Characterization of single-walled carbon nanotubes for environmental implications (2004)

Adsorption capacities of N2 and various organic vapors on select electricarc and HiPco produced SWNTs were experimentally measured at 77 K and 298 K, respectively. The results indicated that the amount of N2 adsorbed on a SWNT sample depended on the sample purity, methodology and on the sample age. Adsorption capacities of organic vapors (100 10000 ppmv) on SWNTs in humid conditions were much higher than those for microporous activated carbons. These results establish a foundation for additional studies related to potential environmental applications of SWNTs. Introduction A wide range in adsorption surface areas (150-1500 m/g) [1, 2] has been reported for single walled carbon nanotubes (SWNTs) for which the most obvious reason is diversities in sample purity and the structure of nanotubes. Previously we reported that slight changes in measuring N2 adsorption data using conventional techniques and the sample age could also affect the characterization results [3]. Also, the organic vapor adsorption capacities of SWNTs are generally assumed inferior to those of activated carbons in dry gas streams [9]. Here we report that SWNTs exhibit much higher adsorption capacities for organic vapors than microporous activated carbons in humid gas streams. Experimental Purified SWNT samples that contained 95-98 wt% electric-arc (EA) produced (EA95), and ~80 wt% and ~95 wt% HiPco produced (CVD80 and CVD95) SWNTs were selected to reflect the properties of nanotubes (and not impurities) as a function of manufacturing process. The characterization techniques included x-ray diffraction (XRD, λ = 0.154 nm) for analysis of SWNT structure, N2 adsorption at 77K for determining adsorption surface area and porosity, thermogravimetric analyzer coupled with a mass spectrometer (TG-MS) for analysis of SWNT surface chemistry, and gravimetric balance to measure the organic vapor adsorption capacities of the samples. Results and Discussion Aging of SWNTs The XRD results showed differences in morphologies of EA and HiPco samples (Fig.1a). The EA sample exhibited two sharp peaks corresponding to the presence of welldefined structures. The 1.47 nm peak (site 1, Fig.1b) represents the diameter of majority of SWNTs in a heterogeneous SWNT bundle. The 0.34 nm peak corresponds to the average size of grooves present on the periphery of SWNT bundles (site 3, Fig.1b). Sample CVD80, however, did not exhibit peaks corresponding to sites 1 and 3, which is most likely due to the presence of inferior quality of SWNTs (i.e. gross structural defects) produced by a low temperature CVD process [2]. Transmission electron microscopy and Raman spectroscopy analyses of these samples showed that sample EA95 contained SWNTs of three different diameters (1.1 nm to 1.52 nm) with majority being 1.52 nm wide [3]. Sample CVD80, on the other hand, was more heterogeneous as it contained five different sized SWNTs (0.9 nm to 11.75 nm) with most nanotubes being 0.9 nm in diameter. The details of the following discussion are presented elsewhere [3], and only a brief description of results is presented for clarity. The surface area and porosity of samples EA95 and CVD80 were measured by N2 adsorption at 77K. The typical procedure for N2 adsorption requires outgassing of a sample (10 milli torr) at 140C to desorb moisture. For freshly produced (< 1 month aged) SWNTs, increasing the outgassing temperature from 140C to 340C increased the N2 adsorption capacities for both, EA and HiPco, SWNT samples (Fig.2a). Oxidative purification processes are known to open the otherwise close-ended SWNTs [4]. The dangling carbon bonds generated in the process are most likely saturated with carboxylic (-COOH) groups, which can be removed by subjecting nanotubes to high temperatures [5]. In fact, decomposition of acidic functional groups on carbon blacks is known to occur at temperature as low as-200-300 C [6, -8]. Therefore, it is very likely that the purified SWNT samples EA95 and CVD80 contained some open-ended SWNTs that were blocked by functional groups, and increasing the outgassing temperature from 140C to 340C removed some of these functional groups, and thus, increased the availability of nanotubes’ porosity. Fig.2. Effect of outgassing temperature on (a) < 1 month and (b) 7 month aged SWNT samples 0 100 200 300 400 500 0.0 0.2 0.4 0.6 0.8 1.0 Relative Pressure (P/Po) V ol um e ad so rb ed (c m 3 (S TP )/g ) CVD80-1m, 340C 832 m2/g CVD80-1m, 140C 695 m2/g EA95-1m, 340C 424 m2/g EA95-1m, 140C 316 m2/g