Conversion of conducting polypyrrole nanostructures to nitrogen-containing carbons and its impact on the adsorption of organic dye

Abstract

New types of materials were produced by gradual heating of a conducting polymer, polypyrrole, to elevated temperatures. Three polymers differing in morphology – globules, nanofibers, and nanotubes – were exposed to temperatures from 100 to 700 °C in an argon atmosphere. The yields always exceeded 50 wt%, and the morphological features of the polymer were preserved. The transformation of polypyrrole salts to the corresponding bases followed by the carbonization was monitored by FTIR spectroscopy. The elemental analysis confirmed the subsequent conversion of polypyrrole to nitrogen-containing carbon. The specific surface areas were of the order of tens of m² g⁻¹; they increased from globules to nanotubes and nanofibers but were virtually independent of the exposition temperature. The conductivity of the powders was compared with that of the pellets when their preparation was possible. As the temperature was increased up to 400 °C, the conductivity decreased for all samples by ca. 5 orders of magnitude, e.g., for nanofibers from 10 to 10⁻⁴ S cm⁻¹ but recovered to 10⁻¹ S cm⁻¹ after the subsequent carbonization up to 700 °C. Polypyrroles exposed to various temperatures were then tested for the adsorption of organic dye, Reactive Black 5, from water. The dye adsorption on original polypyrroles strongly depended on the polymer morphology. Polypyrrole nanofibers were able to remove the dye completely with a capacity of 100 mg g⁻¹, while the adsorption on polypyrrole globules was poor. The adsorption efficiency thus increased from globules to nanotubes and nanofibers. The adsorption performance was reduced after the carbonization, but the general trends were preserved.