Enhanced thermal stability of multi-walled carbon nanotubes after coating with polyaniline salt

Abstract

Multi-walled carbon nanotubes (MWCNT) were coated by polyaniline (PANI) in situ during the oxidation of aniline with ammonium peroxydisulfate in a mixture of ethanol (50 vol.%)–water. PANI alone and PANI-coated MWCNT were carbonized under nitrogen atmosphere at 650 °C and the structural changes were studied by infrared spectroscopy and Raman scattering using 633 and 785 nm excitation lines. MWCNT coated with carbonized PANI salt exhibit a Raman spectrum close to that of the pristine MWCNT before heating. In contrast, the spectra of MWCNT and MWCNT coated with PANI base correspond after exposure to 650 °C to a more disordered carbon-like material according to their Raman spectrum. This means that the coating with PANI salt has a protective effect on the structure of MWCNT when exposed to elevated temperature. During heating, the overlayer of PANI salt thermally deprotonates to PANI base at first, and subsequently converts to a carbonized PANI coating. The integrity of the coating is not damaged. When the PANI coating was chemically deprotonated to PANI base before heating, the accompanying volume contraction reduced the contact between MWCNT and PANI base coating and introduced defects, such as cracks. The protective effect of PANI coating was then lost. The possible interactions between PANI and MWCNT are discussed on the basis of Raman spectra.