Catalyst and catalyst support morphology evolution in single-walled carbon nanotube supergrowth: Growth deceleration and termination

Detailed understanding of growth termination in vertically aligned single-walled carbon nanotubes (SWNTs) made via supergrowth, or water-assisted growth, remains critical to achieving the ultralong SWNTs necessary for next-generation applications. We describe the irreversible catalyst morphology evolution that occurs during growth, and which limits the lifetime of surface supported catalysts. Growth termination is strongly dependent on growth temperature, but not sensitive to C₂H₂:H₂O ratio. In addition to both planar Ostwald ripening of small (sub-5 nm) Fe catalyst particles and diffusion of metal atoms into the alumina support, other features that contribute to growth termination or deceleration are described, including center-of-mass particle motions and coalescence of smaller species of surface supported Fe nanoparticles. Additionally, a temperature-induced structural transition in the alumina catalyst support is found to be coincident with abrupt growth termination at temperatures of 800 °C and higher. In situ electron microscopy observations are used to directly support these observations.