The role of hydrogen in the growth of carbon nanotubes : a study of the catalyst state and morphology

Abstract

The role of hydrogen in chemical vapor decomposition (CVD) of C2H4 for growth of carbon nanotubes (CNTs) was investigated. Fe/A1203 (1/10 nm) catalyst layers were used for growth on Si substrates and the times at which H2 was introduced during the 40 minute temperature ramp, 15 minute annealing (without C2H4), and 15 minute growth (during which C2H4 was flowing) stages was varied. When H2 was introduced before heating, CNTs grew to a length of [approx.] 0.3 mm. However, CNT growth was severely suppressed when H2 was introduced at different points during temperature ramp. Recovery of CNT growth was observed when H2 was introduced during the annealing and growth stages. Under optimum conditions, an [approx.] 1 mm thick carpet of CNTs could be obtained. The chemical state and morphology of the catalysts as a function of the time of H2 introduction were examined using XPS and AFM, respectively. We found that the as-deposited state of Fe was an iron oxide, due to reaction with 02 in the atmosphere, and that the H2 reduced the iron oxide to different oxidation states, depending on the time of H2 introduction. AFM inspection showed that surface roughness could also be correlated with areas of vertical CNT growth.

(cont.) A preliminary model for CNT growth in which the oxidation state of iron determines its catalytic activity is proposed, and it is argued that the effects of H2 seen in this study are the result of the interplay of H2 reduction and oxidation associated with a low partial of 02 in CVD gases.