Results and discussion
4.6 Raman spectra
In general, functional groups are expected to be covalently bonded to free radical bonds on the surface of CNTs [Banerjee 2003-1899]
.
Therefore, higher density of free radical bonds is important for allowing more functional groups to form on nanotube surface. In order to evaluate the formation of free radical bonds on nanotube surface by the various treatments, the Raman spectra are presented in Figs. 4-14, 4-15, and 4-17.Fig. 4-14 Raman spectra of the acid-treated MWCNTs under different treatment times (for
Specimens A1 and A2).As shown in Fig.4-14, the Raman spectra of the MWCNTs treated by HNO3/H2SO4
solution for 6 and 9 h are presented with two characteristic peaks which are attributed to the D and G bands. The spectra have been normalized with respect to the G band for comparison.
Suggested by previous studies, the intensity of the D band, at frequencies around 1,344 cm-1, is correlated with structural disorder of CNTs, which originates from the defects including disordered materials, poor graphitization, functionalized carbon, and the amorphous carbon on
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the sidewall of nanotubes [Shaijumon 2007-75, Dillon 2004-691, Jian 2008-230]
. The G band at frequencies around 1,572 cm-1 is activated by the graphite signal [Jian 2008-230]
. It is also suggested that the ID/IG ratio is closely associated with the defect density on the walls of the MWCNTs [Jian
2008-230]
. Therefore, the intensity ratio can be used to evaluate the formation of free radical bonds which are preferential sites for functionalization. The results of the ID/IG ratio are also listed in Table 4-1. The results show that when the as-purchased MWCNTs are treated by the acid for 6 and 9 h, both the ID/IG ratios are 0.96.
Fig. 4-15 Raman spectra of the 5 min ion-treated MWCNTs under different H
2/O2 flow ratios (for Specimens B1 to B5).In order to evaluate the effects of the H2/O2 gas flow ratio on the formation of free radical bonds on the nanotube surface when the MWCNTs are treated by the ion treatment, the Raman spectra of the as-purchased and the ion-treated MWCNTs are presented in Fig.
4-15. As expected, in contrast to the as-purchased sample, all ID/IG ratios are increased after ion treatment with any H2/O2 gas flow ratio. As shown in Fig. 4-16, the ID/IG ratio increases from 1.03 to 1.27 when H2 concentration increases from 0 to 80 vol. %. The H2/O2 gas of 50/0, 40/10, 25/25, 10/40, and 0/50 (sccm/sccm) are equivalent to H2 concentrations of 100, 80, 50, 20, and 0 vol. %, respectively.
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Fig. 4-16 I
D/IG ratio of the 5 min ion-treated MWCNTs vs. H2 concentration in H2/O2 flow.Note that the ion density is very important for the formation of the free radical bonds on the surface and the oxygen cation in the ion stream is crucial for forming oxygen-containing functional groups. By comparing the ionic current, it is found that current increases from 0.12 to 0.47 A while the H2 concentration increases from 0 to 100%. Note that the ionic current correlates with the ion density and is measured by an ammeter connected to the 150 mm diameter process stage. This shows that the ion density of the cation stream increases as H2
concentration increases. This leads to higher ID/IG ratio when H2 concentration of the gas flow is higher. However, this simultaneously reduces [O]/[C] value due to the decreases on oxygen cations in the ion stream. Apart from their involvement in ion bombardment, the generated oxygen cations can also act as highly reactive chemical species which form covalent bonds with the amorphous carbon and then nanotube surface. More specifically, the amorphous carbon layer is more reactive than the cylindrical walls to form volatile products with the oxygen cations. The products are then pumped out by the vacuum system. Thus, as shown in Table 4-1, the ion treatment using a H2/O2 mixture can increase the concentration of oxygenated functional groups whilst also reducing the sp3 value. On the other hand, treatment with pure O2 gas (with the exception of increasing the ID/IG ratio) does not yield any other obvious effects in regard to the sp2 and [O]/[C] values of CNTs when compared with the
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as-purchased MWCNTs. This indicates that a H2/O2 mixture not only facilitates free radical bonds but also promotes covalent bonding in this case. Therefore, even with the addition of 20 vol. % H2 (H2/O2 = 10/40 (sccm/sccm)) in gas flow, there is still a significant removal of amorphous carbon and formation of oxygen-containing groups on the nanotube surface.
In contrast to the results of the MWCNTs treated by HNO3/H2SO4 solution for 9 h (Specimen A2), as listed in Table 4-1, after the MWCNTs are treated by the ion with H2/O2
gas flow ratio of 25/25 (sccm/sccm) for 5 min, the measurement of Raman spectroscopy displays a higher ID/IG ratio (1.07) than that of Specimen A2 (0.96). This increase may reflect the fact that the ion treatment can introduce higher density of free radical bonds than the nitric/sulfuric acid treatment on the surface of the nanotubes effectively. However, it is observed that when the process time of the ion treatment increases to 20 min (Specimen B6), the ID/IG ratio value decreases to 1.00. As suggested by Osswald et al. [Osswald 2007-728]
, this decrease might be due to the removal of the impurities coating on the surface of the MWCNTs.
Raman spectra of the MWCNTs treated by merely dilute acid treatment for 2 h and the two-step process combining the 5 min ion treatment with various H2/O2 gas flow ratios and the dilute acid treatment are presented in Fig. 4-17. The resultant ID/IG ratios are also listed in Table 4-1. The results show that, when the as-purchased MWCNTs are only treated by the dilute acid treatment, both the ID/IG and [O]/[C] ratios can only slightly increase. However, when the MWCNTs are treated by the ion pretreatment and further treated by the dilute nitric acid treatment for 2 h, the MWCNTs can be with much higher ID/IG values. The increase of the ID/IG ratio indicates that higher density of free radical bonds can be induced by the ion pretreatment. This also supports the hypothesis that the ion treatment is very effective in creating free radical bonds on the nanotubes; and free radical bonds could only be introduced on the outer surface by the treatment because the incident ions do not reach the inner shells of
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the nanotubes.
Fig. 4-17 Raman spectra of the 5 min ion-pretreated MWCNTs and followed by a 0.25 M
HNO3 acid treatment (for Specimens C1 to C5).Additionally, as compared the results of Specimen B3 (cf. Table 4-1), when the MWCNTs are treated by the ion pretreatment with H2/O2 gas flow ratio of 25/25 for 5 min and further treated by 0.25 M HNO3 acid for 2 h (Specimen C3), the ID/IG ratio (1.26) of Specimen C3 is apparently higher than that of Specimen B3 (1.07). The [O]/[C] ratio is also increased from 31.1 % (Specimen B3) to 52.4 % (Specimen C3). These results support the fact that, with the ion pretreatment, high amount of oxygen-containing functional groups can be effectively introduced on nanotube surface by the dilute acid treatment. It is also shown that the ID/IG ratios of the MWCNTs treated by the two-step process are varied with different H2/O2 gas flow ratio. However, it is found that there is no correlation between ID/IG ratio and H2 concentration in gas flow.
It should be noted that, as the presence of measurement uncertainty is found to be relative high, Raman spectroscopy is invalid to quantify the formation of free radical bonds and there is still no sufficient evidence to support this. Therefore, in this study, we are using XPS for the functionalization quantification results. However, as suggested by previous studies [Dillon 2004-691, Osswald 2007-728, Jian 2008-230]
, it is still a good indicator to monitor the
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formation of free radical bonds.