Assessment of catalyst particle removal in multi-wall carbon nanotubes by highly sensitive magnetic measurements

The purity of the multi-wall carbon nanotubes (MWCNTs) from catalyst nanoparticles (cobalt and iron) after conventional nitric acid refluxing has been assessed by standard methods and magnetic measurements. X-ray diffraction and energy dispersive X-ray spectroscopy analyses are not suitable since the catalyst amounts quickly become beyond the detection limit of such techniques. Inductively coupled plasma spectroscopy is useful for this purpose, however it requires a large amount of sample and a long sample processing time since the MWCNTs must be completely destroyed. By contrast, magnetic measurements are a fast and nondestructive method to monitor the catalyst content, showing the existence of a small but detectable signal even for the purest sample, still far from its detection limit. Hence, magnetic measurements are extremely sensitive to detect catalyst impurities and can be used as a quick, first order, tool to evaluate the level of magnetic impurities.