Spectroscopic Characterization of Carbon Nanotube-Polypyrrole Composites

Abstract

The purpose of this study is to investigate the chemical interaction between carboxyl-functionalized single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) with in situ chemically polymerized polypyrrole (PPy) films (approximately 8 nm) by resonance Raman spectroscopy. The composites (SWNT/PPy and MWNT/PPy) were synthesized using nitric acid functionalized nanotubes as the starting material. The resulting composites were characterized by transmission electronic microscopy (TEM) and by resonance Raman spectroscopy (λexc. = 514.5, 632.8, and 785 nm). The TEM images of the nanocomposites revealed that PPy wrapped around the carbon nanotubes. The resonance Raman data showed evidence of charge transfer between composite moieties and a stronger interaction between the carbon material and the polymer in the SWNT/PPy compared to MWNT/PPy. Specifically, an intense charge-transfer process occurred between PPy and the SWNTs, significantly perturbing the electronic structures of both moieties. Analysis of the SWNTs in the SWNT/PPy spectra showed a softening of the C–C bond in both metallic and semiconducting SWNTs and an increase in the Breit–Wigner–Fano (BWF) contribution to the G-band shape of the metallic SWNTs, which resulted from an increase in the electronic density of the SWNT π band. Accordingly, the resonant Raman spectra of PPy in SWNT/PPy showed an increase in the bipolaron/polaron ratio compared to standard PPy, indicating an increase in the polymer’s doping level. Thus, the resonance Raman data were consistent with an increase in the electronic density of metallic and semiconducting nanotubes and a decrease in the electronic density of the polymer, indicating a PPy → SWNT charge transfer in the composite. Our data suggested the formation of a true composite material in the SWNTs with enhanced conductive properties because the interaction with the SWNTs resulted in the stability of the most doped and conductive form of PPy.