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2.6.1. In-Situ TPO/Raman

The combination of TPO and Raman provides interesting details about the structure of nanotubes. For example, the application of this combination of techniques to study raw and purified HipCO materials is illustrated in Figures 25 and 26. The TPO analysis of Figure 25 indicates that the amount of carbon present on the samples was 83% and 99%, respectively; the difference represents the amount of Fe removed in the purification process. A comparison between both profiles shows that there are four different carbonaceous species present on the purified material, each one showing a different behavior toward oxidation by O2. Three of these new carbonaceous species (peak position 600, 700, and 750 0 C in the TPO profile) originated during the purification process since the location of the first maximum present on the profile of the purified material (peak at 400 0C) corresponds to the maximum of the TPO of the raw sample. This would indicate that after the purification step a fraction of the material remains in the same state as the raw sample.

The Raman characterization conducted in combination with the TPO is shown in Figure 26. Raman spectra were obtained after in-situ partial oxidation steps; that is, the spectra were acquired at room temperature and then after oxidation steps at 400, 500, and 630 0C under the same conditions used to get the TPO profiles. The Raman spectra

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