Characterizations and discussion

Figure 4-13 is a TEM image, which shows that the raw sample contains multi-walled carbon nanotubes and catalyst. In this image, metal catalysts of various sizes can be found in the tip or on the tube wall. The diameters from 40 to 60 nm can also be displayed by TEM.

As shown in Figure 4-14, catalysts embedded in the tip were removed. Obviously, the closed tip was opened by acid treatment in microwave digestion. It seems that closed tip was opened first and the catalyst was eliminated by the acid. Another point is that the wall structure was not damaged by acid treatment.

Fig. 4-13 TEM image shows that raw sample contains multi-walled carbon nanotubes and catalysts.

Fig. 4-14 Catalysts embedded in the tip were removed.

Figure 4-15 shows TGA analyses of raw and 120min-purified samples. Curve (a) shows that the original catalyst content is 10.39 wt%. The weight starts to reduce near 478°C and completely evaporate above 773°C. The decomposition temperature at

622°C is defined as the inflection point during oxidation. Besides, there appeared two region at 601°C and 642°C. The former might be carbonaceous particles or amorphous carbon, and the latter is MWCNTs. In curve (b), the weight loss of purified MWCNTs starts from 550°C and completely burn-outs at 728°C. The amount of residual catalysts dropped to 1.03% in 120min-purified samples. The decomposition temperature now is 701°C, which is slightly higher than that of the raw.

The reason might be that the embedded metal catalyst in nanotubes acts as oxidation site and initiated oxidation [179]. It was also reported that the metal impurity would lower the decomposition temperature and increase the decomposition rate [179]. So, the purified sample is thermally more stable towards oxidative destruction than the raw. Another reason is carbonaceous materials contained in raw samples. Carboxyl, aldehyde and other oxygen-containing function groups on the surface of carbonaceous fractions are extremely hygroscopic and reactive towards oxidation. However, in curve (b), there is no any separate region except the main combustion region. This result points out that microwave digestion not only removes catalysts but also has potential to eliminate carbonaceous materials.

As shown in Figure 4-16, digestion time of the residual catalyst content ranges from 10 to 120 min. It is obvious that the content suddenly falls to 1.75% only in 10 min digestion. With increasing purification time the catalyst content slowly decreases to 1.03% in 120 min treatment.

Fig. 4-15 TGA analyses of (a) raw and (b) 120min-purified samples.

Fig. 4-16 Residual catalyst content of different digestion time ranges from 10 to 120 min.

The effect of time on purification efficiency may not be significant. The reason might be that most of metal particles were eliminated within the first few minutes, but other catalysts covered by tens of graphene layer is difficult to remove (Fig. 4-17). So, it is not a genius way to raise the digestion time, but to lower the diameter of carbon nanotubes. The efficiency of microwave digestion could be increased if the diameter of nanotubes is small and uniform.

Fig. 4-17 Catalyst covered by tens of graphene layer is difficult to remove.

Figure 4-18 is Raman analyses of raw and purified samples for 30, 60, 90 and 120 min treatments. The results of Raman analyses show that the ratios of IG/ID increase after purification. Some studies [120,179] have also suggested that the IG/ID ratio would increase after purification because of the improvement in nanotube percentage by eliminating amorphous carbons. TGA analysis proves the existence of some amorphous carbons in raw samples, and there is no such feature in cure b of Fig. 4-15.

Raman spectra and TGA analysis could prove each other that microwave digestion can eliminate both metal catalysts and carbonaceous materials without structure damages.

Fig. 4-18 Raman analysis of raw and purified samples for 30, 60, 90, and 120 min treatments.

Since the discovery of carbon nanotubes, many efforts have been devoted to produce the highest purified carbon nanotubes [126,129,132]. It is obvious that previous chemical purifications usually removed catalysts by long time acid treatments, about 24 h or even longer. However, it was also reported that long acid treatment time would result in the break of tubes [179]. This might be that oxygen containing mineral is very efficient to dissolve metals and polyaromatic solids, such as graphites or amorphous carbons. Although the acid could dissolve polyaromatic materials, the acid in microwave digestion system can rapidly absorb microwave energy and quickly dissolve metal particles in very short time to prevent the damages of tubes.

Different from the chemical treatment, many kinds of physical methods were provided, such as ultrasonically assisted filtration or oxidation combing with acid treatment [125,133]. However, metal catalysts embedded in the tip could not be eliminated by ultrasonication. Besides high efficiency, the other advantage of digestion method is to eliminate the catalyst embedded in tubes.

4.3.3 Summary

Microwave digestion method was successfully developed in our previous work with the advantages of high efficiency, easy operation, short time, damages free on CNTs and little consumption in reagents. However, residual catalyst amount of ECR-synthesized MWCNTs after purification was still high. Investigated in this work is the purification efficiency of MWCNTs synthesized by thermal chemical vapor deposition with different parameters by using TGA, SEM, TEM and Raman spectroscopy and MWCNTs of high purity and fast treatment are expected. The results show that the purification efficiency increases with increasing acid treatment time but the most important parameter might be the diameter of carbon noaotubes.

Microwave digestion method has excellent prospect to yield carbon nanotubes of high purity if carbon nanotubes are small and uniform in diameter. The amount of residual catalysts in purified samples was lowered to only 1.75% for 10 min digestion at 210°C and 1.03% for 120 min digestion. In conclusion, microwave digestion may have great potential in mass purification. Large amount of purified CNTs with high quality would be applied to more intrinsic studies and industrial applications.

4.4 Reaction Model of Microwave-Assisted Purification of MWCNTS