(738i) Surfactant Removal and Purification of Single-Wall Carbon Nanotubes

Single-wall carbon nanotubes (SWCNTs) are attractive for a variety of applications including novel nanoelectronic devices, interconnects, advanced wires and cables, and solar cells. Until recently, the use of SWCNTs in such applications has been limited due to the presence of mixed chiralities, electronic-types, and diameters in as-produced materials, however, advances in post-synthesis processing has allowed for successful SWCNT separation. Utilizing processes such as column chromatography and density gradient ultracentrifugation (DGU), as-produced SWCNTs have been separated into their respective metallic and semiconducting species. These techniques rely on the use of surfactants, commonly sodium dodecyl sulfate (SDS) and sodium deoxycholate (DOC), to exploit differences in the surface chemistry or buoyant density of the SWCNTs, respectively. In an effort to fully probe the intrinsic physical properties of electronic-type-separated SWCNTs, complete surfactant removal must be accomplished.

It had previously been reported that filtration followed by copious water washing was sufficient to remove bulk surfactant from SWCNT materials, however, this did not address the residual surfactant adsorbed to the surface of the SWCNTs. More recently, acidification and thermal oxidation processes have been used to aid in further surfactant removal, however, a universal method for surfactant removal and determination of SWCNT purity is required. In the current study, surfactant removal was systematically examined by employing a variety of acids, bases, and organic solvents to 10 µg/mL SWCNT dispersions in either a 2 wt% solution of SDS or DOC. Upon mixing of the desired reagent, the SWCNT dispersions were bath sonicated, immersed in an ice bath, and observed over the course of 48 hours. The reagents that interrupted the SWCNT/surfactant interaction were selected to allow the SWCNTs to be easily filtered from the bulk solution. Based on a qualitative analysis of approximately 20 reagents, five reagents yielded optimal results, including acetone, acetonitrile, N,N-dimethylacetamide, cyclohexylpyrrolidine, and ethanol. The ratio of reagent:SWCNT dispersion was investigated to minimize the amount of solvent needed to precipitate the SWCNTs. Subsequently, the SWCNTs were filtered into bulk papers containing 2 mg of SWCNTs with an areal density of approximately 1 mg/cm². The SWCNT papers were rinsed with the appropriate solvent, thermally oxidized, and were characterized via thermogravimetric analysis (TGA), Raman spectroscopy, optical absorbance spectroscopy, and scanning electron microscopy (SEM). Additionally, electrical characterization was conducted using 4-point probe measurements and the van der Pauw method. The purity of the treated SWCNT samples were quantified in relation to control samples that were rinsed from the surfactant only with water, and to traditionally purified SWCNT papers, which had not been exposed to surfactants. Results demonstrating the successful removal of SDS and DOC from SWCNT dispersions will be discussed.