Purification of Multi-Walled Carbon Nanotubes Synthesized by Fluidised-Bed Chemical Vapour Deposition
Carbon nanotubes (CNTs) are crystalline carbon materials with the properties that make them suitable for use in composite materials, field emission devices, molecular electronic devices and many other applications. Fluidised-bed catalytic chemical vapour deposition (CCVD), where the CCVD reaction occurs within a fluidised bed of catalyst particles, has been investigated as a promising technique for the economically viable, large-scale synthesis of CNTs. However, the CNT products synthesised via this technique are a complex mixture of carbon nanotubes, metal catalyst, catalyst support and reaction by-products, e.g. amorphous carbon. Separating the CNTs from this mixture has been problematic to date.
In this work, we report on a multi-step purification process to separate metal catalyst and support materials from carbon nanotubes synthesized via fluidised-bed CCVD. It involves: i) refluxing as-synthesised bed materials in either HNO3, HNO3/H2SO4 (v/v = 1:3) or H2SO4. ii) filtering the samples using a 2-stage membrane filtration system, iii) drying in air and iv) selective oxidation in air. This combination of steps successfully separated the amorphous carbon, metal catalysts and their alumina supports from the carbon nanotubes. Carbon nanotube purity was inferred from TGA measurements. These showed an increase from 2.0wt% in the as-synthesised samples to 40.0wt% in the samples obtained from the purification process of 12-hour H2SO4 reflux, 2-stage filtration, dry in air and gas oxidation. Of particular interest was the successful purification of multi-walled spiral nanotubes, which were identified in the TEM images in both as-synthesised and purified samples.