Electronic supplementary material

On the merits of Raman spectroscopy and thermogravimetric analysis to asses carbon nanotube structural modifications

R. Schönfelder 1,2*, F. Avilés 1, A. Bachmatiuk 2, J.V. Cauich-Rodriguez 1, M. Knupfer 2, B. Büchner 2, M.H. Rümmeli 2,3

1Centro de Investigación Científica de Yucatán, A.C., Unidad de Materiales,

Calle 43 # 130, Col. Chuburná de Hidalgo, 97200 Mérida, Yucatán, México

2IFW Dresden, PO Box 270116, D-01171 Dresden, Germany

3Technische Universität Dresden, Dresden, Germany

*Corresponding author. Tel: +49 351 4659 740. Fax: +49 351 4659 313.

E-mail:

Figures S1 and S2. Relative peak areas of functional groups measured by FTIR for the examined MWCNTs (S1) and SWCNTs (S2).

Table S1. Weight loss for the examined MWCNTs.

Weight loss (%)
50-150 °C / Weight loss (%)
150-350 °C / Weight loss (%)
350-550 °C
Untreated (U) / 0.1 / 0.4 / 3.0
A / 0.4 / 1.1 / 2.7
B / 0.3 / 1.3 / 0.7
C / 0.3 / 1.1 / 1.0
D / 0.8 / 2.6 / 4.7

As seen in Fig 4a, the thermogram of MWCNTs is conceptually divided into four temperature ranges. Weight losses are reported for the first 3 regions as seen in Table S1. Weight losses between 50 and 150 °C are explained based on physi-adsorbed water and maybe evaporation of a few functional groups present in the CNTs. Dehydration and decarboxylation of bonded water or functionalized groups is expected to occur from 150 to 350 °C [1,2]. Oxidation of amorphous carbon occurs between 350 and 550 °C [3,4]. Finally, above 550 °C oxidation of the graphitic structure of MWCNTs occurs. A quite different behavior is observed in Fig. 4a between untreated and oxidized MWCNTs evidencing the sensitivity of TGA. The behavior of MWCNTs can be better explained with the aid of Table S1, which presents the weight losses corresponding to each temperature range examined in Fig. 4a. During the first region (50 to 150 °C), untreated MWCNTs loose only 0.1% of their weight, which is associated to physi-adsorbed water and/or evaporation of some functional groups. For the same temperature range, a weight loss of 0.3-0.4% was found for samples A, B, and C, while 0.8% weight was lost for sample D. Since all samples were dried under similar conditions before the analysis, this suggests a more hydrophilic behavior of the oxidized MWCNTs as a consequence of the large density of functional groups (see Fig. 1a). Evaporation of a few of those functional groups may also be possible in this temperature range (50 to 150 °C). The loss of functional groups (dehydration and decarboxylation) is more evident for the second region (150 to 350 °C) where the weight loss for as-received MWCNTs was found to be ~ 0.4% while for treatments A, B, and C was 1.1-1.3 %. Once again, a marked difference is observed for treatment D with a weight loss of 2.6%. The increased weight loss of treatment D for this temperature is consistent with the larger density of functional groups observed by FTIR. Weight losses in the range of 350 to 550 °C are related to the oxidation of amorphous carbon. Within this range, 3% weight loss was detected for the as-received MWCNTs. This weight loss was reduced for samples that were treated by acid oxidations A and B down to 2.7% and 0.7%, respectively, given the established purification power of nitric acid to remove some of the amorphous carbon [4]. However, for oxidative treatments that were extended in time (such as for the case of treatment C, which was applied for 6 h, see Table 1) or more aggressive in concentration (treatment D, see Table 1), the weight losses increased back again (1.0% and 4.7%, respectively) with respect to treatment B (0.7 %). This implies that there is a compromise with the use of acids with respect to exposure time and concentration. Moderate exposure times and concentrations eliminate the amorphous carbon and maybe some defective layers existing in the as-produced MWCNTs leaving more crystalline tubes, but once this amorphous carbon is eliminated, continued exposure times or high acid concentrations generate further unstructured carbon, which is less thermally stable than the graphitic structure of MWCNTs. This effect was much stronger for MWCNTs treated with the most concentrated acids (D) than for those exposed to prolonged periods of time (C). Finally, after 550 °C (fourth section in Fig. 4a), a steep decrease in the weight of the sample is observed for all MWCNTs.

Table S2. Weight loss for the examined SWCNTs.

Weight loss
50-150 °C / Weight loss
150-400 °C
Untreated (U) / 0.5 / 6.3
A / 1.1 / 19.8
B / 2.0 / 15.1
C / 0.9 / 21.4
D / 5.5 / 27.1

The weight loss curves of SWCNTs in Fig. 5a was divided only in three regions (instead of four as in the case of MWCNTs), corresponding to physi-adsorbed water evaporation (and maybe loss of a few functional groups) between 50 and 150 °C, loss of functional groups and oxidation of amorphous carbon between 150 and 400 °C, and oxidation of the SWCNT graphitic structure above 400 °C. The weight losses of the first 2 sections are shown in Table S2. The weight loss for as-produced SWCNTs in the first region (50-150 °C) was 0.5%. As for the case of MWCNTs, this weight loss increased to ~ 2% for the samples oxidized by treatments A, B and C and up to 5.5% for the most aggressive treatment (D). This result consistently evidences a more hydrophilic behavior and the presence of more functionalities after the acid treatments, especially for treatment D. In the temperature range from 150 to 400 °C, the weight loss of the as-produced SWCNTs is 6.3%, which is associated to loss of functional groups and amorphous carbon. However, since the amount of functional groups in the as-produced SWCNTs is expected to be minor, it is assumed that the majority of this weight loss comes from the oxidation of amorphous carbon. The weight loss of the SWCNTs treated by A, B, C and D in the temperature range of 150 to 400 °C increased significantly with respect to the as-received ones. This shows a clear increment of functionalized groups which may be convoluted with the creation of new amorphous carbon due to damaging of the nanotubes by the acid treatments. It is worth noticing that the SWCNTs oxidized by treatment D has the highest weight loss in the temperature range of 150 to 400 °C (functional groups) with respect to the as-produced SWCNTs, which is consistent with the FTIR peak area measurements (Fig. S2). The onset of oxidation of the graphitic structure of SWCNTs is expected above 400 °C, although certain amorphous carbon in the SWCNT samples may also still be present at that temperature. It is noticed that the TGA curves of all SWCNTs do not go down to zero at 750 °C (as in the case of MWCNTs), since there are still important quantities of metal catalyst impurities (mainly Pt and Rh) as well as other graphitic species present in the TGA crucible at that temperature.

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