Characterization of the Purified MWCNTs and Discussions

After purification, the morphology of MWCNTs and purification degree were observed by TEM The amount of residual catalyst metals in samples will be estimated with thermogravimetric analysis (TGA) by using a thermal analysis system of PERKIN ELMER 1020 Series TGA 7 with a rate of 20°C /min from 30°C to 900°C at

the air flow rate of 10 sccm. Acid treated MWCNTs were oxidized by air at the temperature determined by TGA for 45 min.

Figure 4-1 shows a low magnification TEM image of raw carbon nanotubes. In this image, there appeared impurities such as amorphous carbons, graphite and metals in multi-walled carbon nanotubes. Metal particles were evidently embedded in the tip or in tube core of MWCNTs. Many bundles with diameters ranging from 10 to 30 nm can be observed in the TEM image.

Fig. 4-1 A low magnification TEM image of raw carbon nanotubes.

Figure 4-2 shows a low magnification TEM image of MWCNTs after purification by microwave digestion. It indicated that most of the metal particles were removed.

The structure and wall of MWCNTs were not destroyed. It is well known that HNO3

is very efficient in solving metal particles and HCl is good in solving metal oxide.

Amorphous carbon can be removed by nitric acid because it is a strong oxidant.

Fig. 4-2 A low magnification TEM image of MWCNTs after purification by microwave digestion.

However, in the microwave system, inorganic acid such as HNO3 and HCl rapidly absorbed microwave heat and energy without agitation and rapidly dissolved metals.

The processing time of the two step microwave-assisted and acid treated approach to dissolve metals in the MWCNTs was less than one hour. In a microwave digestion system without agitation, heat was absorbed rapidly so that metal catalysts could be eliminated from MWCNTs rapidly without damage.

Chen et al. [129] reported a three step non-destructive purification of MWCNTs by which the raw material can be purified completely without damage. But their procedure was crudely stirred in 3M nitric acid and refluxed for 24 h at 60°C, and then suspended and refluxed in 5M HCl solution for 6 h at 120°C. The total acid treatment processing time was above 30 hours. Moon et al [126] proposed a two step process of thermal annealing in air and acid treatment to purify single walled carbon nanotubes. This purification process used an acid treatment with HCl for 24 h to etch

away the catalytic metals and obtained SWCNTs with metals less than 1%. Kajiura et al. [130] reported a three-step purification process consisting of soft oxidation with 2.8 N HNO3 for 6-24 h, air oxidation for 10 min at 550°C and a high-temperature vacuum treatment for 3h at 1600°C. After the final step, about 20 % weight of the initial raw soot remained and the final product contained metals less than 1%. Most purification methods removed metal catalysts with acid for more than 24 h. Too long nitric acid treatment will break down CNTs to small pieces [126].

Figure 4-3 shows a TEM image of MWCNTs treated by acids. It indicates the open end of MWCNTs and reveals that the cap is etched off and the wall of the graphite structure is not damaged. The tube diameter is about 20 nm. So, lower acid concentration and immersing time are available to completely retain carbon nanotube walls.

Fig 4-3 TEM image of acid treated MWCNTs.

Microwave-assisted digestion system was used in this research to dissolve metal catalysts. Since the total acid treatment time of the two steps digestion system was less than 1h, microwave digestion is effective and fast to remove metal particles from carbon nanotubes.

Combustion of acid treated samples proceeds to purify CNTs according to the

oxidation temperature difference between non-carbon nanotubes and CNTs. The burning temperature of CNTs is related to pre-treatment process and graphitization degree, so there is no general combustion temperature of CNTs. TGA is an effective method to detect the combustion temperature in air.

Figure 4-4 shows TGA graphs of raw samples and purified MWCNTs. Figure 4-4(a) shows the TGA of raw samples and indicates that the weight starts to reduce near 410°C. MWCNTs are completely evaporated at 730°C. The remaining materials are metal catalysts, the amount of which is about 30 % of the whole weight. The TGA graph indicates the existence of three phases in the sample. A peak at 520°C in the differential TGA suggests the presence of amorphous carbons and the other small peak at 630°C indicates that high temperature oxidation damages MWCNTs.

Figure 4-4(b) is the TGA graph of the sample after microwave digestion and acid purification treatment. It shows the correspondence between the slow weight loss from 30 to 450°C and the loss of water and amorphous carbon. In the temperature range from 450 to 650°C, the weight decreases sharply to 5.25 wt %. The broad peak at 520°C in the differential TGA is assumed to be amorphous and another peak at 610°C indicates the damage of MWCNTs due to high oxidation. Combustion temperature of MWCNTs begins at 600°C. The curve slope keeps almost constant in the temperature range between 490°C and 650°C. It shows a constant combustion speed. After 650°C the weight of MWCNTs remains constant, and the remainders may be metals and metal oxides which reside inside the tube before combustion. So, the optimum amorphous combustion temperature is about 520°C. The burning temperature of CNTs is related to pre-treatment process and graphitization degree, so it has little in common. TGA is an effective method to detect the combustion temperature in air. Dillon et al. [179] reported that the combustion temperature of carbon nanotubes is 785°C by TGA.

Fig. 4-4(a) TGA graphs of raw samples and purified MWCNTs.

Fig. 4-4(b) TGA graphs of raw samples and purified MWCNTs.

In their procedure, carbon nnanotubes were synthesized by using laser vaporization method. Colomer and coworkers [180] proposed that the optimum reaction temperature in air is 500°C for CNTs synthesized by catalystic chemical vapor deposition. Chen et al. [129] reported that raw materials of CNTs produced by different catalyst and synthesis methods are different in component and in graphitization degree. Therefore, the combustion temperature of raw carbon nanotubes synthesized by ECRCVD in our purification procedure begins at 520°C.

This conclusion matches that of Colomer et al. [180].They reported the burning temperature in air is 500°C for CNTs synthesized by catalystic chemical vapor deposition.

While microwave digestion purification procedure for MWCNTs synthesized by catalyzed CVD is an effective purification process, TGA is a good and accurate approach to evaluate the purity of MWCNTs on a weight percentage basis.