On the role of surface oxygen during nascent single-walled carbon nanotube cap spreading and tube nucleation on iron catalysts

Abstract

Single-walled carbon nanotubes (SWCNTs) are commonly grown in oxygen-rich environments. The precursor used as input for the SWCNT synthesis reaction typically provides oxygen atoms either as a pure gas or as an oxygen-containing compound. Similarly, metal oxide substrates are used to support the catalysts or as reactor wall materials and can transfer oxygen atoms while in contact with the catalytic particles. It has been proposed and experimentally observed that the interaction of the metal catalyst with specific promoters (e.g., oxygen, sulfur, hydrogen) triggers significant changes in the SWCNT properties during spreading, nucleation and growth. However, very few studies address the carbon-catalyst surface interactions in rich oxygen environments. Here we show that an increased surface oxygen concentration in the neighborhood of a spreading nascent carbon shell reduces the strength of interaction of the carbon shell with the iron catalyst's surface and increases the probability of carbon nucleation and SWCNT lift-off. The evolution of the carbon-surface interaction during the spreading and nucleation phases suggests that closed pentagons at the rim are preferentially formed in oxidized iron surfaces, and they exhibit the strongest and most stable interaction with the iron surface.