Surfactant Removal from Single-Wall Carbon Nanotube Dispersions

Until recently, the use of single-wall carbon nanotubes (SWCNTs) in nanoelectronic devices has been limited due to the presence of both metallic and semiconducting SWCNT electronic-types.  Advances in post-synthesis processing, using techniques such as column chromatography and density gradient ultracentrifugation (DGU), has allowed for successful SWCNT separation.  Many separation techniques rely on the use of surfactants, commonly sodium dodecyl sulfate (SDS), to exploit differences in the surface chemistry or buoyant density of the SWCNTs, respectively.  Therefore, complete surfactant removal from the SWCNT species is critical in order to fully probe the intrinsic physical and electrical properties of the electronic-type-separated SWCNTs. 

     In the current study, an approach to surfactant removal is described whereby purification was systematically examined by employing a variety of acids, bases, and organic solvents.  Upon mixing of the desired reagent, the SWCNT dispersions were bath sonicated, immersed in an ice bath, and observed over the course of 24 hours.  Reagents were qualitatively deemed successful if they interrupted the SWCNT/surfactant interaction, yet did not change the solubility of the surfactant.  Of those examined, five reagents yielded optimal results, including acetone, acetonitrile, N,N-dimethylacetamide, cyclohexylpyrrolidine, and ethanol.  The SWCNTs were filtered into bulk papers, each containing 1 mg of SWCNT material, which was followed by thermal oxidation in air.  Purification temperatures were determined based on the decomposition profile of the as-produced SWCNT paper obtained by thermogravimetric analysis (TGA).  The purity of the treated SWCNT samples was characterized via Raman spectroscopy, optical absorbance spectroscopy, scanning electron microscopy, and TGA.  The use of organic reagents has led to a method for surfactant removal that also reduces filtration time, which may prove useful in large scale batch processing.  Results demonstrating the successful removal of SDS from SWCNT dispersions will be discussed and compared to prevalent strategies previously reported in the literature.