Redox Sorting of Carbon Nanotubes

Abstract

This work expands the redox chemistry of single-wall carbon nanotubes (SWCNTs) by investigating its role in a number of SWCNT sorting processes. Using a polyethylene glycol (PEG)/dextran (DX) aqueous two-phase system, we show that electron-transfer between redox molecules and SWCNTs triggers reorganization of the surfactant coating layer, leading to strong modulation of nanotube partition in the two phases. While the DX phase is thermodynamically more favored by an oxidized SWCNT mixture, the mildly reducing PEG phase is able to recover SWCNTs from oxidation and extract them successively from the DX phase. Remarkably, the extraction order follows SWCNT bandgap: semiconducting nanotubes of larger bandgap first, followed by semiconducting nanotubes of smaller bandgap, then nonarmchair metallic tubes of small but nonvanishing bandgap, and finally armchair metallic nanotubes of zero bandgap. Furthermore, we show that redox-induced surfactant reorganization is a common phenomenon, affecting nanotube buoyancy in a density gradient field, affinity to polymer matrices, and solubility in organic solvents. These findings establish redox modulation of surfactant coating structures as a general mechanism for tuning a diverse range of SWCNT sorting processes and demonstrate for the first time that armchair and nonarmchair metallic SWCNTs can be separated by their differential response to redox.

Origin of the Surfactant‐Dependent Redox Chemistry of Single‐Wall Carbon Nanotubes

“…The oxidation of the SWCNTs observed in SDS and SUS solution should alter the density of the coating layer of the surfactants on the SWCNTs because the adsorbability of SWCNTs dispersed by SDS onto the hydrogel is reduced by decreasing the pH value, that is, oxidation with the O 2 /H 2 O couple . Recently, it was also reported that the adsorbability onto hydrogels was reduced by the addition of NaClO . On the basis of these results, we suggest that more oxidized SWCNTs have more surfactant molecules on the SWCNT surfaces.…”

“…[23] Recently, it was also reported that the adsorbability onto hydrogels was reduced by the addition of NaClO. [22] On the basis of these results, we suggest that more oxidized SWCNTsh ave more surfactantmolecules on the SWCNT surfaces. In fact, the MD simulationsr evealed an increased number of the surfactants in the vicinity of the oxidized SWCNT (Table 2).…”

“…We and other researchers have developed a technique of separating SWCNTs into metallic and semiconducting species or each chiral species by using hydrogel columns in SDS solutions . Other separation methods using density gradient ultracentrifugation systems and aqueous two phase systems have been reported, which also require SDS as a coexisting solute. Our previous comprehensive examination of surfactants showed that only SDS and its close analogs are effective in the separation; in addition, SUS is found to be effective in this study.…”

“…For example, although SWCNTse xhibit light absorptiona nd fluorescencei nS DS solutions, the intensities are relativelyw eak compared with other well-used anionic surfactants, such as SDBS, SC, and DOC, which is suggested to partially be associated with the redox chemistry of SWCNTs, described above, and with the exciton energy transfer from small-to large-diameter nanotubes. [14] Interestingly,i tw as reported that separation of the SWCNTsw as successfully performed in SDS solution by using hydrogel columns, [15][16][17][18] density gradientu ltracentrifugation, [19,20] and polymer solutions, [21,22] which enable the extractiono fm etal and semiconductor species, each chiral species, and enantiomers from the SWCNT mixture.S uch separation has also been suggested to occur as ar esult of the redox chemistry of the SWCNTs. [22,23] SDS is thus ak ey factor in elicitingt he band-gap-dependentp roperties of the SWCNTsi na queouss olutions.…”

“…[14] Interestingly,i tw as reported that separation of the SWCNTsw as successfully performed in SDS solution by using hydrogel columns, [15][16][17][18] density gradientu ltracentrifugation, [19,20] and polymer solutions, [21,22] which enable the extractiono fm etal and semiconductor species, each chiral species, and enantiomers from the SWCNT mixture.S uch separation has also been suggested to occur as ar esult of the redox chemistry of the SWCNTs. [22,23] SDS is thus ak ey factor in elicitingt he band-gap-dependentp roperties of the SWCNTsi na queouss olutions. In spite of such unique properties of SDS for the redoxc hemistry of the SWCNTs, little is knownregarding the origin of the uniqueness.…”