The influence of reagent stoichiometry on the yield and aspect ratio of acid-oxidised injection CVD-grown multi-walled carbon nanotubes

Injection chemical vapour deposition (ICVD) was used for the preparation of multi-walled carbon nanotubes (MWCNTs) with varying lengths and diameters. The concentration of iron injected with the feedstock is related to the resulting average MWCNT dimensions, mass yield, packing density, crystallinity, and iron content. An optimum set of conditions is identified that maintains a constant diameter during growth; higher iron injection concentrations lead to an increasing MWCNT diameter as growth proceeds. Carbon nanotubes are frequently oxidised in mixtures of concentrated nitric and sulphuric acids in order to purify, functionalise, and shorten as-synthesised materials. Here, the carbon:acid stoichiometry is systematically varied, and shown to control the yield and dimensions of the resulting oxidised MWCNTs in a predictable manner, for the various ICVD starting materials. With increasing acid content, the yield of nanotubes decreases, the fraction of oxidised debris increases, and the initial, defect-driven, cutting reaction saturates, whilst thinning reactions continue. The results provide insight into both ICVD synthesis and acid oxidation of MWCNTs, as well as a means to prepare well-dispersed samples with controlled dimensions.