Elucidation of Features in the Separation of Carbon Nanotubes by Gel Media and a System Model to Describe Those Interactions

Abstract

Single Walled Carbon Nanotubes (SWNTs) are an intriguing class of materials because their electronic and optical properties can be finely tuned by selection of nanotubes with specific morphology and certain qualities. For example, SWNTs may exhibit structure-dependent semiconducting or metallic character, with absorbance and emission features ranging from the visible to near infrared regions. In many applications, it has been theorized or demonstrated that SWNT-based technologies and schemes can outperform their counterparts comprised by classical materials. However, such levels of performance tend to require purification of SWNT material to a single atomic structure, or chirality. Purification of SWNTs is complicated by the fact that typical methods of purification cannot separate SWNT by chirality.

Instead, novel methods have been developed which take advantage of small differences in the surfactant shell surrounding SWNTs of varied structure to afford separation. A promising technique among these is hydrogel-based separation of SWNT, wherein certain types of SWNT are selectively adsorbed to a hydrogel medium and later recovered in purified form by induced desorption. While this procedure has afforded the production of many single-structure samples of SWNTs, the process mechanism is not well understood. For instance, it is not known how the gel selects for certain SWNT structures or by what mechanism the gel is able to irreversibly retain some nanotubes and reduce process efficiency.

This work provides an investigation into the process of SWNT-hydrogel separation schemes which employ Sephacryl hydrogels. A series of experiments are reported which provide insight into the distinct processes of SWNT adsorption to and desorption from a hydrogel. First, deconvolution of the roles of gel surface area and volume revealed that SWNT-gel interaction scales with total gel surface are rather than volume, which supports an adsorption-driven mechanism of SWNT separation. Second, study of the effect of charged components in the gel during separation revealed that all stages of the SWNT-gel interaction process are inhibited by increasing charge in the gel. Finally, investigation of the interaction of SWNT with gels of varied porosity revealed that non-porous regions act as the adsorption sites for SWNT, and that increasing levels of porosity mitigate the interaction between SWNT and gel. These results were integrated into a system model of SWNT-hydrogel interaction, which accounts for charge interactions in the system, the role of porous locations on the hydrogel, and reveals unique system pathways which can explain the irreversible retention of SWNTs by Sephacryl gels.