Iron changes in natural and Fe(III) loaded montmorillonite during carbon nanotubegrowth

Abstract

A detailed elaboration of the transformations of iron species, present in natural andFe(NO3)3 loaded montmorillonite, during carbon deposition and carbon nanotube growthis described. According to transmission electron microscopy results, depositedcarbon atoms form fibres in the case of pristine montmorillonite and multiwalledcarbon nanotubes in the case of Fe(III) loaded montmorillonite. Mössbauer andx-ray diffraction analysis results point to an extensive reduction of structural andintercalated Fe(III) cations to Fe(II) with the latter migrating from the interlayerspace to the vacant octahedral sites of the mineral’s lattice. Such migration of thenon-structural iron catalyst prohibits extensive contamination of the final compositewith various metal catalyst impurities. The crucial role of the active catalyticcentres in the formation of carbon nanotubes is ascribed to a minor quantity ofiron, found entrapped in the carbon nanostructures, which, at the end of thereaction, is identified as iron carbide. The interesting formation of a nanometricγ-iron precipitate is also detected, which is probably stabilized through stronginteractions with the lattice of montmorillonite. Finally, it is demonstrated thatiron-rich natural clay minerals can serve as direct catalysts for carbon nanotubegrowth.