Synthesis of multiwalled carbon nanotubes using methane/air diffusion flames

Abstract

An experimental investigation has been conducted on the synthesis of multiwalled carbon nanotubes (MWCNTs) using methane/air laminar diffusion flames, with an emphasis on the effects of flame temperature and catalyst particle size on nanotube growth. The yield of MWCNTs occurred on a substrate consisting of a 0.4 mm Ni−Cr (60% Ni, 26% Cr, and 14% Fe) catalyst wire connected to a stainless steel grid. The self-formed catalyst particles in the flame were analyzed by energy dispersive X-ray to be mostly nickel oxide with some iron oxide. A high-resolution transmission electron microscope analysis indicates well-graphitized layers of MWCNTs consisting of continuous lattice fringes along the cylindrical section of the nanotube. We studied the effects of flame temperature on nanotube growth by changing the sampling location along the flame height and diluting methane with nitrogen. High temperature was found to favor the growth of large-diameter nanotubes, and nanotube growth was influenced by chemical species environment in the flame. Nitric acid treated Ni−Cr wire produced smaller sized catalyst particles that can be encapsulated within the nanotubes. The growth of well-aligned, bean-sprout-like bundles of nanotubes with catalyst particles pushed up at the tube top was found to occur: the mechanism of this unique synthesis is unknown and needs further study.