Rapid growth of vertically aligned multi-walled carbon nanotubes on a lamellar support

Abstract

Vertically aligned carbon nanotubes offer several advantages over nanotubes synthesized using traditional techniques. An inherent disadvantage of vertically aligned nanotubes is the necessary use of low surface area flat catalyst supports during synthesis that are not easily amenable to scale up. Attempts to synthesize nanotubes within layers of a lamellar support in a scalable manner in a fluidized bed while protecting the catalyst surface from attrition have been limited to lower reaction temperatures and slow growth rates due to rapid particle sintering. This article reports on the synthesis of vertically aligned multi-walled carbon nanotube (V-MWNT) forests with heights over 80 μm and growth rates eight times faster than previously reported in a fluidized bed on lamellar supports. The addition of an Al precursor to Fe and Co catalyst precursors between layers of mica clay was found to play a key role in the promotion of nanotube forest growth. The volume expansion in the catalyst after synthesis of the V-MWNTs was correlated to the average nanotube forest length to obtain a simple bulk descriptor of forest height. Modifications to the reduction and reaction temperatures enabled control over the carbon nanotube diameters. Forests with V-MWNT diameters as large as 33 nm and small as 7 nm were obtained, with the smallest diameter V-MWNTs being double and triple walled.