role of the active catalytic centres in the formation of carbon nanotubes is ascribed to a minor quantity of iron, found entrapped in the carbon nanostructures, which, at the end of the reaction, is identified as iron carbide. The interesting formation of a nanometric gamma-iron precipitate is also detected, which is probably stabilized through strong interactions with the lattice of montmorillonite. Finally, it is demonstrated that iron-rich natural clay minerals can serve as direct catalysts for carbon nanotube growth.
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