Effect of acid treatment duration and hydrochloric acid molarity on purification of multi-walled carbon nanotubes

Since the discovery of carbon nanotubes (CNTs) by Iijima in 1991 [1] they have attracted an enormous amount of interest because of their unique physical properties and many potential applications; such as catalyst’s support, hydrogen storage, etc. [2]. For many carbon nanotubes applications, purified nanotube material is often desired. Hence, to exploit the properties of nanotubes the purification is a crucial step in order to remove the metal catalyst from undesired by products without damaging target compound structure. The presence of impurities like amorphous carbon, graphitic particles, catalyst and support materials affect the properties of the nanotubes, and the behavior of any device built from them. Several purification processes, both chemical and physical, have been reported in the literature survey depending on type of CNTs (MWCNTs or SWCNTs), growth method and metal catalyst [3]. The chemical methods separate the synthesis products as a function of their reactivity, generally resulting in CNTs of higher purity but causing remarkable damages to nanotubes morphology [4]. On the other hand, the physical methods separate the synthesis product as a function of their size and are non-destructive not damaging tubes, but are more complex and less effective leading to a lower purity [5, 6]. In order to approach a final product with good purity without wide material loss, it is often useful to combine chemical and physical methods [3]. Although it is well-known that acid treatment are effective in removal of metallic catalysts but at the same time, the acid treatment could create some unwanted damages such as oxidation of tube walls and introduction of chemical functional group to the CNTs [3, 5]. Therefore, finding a suitable condition for purification of CNTs is very important.