Controlled Multistep Purification of Single-Walled Carbon Nanotubes

Abstract

A controlled and scalable multistep purification method has been developed to remove iron impurity and nonnanotube carbon materials from raw single-walled carbon nanotubes (SWNTs) produced in the HiPco (high-pressure CO) process. In this study, iron nanoparticles, coated by carbon, are exposed and oxidized by multiple step oxidation at increasing temperatures. To avoid catalytic oxidation by iron oxide of carbon nanotubes, the exposed and oxidized iron oxide is deactivated by reaction with C 2 H 2 F 4 or SF 6 . The iron fluorides are removed by a Soxhlet extraction with a 6 M HCl solution. The purity and quality of each sample were determined by thermogravimetric analysis (TGA), Raman spectrometry, ultraviolet−visible-near-IR (UV−vis−near-IR) spectrometry, fluorescence spectrometry, and transmission electron microscope (TEM) spectroscopy. The purity and yield of SWNTs are improved due to reduced catalytic activity of the iron oxide. Greater iron oxide removal also resulted from oxidation at higher temperatures.In recent years, single-walled carbon nanotubes (SWNTs) have been intensively studied because of their many potential applications. The high-pressure CO (HiPco) process, where Fe(CO) 5 is used as catalyst, is one of the most productive methods for SWNT production. 1-3 However, the iron and nonnanotube carbon impurities in the produced material need to be removed without damaging the SWNT. To remove catalyst (typically iron, cobalt, and nickel) and obtain highpurity SWNTs, many purification methods have been reported previously. A common approach has been to use strong oxidation followed by an acid treatment. An oxidative treatment of raw SWNT material is effective in removing nonnanotube carbon and exposing the metal catalysts by removing carbon coating. However, nanotubes can be lost or damaged during the oxidation process. It is desirable that a scalable cleaning method only removes carbon impurities and metal catalysts without damaging nanotubes.In this study, we report a scalable multistep purification method to remove metal catalysts and remove nonnanotube carbon from raw HiPco SWNTs. Our scalable multistep purification method includes two processes: oxidation and deactivation of metal oxides. In the oxidation process, metal catalysts coated by nonnanotube carbon are oxidized by O 2 and exposed in multiple steps with increasing temperature steps (150°C through 350°C). In the deactivation step, exposed metal oxides are deactivated by conversion to metal fluorides through reacting with C 2 H 2 F 4 , SF 6 , or other fluorinecontaining gases to avoid the catalytic effect of iron oxide on SWNT oxidation. The proposed mechanism of purification is shown in Scheme 1. Figure 1 shows the thermal gravimetry analyses of raw HiPco SWNTs in N 2 /O 2 , N 2 /O 2 /C 2 H 2 F 4 , and N 2 /O 2 /SF 6 as function of time during heating. Weight gain during the initial heating is caused by the oxidation of iron particles. We found that all raw HiPco material is burned out at 325°C in air within less than 5 min witho...

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