Purification of carbon nanotubes

Many kinds of synthetic techniques have been developed, and metal catalysts are generally necessary to activate carbon nanotube growth. Carbon nanotubes may find their limited use in some applications as they contain a small fraction of metal catalyst in tubes and tend to have defects along the graphene tube wall. Defects within the multi-walled carbon nanotubes would reduce electrical and structure properties.

Recently, many purified methods have been investigated and have been used successfully to remove impurities from carbon soots [115-120].

2.1.1 Thermal oxidation

One of the efficient purification methods reported by Tsang et al. [121] was oxidation in air at 750°C. Due to the small difference in reactivity between Multi-walled carbon nanotubes (MWCNTs) and carbon nanoparticles, pure MWCNTs were obtained after prolonged oxidation. Many following up researchers [122-124]

adopted thermal annealing and similar thermal oxidation method to purify CNTs but with low yield.

2.1.2 Microfiltration and ultrasonically assisted filtration

In addition to thermal oxidation, Shelimov and coworkers [125] proposed a method to purify single walled carbon nanotubes by ultrasonically assisted filtration. These methods were based on physical phenomena. In this method, sample sonication during filtration prevents filter contamination and provides a fine nanotube-nanoparticle suspension through purification. Amorphous and crystalline carbon impurities and metal particles are removed from single walled carbon nanotube samples by ultrasonically-assisted microfiltration. The process generates SWNT material with purity above 90% and yields of 30-70%. Although this method

could separate coexisting carbon nanospheres, metal nanoparticles, polyaromatic carbons and fullerenes from the carbon nanotube fraction, metal catalysts embedded in the tip and wall structure could not be eliminated by this method. One advantage of microwave digestion method was that the embedded metal catalyst would be eliminated and the purity of carbon nanotubes could be higher.

2.1.3 Acid treatment

A two step process of thermal annealing in air and acid treatment, proposed by Moon et al. [126], was used 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 below 1%. The result showed that the reproducible optimal purification process provided a total yield of about 25~30 wt

% with transition metals less than 1%.

Zhang et al. [127] investigated the effect of PMMA and MCB on the purification and cutting of SWCNTs by thermogravimetric analyses. Chattopadhyay et al. [128]

proposed a method of complete elimination of metal catalysts from single walled carbon nanotubes. Chen and coworkers [129] investigated a three steps purification of MWCNTs by which the raw material can be purified completely without damage.

Various acids such as HF, H2SO4, HNO3 and HCl have been used to remove metal catalysts mostly. These processes involved repeated steps of filtering and ultra-sonication in acid solution, for example, 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. After acid treatment, samples were calcined in static air at 510°C for about 60 min. The total acid treatment processing time was above 30 hours. While metals are dissolved in solution, CNTs are cut into small length and even cause destruction.

Walls of CNTs are always damaged by strong acid. 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 % of the weight of the initial raw soot remained and the final product contained metals less than 1%. Bandow et al. [131]

investigated a purification method of microfiltration which could separate carbon nanospheres, metal nanoparticles, polyaromatic carbons and fullerenes from single-walled carbon nanotube (SWNTs) fraction. Ando et al. [132] reported that MWCNTs were ground and boiled with 20% H2O2 in a reflux condenser for 45 h.

Then the residual material was refluxed for 24h in a mixture of sulfuric acid (96%) and nitric acid (61%) with the ratio of 3:1.

2.1.4 Thermal oxidation combined with acid treatment

Another purification method to eliminate metal catalyst was proposed by Chiang et al. [133]. This method suggests a purification strategy to oxidize Fe and then dissolve the oxide. Raw material was heated in static air at 200°C for 24 h and followed by sonication in concentrated HCl (37%) in 80°C water bath for 15 min. Although the HCl treatment time was 15 min, the total purification time was obviously higher than 24 h. In this report, the total acid treatment time was below two hour. It was apparently that microwave digestion could effectively eliminate catalysts from carbon nanotubes and would not introduce structure defect.

In this work, a microwave-assisted digestion system was used to dissolve the metal catalyst. Inorganic acids such as H2SO4, HNO3 and HCl can rapidly absorb microwave heat and energy and completely dissolve metals in carbon nanotubes.

Since Environmental Protection Agency (EPA) recommended the microwave-assisted method with nitric acid [134], this leaching procedure of metals has been widely applied in sediments of soils and sludges. Nitric acid is strong enough to solubilize metals from materials. In closed microwave digestion system, metal catalysts are dissolved in acid solution rapidly without agitation. Therefore, lower concentration of

acids and acid immersing time are available to completely retain walls of carbon nanotubes. After purification, morphology of carbon nanotubes and purification degree are evaluated by SEM and TEM. The amount of residual catalyst metals in samples is estimated by thermogravimetric analysis (TGA). A high-yield and no destructive multi-walled carbon nanotubes in high purity are obtained. Metal content is less than 5 wt%

2.2 Fundamental and structure of DMFC