Effects of Solution Properties on Iodine Monobromide Doping for Enhanced Bulk Carbon Nanotube Electrical Conductivity

Abstract

Purified commercially available carbon nanotube (CNT) sheet and yarn materials were chemically doped in solutions of IBr using varying solvents to enhance the CNT bulk electrical conductivity. Time-dependent optical absorption spectroscopy was employed to quantify IBr adsorption onto the CNT samples, and results correlate dopant adsorption with CNT conductivity enhancement. Two independent solvent systems were evaluated (hexanes and ethanol) leading to 40% greater conductivity for CNTs doped with IBr in hexanes compared to CNTs doped with IBr in ethanol. A comparative analysis of IBr in nine solvents with varying polarities at 2.1 g of IBr per liter of solvent was employed to evaluate CNT doping efficacy and supports a mechanism whereby saturated solutions in hexanes and water favor dopant–CNT interactions that shift the equilibrium in favor of dopant adsorption and yield the highest CNT electrical conductivity. Saturated dopant solution loadings of 10–20 g IBr/L hexanes resulted in the maximum electrical conductivity of 0.85 MS/m compared to an initial CNT conductivity of 0.1 MS/m. The optimal IBr doping conditions from the work (60 min exposure of 20.7 g IBr/L hexanes) were applied to commercial CNT yarns leading to an improvement in conductivity of 14× to a value of 1.4 MS/m. High-voltage testing in air shows a 36% increase in maximum current carrying capacity at failure compared to as-received yarn. Thus, the proper combination of dopant and solvent leads to enhanced electrical transport properties in advanced carbon conductors.