Low-Temperature Phase Transformation Accompanied with Charge-Transfer Reaction of Polyiodide Ions Encapsulated in Single-Walled Carbon Nanotubes

Abstract

In order to understand the reason why polyiodide ions encapsulated in single-walled carbon nanotubes (SWCNTs) drastically improve the electric conductivity and the water dispersibility of SWCNTs at low temperature, we performed in situ Raman measurements of polyiodide ions encapsulated in three kinds of SWCNTs having different mean tube diameters under low temperature down to −100 °C. It was found that, for all the three samples, the Raman peak intensities of polyiodide ions increase and the G-band peak position of SWCNTs shifts toward the higher-wavenumber side with decreasing temperature. It means that the charge-transfer from SWCNTs to encapsulated iodine molecules increases with decreasing temperature and that hole-doping level of SWCNTs increases at low temperature. Furthermore, the peak profiles changed with temperature drastically for polyiodide ions encapsulated in the SWCNT sample having the largest mean tube diameter of 2.5 nm. This change indicates the structural transformation of polyiodide ions in SWCNTs. These experimental results can be explained by the promotion of the chain-like polyiodide ion formation at low temperature. It was discussed with the control experiment using amylose that the promotion of the polyiodide ion formation at low temperature is characteristic for iodine molecules encapsulated in SWCNTs.