Growth of multi-walled C nanotubes from pre-heated CH4 using Fe3O4 as a catalyst precursor

The present study aims to investigate influence of pre-heating of CH4 on the growth of multi-walled C nanotubes (MWCNTs) on a Si (100) substrate by chemical vapor deposition technique using Fe3O4 powder as catalyst precursor. Reduction behavior of Fe3O4 was also studied in a flowing undiluted CH4 atmosphere in order to gain better insight into MWCNT synthesis. Mass measurements, XRD and thermodynamic analyses were carried out to determine the extent of reduction of Fe3O4 by CH4. It was found that Fe3O4 initially transformed to Fe via FeO within 30 min at 1200 K. Fe3C and C then formed as reaction time increased to 60 min. It was postulated that reduction of Fe3O4 took place by H-2, a product of CH4 decomposition. The overall reactions leading to the formations of Fe and Fe3C phases were proposed using equilibrium thermodynamic analysis and the experimental results. Undiluted CH4 was used to synthesize MWCNTs at temperatures in the range of 1050-1300 K. It was observed that a dense carbon coating was formed at 1300 K owing to self pyrolysis of CH4, while at 1200 K individual MWCNTs were observed on the Si substrate. Growth of MWCNTs did not take place at the temperature range of 1050-1150 K. The use of CH4 pre-heated at 1200 K, however, yielded MWCNTs at this temperature range. Experimental results and thermodynamic analysis of the C-H system (excluding graphite) indicated that pre-heating treatment of CH4 promoted Fe3O4 reduction by H-2 and C formations from active intermediate hydrocarbon species of high molecular weights (especially C6H6). (C) 2012 Elsevier B.V. All rights reserved