Evaluating the Purity of Single-Wall Nanotube Materials

Abstract

Carbon single-wall nanotubes (SWNTs) have a variety of unique physical, electronic and mechanical properties. However, the SWNTs must be thoroughly purified if they are to be used in the wide array of projected applications and basic studies. Although numerous purification schemes have been employed in the literature, none of them provides an accurate estimate of the SWNT content in the final materials. Here we describe a simple 3-step purification process coupled to an accurate method for determining SWNT wt.% contents in both the crude and purified samples. We employ a laser vaporization synthesis technique and take care to avoid both forming graphite-encapsulated metal particles and incorporating sputtered target material into the collected soot. It is then possible to employ a dilute nitric acid reflux to digest the metal particles and to functionalize and redistribute the non-nanotube carbon fractions in the soots forming a uniform and reactive coating on the SWNTs. This coating is selectively removed by oxidation in air at 550 °C. Thermogravimetric analysis (TGA) and inductively coupled plasma spectroscopy (ICPS) are used to evaluate the purity of the material at each step of the process, and illustrate that the crude materials contain 6 wt.% metal and 10-35 wt.% SWNTs. The purified materials are found to be >98 wt.% pure with metal contents of < 1 wt.%. Using the 98 wt.% pure materials as a yardstick, we are able to evaluate the accuracy of several other methods commonly employed in determining nanotube purity levels including transmission and scanning electron microscopies (TEM and SEM) and Raman spectroscopy. We show that determining nanotube contents in various materials requires careful scrutiny and the application of multiple techniques.