Fe/Co Alloys for the Catalytic Chemical Vapor Deposition Synthesis of Single-and Double-Walled Carbon Nanotubes(CNTs).1.The CNT-Fe/Co-MgO System

摘要

Mg_(0.90)Fe_xCo_yO(x + y = 0.1)solid solutions were synthesized by the ureic combustion route.Upon reduction at 1000 deg C in H_2-CH_4 of these powders,Fe/Co alloy nanoparticles are formed,which are involved in the formation of carbon nanotubes,which are mostly single and double walled,with an average diameter close to 2.5 nm.Characterizations of the materials are performed using ~(57)Fe Mossbauer spectroscopy and electron microscopy,and a well-established macroscopic method,based on specific-surface-area measurements,was applied to quantify the carbon quality and the nanotubes quantity.A detailed investigation of the Fe/Co alloys' formation and composition is reported.An increasing fraction of Co~(2+)ions hinders the dissolution of iron in the MgO lattice and favors the formation of MgFe_2O_4-like particles in the oxide powders.Upon reduction,these particles form alpha-Fe/Co particles with a size and composition(close to Fe_(0.50)Co_(0.50))adequate for the increased production of carbon nanotubes.However,larger particles are also produced resulting in the formation of undesirable carbon species.The highest CNT quantity and carbon quality are eventually obtained upon reduction of the iron-free Mg_(0.90)Co_(0.10)O solid solution,in the absence of clusters of metal ions in the starting material.