2.3 Characterization
3.1.2 Optical spectroscopic results
In the investigation of diameter selectivity of addition reactions on carbon nano-tubes, the key tool is wide range optical spectroscopy, since exact, quasi-quantitative conclusions can be drawn from the special features of the spectra [87–89].
The most precise procedure to extract changes from solid-state spectra is to compare the optical conductivity. This quantity is additive when several independent processes are involved, like light absorption by different nanotubes, and its calculation takes into account the reflectance at the interfaces, which can heavily influence the optical density calculated from transmittance [84]. Contrary to Raman spectroscopy, there is no resonance process which prefers certain nanotubes over others with selective increase in the scattering intensity.
The energy of interband transitions scales inversely with tube diameter. These ener-gies are very close to each other if the tube diameters are so, which causes an enlarged width of the peaks. Upon addition reactions, sp3 C atoms are formed in the nano-tube sidewall, which decreases the intensity of interband transitions, because a smaller number of electrons will be involved in the excitation process. The change in the peak shape of a functionalized sample compared to its starting material and reference sample reflects the relative amount of electrons localized on sp3 orbitals in the nanotubes with different diameters.
The results of wide range optical transmission measurements on samples prepared by modified Birch reduction are shown in Figure 3.5. They confirm the above sketched picture of quasi-homogeneous reaction on loosened bundles (Section 3.1.1), since they show the expected diameter selectivity (described in Section 1.2.1): higher reactivity of smaller diameter tubes (which have larger curvature, so larger structural strain, higher reactivity towards addition reactions on through sp3 carbon atoms can be formed).
Figure 3.5a represents the Drude-Lorentz fit of calculated optical conductivity spec-tra from wide range spec-transmission data. The single groups of peaks are related to the single types of transitions as noted in the figure. They have been obtained by subtract-ing all other groups of peaks as background (as shown in Figure 2.5). By observsubtract-ing the S11 peak shown in Figure 3.5b, it can be clearly seen that the high-wavenumber part of the peaks shows larger decrease in intensity than the low-wavenumber part. This means that nanotubes with higher transition energies (these are the smaller diameter tubes) have more electrons removed from the sp2 electronic system and are localized in a sidewall C–H bond on an sp3 orbital.
As a conclusion, it can be stated that there is practically no difference between the efficiency of hydrogenation of HiPco single-walled carbon nanotubes either by modified Birch reduction in liquid phase, or by intercalation of potassium into the bundles in solid phase, although the degree is very small (as detected by TG-MS), 2-4 H/100 C.
0 5 0 0 0 1 0 0 0 0 1 5 0 0 0 2 0 0 0 0 2 5 0 0 0
0
2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0
Optical conductivtiy (Ω-1 cm-1 ) W a v e n u m b e r ( c m - 1)
M 0 0 S 1 1 S 2 2 S 3 3+ M 1 1
a )
6 0 0 0 6 5 0 0 7 0 0 0 7 5 0 0 8 0 0 0 8 5 0 0
7 0 0 8 0 0 9 0 0 1 0 0 0 1 1 0 0
Optical conductivtiy (Ω-1 cm-1 ) W a v e n u m b e r ( c m - 1)
b )
Figure 3.5: a) Drude-Lorentz fit of calculated optical conductivity spectra from wide range transmission measurements of HiPco nanotubes hydrogenated by modified Birch reduction. The groups of peaks related to the single transitions are indicated. Notations:
(black line) 1x; (red line) 2x; (green line) 3x hydrogenated sample. b) zoom in to S11 transitions.
On the other hand, by looking at the optical conductivity spectra of samples hydro-genated by intercalating potassium shown in Figure 3.6, one can observe the difference between the S11 peaks compared to those in Figure 3.5.
In this case, the lower wavenumber part of the S11 peaks decreases more with the degree of hydrogenation. This surprising dissimilar behaviour of S11 peaks can be explained by the different reaction mechanism and driving forces. According to the detailed work of Kukovecz et. al [68], the rate of potassium intercalation into nanotube bundles strongly depends on the size of the intertube channels. Since a tight bundle of nanotubes consists of tubes with similar diameters nestled tightly parallel with each other, almost as much ordered as in a crystal, the size of these interstitial channels can be well estimated from the tube diameter (as described in Section 3.3.1), and they become larger with increasing tube diameter. The larger interstitial channels infer less tight bundles, which eases the intercalation of potassium cations.
The decreasing trend of S22 transition peaks at both series of samples is apparent.
In the case of S33 and M11 peaks we cannot make such a clear statement, since their energies are very close to each other and mixed very much. However, in the case of M00 (Drude) peaks at zero wavenumber, the changes in intensity seem to be independent of functionalization rate. This is because the intensity of this peak is connected to (beside the rate of covalent functionalization of metallic tubes) the amount of free charge carriers [58, 90]. Exposing carbon nanotube thin films to air (specifically to oxygen), weak p-doping occurs. This increases the concentration of free charge carriers as a function of exposure time and causes also an increase in M00 peak intensity at wavenumbers below 400 cm−1. This increase, and the intensity decrease from covalent functionalization of metallic tubes are added, and after all, since we did not take care of exposure time in our experiments, they make this peak not representative to either case.
0 5 0 0 0 1 0 0 0 0 1 5 0 0 0 2 0 0 0 0 2 5 0 0 0
Figure 3.6: a) Drude-Lorentz fit of calculated optical conductivity spectra from wide range transmission measurements of HiPco nanotubes hydrogenated by alkali metal (potassium) intercalation. The groups of peaks related to the single transitions are indicated. Notations: (black line) 1x; (red line) 2x; (green line) 3x hydrogenated sample.
b) zoom in to S11 transitions.
Figure 3.7: Typical TG-MS curve of HiPco n-butylated by intercalating potassium into the bundles. Black solid lines represent the mass loss (TG curve) and its derivative (DTG curve). Notations: (red circle) m/z 2 hydrogen; (blue triangles) m/z 18 water;
(green squares) m/z 43 C3H7; (magenta triangles) m/z 92 toluene.