In-Flight Dimensional Tuning of Metal Nanoparticles by Microplasma Synthesis for Selective Production of Diameter-Controlled Carbon Nanotubes

Diameter-controlled carbon nanotubes (CNTs) are continuously grown in a gas-phase process consisting of two sequential stages. Metal nanoparticles with tunable mean diameters between 2 and 5 nm and standard deviations less than 15% are initially prepared from metallocene vapors using an atmospheric-pressure microplasma. The narrowly dispersed catalyst particles are then introduced with acetylene and hydrogen into a flow furnace reactor to selectively grow single-, double-, triple-, or higher order multiwalled nanotubes. High-resolution transmission electron microscopy and micro Raman spectroscopy reveal the morphology of as-grown CNTs and confirm that the number of walls is controlled by catalyst size. Suspended CNTs are characterized by UV-visible-NIR absorbance spectroscopy and found to contain a high-purity (similar to 75%) of single-walled CNTs when the catalyst diameter is reduced to 2.2 nm.