Results and discussion
4.5 XPS spectra
In order to investigate the functionalization extent, the C1s spectra of the nitric/sulfuric acid-treated MWCNTs are shown in Fig. 4-9, and the deconvolution results are given in Table 4-1. Note that the [O]/[C] ratio in the table is based on the relative percentage of three carboxyl groups to all the carbon species detected [Ago 1999-8116, Xu 2007-8945]
. As can be seen in Fig. 4-9 and Fig. 3-3, it is observed that when the as-purchased MWCNTs (Fig. 3-3) are treated by HNO3/H2SO4 solution for 6 and 9 h, the C1s spectrum is apparently shift to higher binding energy. More specifically, the deconvolution results reveal that the share percentages of the oxygenated carbon species are higher after the nanotubes are treated by the acid treatment; and the [O]/[C] ratio increases as the process time increases from 6 to 9 h (46.2%
to 52.7% respectively) while the sp3 carbon are reduced to 26.4% and 24.5% and sp2 are reduced to 27.5% and 22.6%, respectively. Suggested by Felten et al. [Felten 2005-074308]
, the sp3 content peaking at 281.5 can be attributed to amorphous carbon.
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(a)
(b)
Fig. 4-9 XPS C
1s spectra of the acid-treated MWCNTs under different treatment times, (a) 6h, and (b) 9 h (for Specimens A1 and A2, respectively).After the as-purchased MWCNTs are treated by the ion treatment for 5 min using various H2/O2 gas flow ratios, the XPS survey spectra are shown in Fig. 4-10. It is noted that the spectra showing the presence of carbon and oxygen on the treated and untreated samples are normalized with respect to C1s intensity for comparison purposes. In contrast to the spectrum of the as-purchased MWCNTs, a higher concentration of oxygen is introduced to the surface of the nanotubes treated by the ion treatment using any gas flow ratio.
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Fig. 4-10 XPS survey spectra of the 5 min ion-treated MWCNTs under different H
2/O2 flow ratios (for Specimens B1 to B5).Fig. 4-11 XPS C
1s spectra of the 5 min ion-treated MWCNTs under different H2/O2 flow ratios (for Specimens B1 to B5).Fig. 4-12 XPS C
1s spectrum of the 20 min ion-pretreated MWCNTs (Specimen B6) and its deconvoluted spectra.54
Table 4-1 Comparisons of the XPS, Raman I
D/IG, and TGA peak temperature for MWCNTs specimens merely treated by ion and/or acid processes.Specimens
# [O]/[C]: total percentage of oxygenated carbon species to the total number of carbon species detected.
+ Corresponding to the peak temperature of weight derivative curve.
*Represents the percentage of each carbon containing species in the MWCNTs specimens.
For a detailed comparison, all C1s spectra of the MWCNT samples are presented in Fig.
4-11 and the deconvolution results are summarized in Table 4-1. After the samples are treated by the ion treatment, the XPS measurements show that the concentrations of the graphite, sp2 and sp3 carbons, and oxygen-containing functional groups are different according to the gas
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mixture composition. Also, it is clear that when the samples of the MWCNTs are treated with a H2/O2 gas flow ratio of 25/25 (sccm/sccm), the highest concentration of oxygenated functional groups is achieved whilst the concentration of sp3 carbon is minimized. Meanwhile, the sp2 carbon is insignificantly decreased.
The C1s spectrum of the MWCNTs treated by the ion treatment with H2/O2 gas flow ratio of 25/25 (sccm/sccm) for 20 min is presented in Fig. 4-12. The results listed in Table 4-1 reveal that the [O]/[C] ratio is extremely high up to 59.8% and the sp3 carbon is only with 13.1% while the morphology (cf. Fig. 4-3f and 4-4f) is almost the same as that of the as-purchased MWCNTs (Fig. 3-2). In contrast to the results of the MWCNTs merely treated by nitric/sulfuric acid for 9 h, the MWCNTs treated by the ion treatment with H2/O2 gas flow ratio of 25/25 (sccm/sccm) for 20 min are with higher [O]/[C] and sp2 values (Table 4-1).
As shown in Fig. 4-13, all XPS C1s spectra of the MWCNTs pretreated by the ECR ion pretreatment with various H2/O2 gas flow ratios and followed by a 0.25 M HNO3 acid treatment for 2 h are presented. It is seen that there are shifting for all MWCNTs specimens on the carbon signals of XPS spectra varied by various flow ratios. The results in Fig. 4-13 and Table 4-1 also show that when the MWCNTs are pretreated with H2/O2 gas flow ratios of 25/25 and 10/40 (sccm/sccm) (for Specimen C3 and C4 respectively) the [O]/[C] ratios are 42.4% and 52.4% while sp3 carbon are reduced to 10.6% and 19.1% respectively. Meanwhile, the sp2 carbon of both the specimen are significantly higher than the results of the MWCNTs only treated by nitric/sulfuric acid for 6 and 9 h (Specimen A1 and A2 specifically). These results depict that the two-step method is significantly capable of facially functionalization and purification for removing the carbon impurities generated by synthesizing process simultaneously on the nanotube surface. In contrast to the MWCNTs merely treated by the ion treatment with H2/O2 gas flow ratio of 25/25 (sccm/sccm) for 5 min (Specimen B3, cf.
Table 4-1), the results support the fact that the two-step method, which combines the ion
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pretreatment with H2/O2 gas flow ratio of 25/25 (sccm/sccm) for 5 min and followed by a 0.25 M HNO3 acid treatment for 2 h (Specimen C3), has higher capability of facially functionalizing and purifying nanotubes. In addition, by comparing the results of the MWCNTs treated by the ion treatment with H2/O2 flow ratio of 25/25 (sccm/sccm) for 20 min (Specimen B6), the [O]/[C] ratio and sp3 values of the MWCNTs treated by the two-step process with using the same H2/O2 gas flow ratio (Specimen C3) are almost the same as that of Specimen B6 but the sp2 carbon of Specimen C3 is significantly higher than that of the Specimen B6. This further confirms that the functionalization and purification capability of this method is significant, only the process time is longer than that of the ion treatment.
Fig.4-13 XPS C
1s spectra of the 5 min ion-pretreated MWCNTs and followed by a 0.25 M HNO3 acid treatment (for Specimens C1 to C5).As presented in Table 4-1, it is shown that the [O]/[C] ratio of the MWCNTs treated only with the 0.25 M HNO3 acid treatment (Specimen A3) is obviously smaller than the results of the MWCNTs that underwent ion pretreatment alone by H2/O2 flow ratio of 25/25 (sccm/sccm) for 5 min. This supports and confirms that in this case, the effect of the ion pretreatment is significant on the functionalization of MWCNTs by the dilute acid treatment.
As compared to the results of the MWCNTs treated by only ion treatment with H2/O2 gas flow ratio of 25/25 and the two-step process with the same gas flow ratio, the HNO3/H2SO4
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acid treatment reveals to be low facially functionalization and purification efficiencies. Thus, it is found that both the two presented methods are effective ways to graft polar functional groups onto the nanotubes while the removal of sp3 and the increase of sp2 are also significant.