Structural and Electrode Properties of Iodine Molecules Encapsulated in Single-Walled Carbon Nanotubes

Abstract

Although transparent conductive films such as indium tin oxide (ITO) or fluorine tin oxide (FTO) are in high demand for use in optoelectronic devices, including solar cells, ITO and FTO are expensive and inflexible. On the other hand, we can fabricate transparent, conductive, and flexible films by placing single-walled carbon nanotubes (SWCNTs) on the surface of transparent and flexible polymer films. Such nanotube-based transparent conductive films are expected to replace ITO and FTO. However, the conductivity of pristine SWCNTs is not sufficient for typical transparent conductive film applications. To improve the conductivity, doping alkali metals (K, Rb) or halogen atoms (Br, I) would be an effective technique. Among the doped SWCNTs, iodine-doped SWCNTs are particularly interesting. Iodine doping can lead to a reduction in current resistance of SWCNTs by an order of magnitude, and the iodine-doped SWCNTs are known to be air-stable charge-transfer compounds. However, it is not very easy to control iodine doping level by conventional doping methods. Recently, we found that iodine molecules can be encapsulated (here we write iodine molecule encapsulated in SWCNTs as I@SWCNTs) by an electrochemical method [1]. This method is very easy to control the doping level. We also found that iodine doping improves not only the conductivity but also the dipersibility of SWCNTs. Interestingly, I@SWCNTs were dispersed in water very well at low temperature. In order to investigate the temperature dependence of I@SWCNT dispersion, we measured Raman spectra of SWCNTs encapsulating iodine molecules at low temperatures. Three kinds of SWCNTs having mean tube diameters of 1.0, 1.5 and 2.5 nm were used in this study. For electrochemical iodine doping, we fabricated three-electrode configuration cells: SWCNT working, Pt counter, and Ag/AgCl reference electrodes. Iodine-doping processes were investigated by in situ Raman scattering and synchrotron XRD measurements of SWCNTs under an applied potential. For the in situ measurements, we fabricated special cells. Raman scattering measurements were performed using a micro-Raman spectroscopy system (JASCO) equipped with a temperature control stage (JHT 10036 L). It was found by Raman measurements that G-band peak position of I@SWCNTs shifts toward higher wavenumber side with decreasing temperature. It indicates that charge transfer from SWCNT to iodine molecules increases with decreasing temperature. Raman peaks of polyiodine molecules and their overtone peaks were also observed in the low wavelength region. The peak at around 175 cm-1 and its overtones are assigned to the Raman band of I5 -, while the peak at around 110 cm-1 is assigned to I3 -. The presence of poly iodine molecules was also confirmed by XPS measurements. It is very interesting that the peak intensity ratio of the I5 - Raman band to the G-band of SWCNTs observed at around 1600 cm-1 increased with decreasing temperature. It indicates that I5 - ion formation in SWCNTs by the reaction (5I2+5e→I5 -) is promoted at low temperature. In the conferece, I will discuss the structural changes of iodide ions in SWCNTs having different diameter with decreasing temperature. Reference [1] H. Song, Y. Ishii, A. Al-zubaidi, T. Sakai, S. Kawasaki, Temperature-dependent water solubility of iodine-doped single-walled carbon nanotubes prepared using an electrochemical method, Phys. Chem. Chem. Phys. 15, 5767-5770 (2013).