Polyelectrolyte-Assisted Solubilization and Metal Decoration of Carbon Nanotubes. Dual role of PEDOT-PSS.

M. SERGIO MORENO; HORACIO J. SALAVAGIONE; JUAN C. HERNÁNDEZ; PAUL A. MIDGLEY; EMILIA MORALLÓN; CESAR BARBERO; GUSTAVO M. MORALES

Rosario - Argentina

Congreso; 10th Inter-American Congress of Electron Microscopy 2009 CIASEM 2009; 2009

Inter-American Committee of Societies for Electron Microscopy

Molecular engineering of carbon nanotubes (CNTs) will play a vital role in developing new applications. Non-covalent functionalization of CNTs is of interest because it enables to tailor their properties preserving nearly all of the intrinsic properties of CNTs. We report a new approach to non-covalent engineering of the surface of multi-walled CNTs (MWCNTs). MWCNTs were wrapped by PEDOT-PSS, a symplexe formed by a conjugated olygomer and a water-soluble polyelectrolyte. The strategy yields a product free of carbonaceous impurities and stable in water solution by months. The conjugated olygomer present in the composite can be used to reduce cations in solution to metallic nanoparticles (MeNPs), which are deposited on the sidewalls of the wrapped CNT to produce the hybrid MWCNT/PEDOT-PSS/MeNPs. The composite was synthesized by dispersing under continuous stirring as-received MWCNTs in an aqueous solution of PEDOT-PSS. Afer 48 h, MWCNT/PEDOT-PSS was purified by centrifugation. Dispersions of the MWCNT/PEDOT-PSS composite in water were analyzed on the time by dynamic light scattering (DLS), the hydrodynamic diameter and size distribution estimated by DLS was constant for more than one month. In order to explore the possibility to deposit MeNPs on the surface of the MWCNT the reduction of Pt4+ was attempted. The purified MWCNT/PEDOT-PSS was redispersed in a solution of H2Cl6Pt. after 24 h the mixture was centrifuged to yield MWCNT/PEDOT-PSS/PtNPs. It should be noticed that using a similar methodology Au and Pd has been also deposited on the CNT. High resolution TEM (HRTEM) was done using a Philips CM 200 microscope operated at 200 kV. Energy-loss spectra were measured with a collection semiangle of ~ 6 mrad. A Tecnai F20 (G2) TEM was used coupled to a GIF 2002 spectrometer and operated at 200 kV, with the specimen at room temperature. The energy resolution was 1.0 eV. In Figure 1 are shown the starting CNT (left) and the polymer processed ones (right), the inset present dispersions in water of both materials respectively. From the observed solubility and the image we can appreciate that polymer wrapping gave water-dispersable MWCNTs free of carbonaceous species. The polymer-wrapped nanotubes were characterized by HRTEM, EELS and energy-filtered TEM (EFTEM). Figure 2 left shows an image of a wrapped CNT. The bright-field image shows an amorphous layer surrounding the pure-carbon core which is attributed to the polymer. Our general observation was that the thickness of the amorphous layer is approximately constant in the straights parts of the CNT but increases in the bended or deformed regions. In order to confirm that the amorphous layer corresponds to the polymer we have started by studying the spatial distribution of the non-graphitic phase by EFTEM since a ratio of energy filtered images allows to distinguish between the graphitic and amorphous C. In the places were we found the amorphous layer (figure 3) we look for the presence of especific cations corresponding to the polymer like oxygen by using Electron Energy-Loss Spectroscopy (EELS). In the inset is shown the O-K edge EEL spectrum acquired. EELS clearly reveal the presence of oxygen, confirming that the amorphous layer wrapped around the CNT´s is the polymer and not other carbon material. A characterization by Raman spectroscopy (not showed) confirms this finding. Figure 2 (right) shows a TEM image of Pt NPs anchored onto the external walls of the MWCNT/PEDOT-PSS. Pt NPs are deposited throughout the full length of the CNTs. Although some aggregates could be distinguished, most of the NPs have a diameter smaller than five nanometres.