KOH and NaOH activation mechanisms of multiwalled carbon nanotubes with different structural organisation

Abstract

New information was obtained on the mechanism of porosity development during chemical activation by KOH and NaOH using various multiwalled nanotubes (MWNTs) of different structural organization. The high purity MWNTs were prepared by acetylene decomposition on a cobalt-based catalyst at different temperatures. The obtained samples ranged from MWNTs with well organised graphitic walls to nanotubes with disorganised layers mixed with some pyrolytic carbon when decreasing synthesis temperature. The results of transmission electron microscopy (TEM) observations were linked with gas adsorption measurements and X-ray diffraction data. They show that NaOH is only effective with disordered materials whereas KOH is effective whatever the structural order. After reaction of the poorly ordered precursor with KOH, the nanotubular morphology is completely destroyed, whereas it is preserved when NaOH is used. However for the more ordered materials, the morphology remains unchanged with both reactants. Effects of activation are only seen with KOH, which generated a large concentration of defects in the nanotubes walls. The differences found between KOH and NaOH during activation are related with an additional intercalation step of metallic K or Na produced during the redox reactions. It is shown that metallic K has the ability to be intercalated in all materials in contrast with Na which can only intercalate in the very disorganised ones. The conclusions obtained from the study on ordered nanotubes were confirmed with an ordered carbon black, demonstrating that the structural organization of the carbon precursor is an important parameter which must be taken into account when alkali reactants are used for the activation.