Role of Catalyst Oxidation State in the Growth of Vertically Aligned Carbon Nanotubes

Abstract

The impact of gas-phase pretreatment of supported iron-oxide catalyst utilized in aligned carbon nanotube (CNT) growth is studied to understand the correlation between the catalyst oxidation state and the growth characteristics of the aligned CNT forests. By varying the pretreatment conditions from a reducing to an oxidizing environment, notable changes are observed in both the collective CNT array growth behavior and the individual CNT characteristics. Although the greatest catalytic activity was observed following a full reduction to the zerovalent (metallic) Fe catalyst, growth is also observed from a catalyst composed of both Fe2O3 and Fe3O4 particles. XPS core-level analysis, following pretreatment of the catalyst, emphasizes the critical nature of the combined catalyst–underlayer interaction to achieve optimal catalyst activity during growth and hence the most efficient catalyst reduction process. Additionally, CNT diameters during growth were strongly affected by the pretreatment process. Overall, this work gives a collective picture of how the catalyst oxidation state affects the CNT growth based on the catalyst pretreatment environment and the nature of the catalyst–underlayer interactions. Such concepts are critical for the rational design of alternative catalyst–underlayer systems for efficient CNT synthetic processes.