Evaluating the Purity of Single-Wall Nanotube Materials
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Evaluating the Purity of Single-Wall Nanotube Materials A.C. Dillon, T. Gennett†, P.A. Parilla, J.L. Alleman, K.M. Jones, Heben
and M.J.
National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO 80401 † working at NREL on sabbatical from: Chemistry Department, Rochester Institute of Technology, 85 Lomb Memorial Drive, Rochester, New York 146235604, USA 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. INTRODUCTION Single-wall carbon nanotubes were initially produced by the co-evaporation of graphite and metal in an arc-discharge apparatus1, 2. More recently, carbon single-wall nanotubes were produced at high density by direct laser-vaporization of transition-metal / graphite targets at 1200 ˚C 3. Multiple reports of methods to purify these new laser materials quickly followed4, 5, 6. However, a rigorous technique to accurately establish the SWNT content in the final materials was not developed. Knowing the precise content of SWNTs in nanotube soots is essential to quantification in the numerous basic research studies which are currently underway. In certain cases it is also necessary to obtain highly purified materials so that true SWNT characteristics are ascertained. We have developed a simple 3-step purification process. It is coupled to TGA and ICP analyses which enable accurate wt.% estimates of S
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