Dispersant Effects in the Selective Reaction of Aryl Diazonium Salts with Single-Walled Carbon Nanotubes in Aqueous Solution

Abstract

Current methods of synthesis for carbon nanotubes (CNTs) usually produce heterogeneous mixtures of different nanotube diameters and thus a mixture of electronic properties. Consequently, many techniques to sort nanotubes according to their electronic type have been devised. One such method involves the chemical reaction of CNTs with aryl diazonium salts. Here we examine the reactions of electric arc produced CNTs (dispersed by a variety of surfactants and polymers in aqueous solution) with 4-bromo-, 4-nitro-, and 4-carboxybenzenediazonium tetrafluoroborate salts in order to find conditions for maximum selectivity. Reactions were monitored through the semiconducting S22 and metallic M11 transitions in the UV–vis–NIR absorbance spectra of the nanotube dispersions. Selectivity was observed to depend heavily on the type of surfactant, the type of diazonium salt and its concentration, the reaction temperature, and the solution pH. Additionally, the surfactant concentration was found to exert a significant influence as the dediazoniation product yields are affected by this parameter. For certain combinations of surfactant and diazonium salt the selectivity is markedly improved, particularly in dispersions of nonionic surfactants Pluronic F-127 and Brij S-100, which are similar in structure. Smaller diameter HiPCO nanotubes were better functionalized in dispersions of Triton X-405. The greater selectivity afforded by these poly(ethylene oxide) containing polymers is postulated to arise from electron donation provided by their ether oxygens. The ionic surfactant sodium dodecyl sulfate was found to display unique behavior in that semiconducting nanotubes were preferentially functionalized at natural pH, likely due charge localization interactions with the surfactant.

The role of sodium dodecyl sulfate concentration in the separation of carbon nanotubes using gel chromatography

“…Raising the pH of the solution is able to deprotonate the surface oxides, thus recovering the nanotubes' optical properties. This behaviour is known to occur in SDS-based suspensions [28], as well as for DNA [29,30] and poly(maleic acid/octyl vinyl ether) [16], however it does not appear to occur in dispersion of CNTs with many other surfactants [31], at least not to the same extent. This is most likely due to the density of the adsorbed surfactant layer and the corresponding ability of that layer to exclude oxygen from the nanotube surface [29].…”

Surfactant Concentration Dependent Spectral Effects of Oxygen and Depletion Interactions in Sodium Dodecyl Sulfate Dispersions of Carbon Nanotubes

“…18,25 In cases where the dispersant forms a wrapping layer that provides a sufficient barrier to oxygen, the optical properties of the CNTs are greatly enhanced. 26 It is suggested that the lability and loose organization of SDS molecules on the nanotube surface allows for such oxidation to take place.…”

Optical absorption response of chemically modified single-walled carbon nanotubes upon ultracentrifugation in various dispersants

“…Ideally, the best dispersion is achieved when all the SWCNTs are individualized and isolated. Different aqueous solutions including ionic, non-ionic, biological and polymeric surfactants have been probed as the liquid media for SWCNT dispersion [6][7][8]. Among them, SDBS has been widely used in the past and it is still considered as a quite efficient medium for dispersing SWCNTs [7].…”

“…Other important SWCNT features can be studied from absorption spectra, including the influence of covalent functionalization on the electronic properties [6]. On the other hand, exfoliation of aggregates and bundles of a given SWCNT powder material during ultrasound treatments in a liquid dispersion medium can be monitored through absorption spectroscopy, and the maximum achievable exfoliation coincides with the maximum absorbance [13].…”