Carbon Nanotubes by a CVD Method. Part I: Synthesis and Characterization of the (Mg, Fe)O Catalysts

The controlled synthesis of carbon nanotubes by chemical vapor deposition requires tailored and well-characterized catalyst materials. We attempted to synthesize Mg 1 - x Fe x O oxide solid solutions by the combustion route, with the aim of performing a detailed investigation of the influence of the synthesis conditions (nitrate/urea ratio and the iron content) on the valency and distribution of the iron ions and phases. Notably, characterization of the catalyst materials is performed using 57 Fe Mössbauer spectroscopy, X-ray diffraction, and electron microscopy. Several iron species are detected including Fe 2+ ions substituting for Mg 2+ in the MgO lattice, Fe 3+ ions dispersed in the octahedral sites of MgO, different clusters of Fe 3+ ions, and MgFe 2 O 4 -like nanoparticles. The dispersion of these species and the microstructure of the oxides are discussed. Powders markedly different from one another that may serve as model systems for further study are identified. The formation of carbon nanotubes upon reduction in a H 2 /CH 4 gas atmosphere of the selected powders is reported in a companion paper.