Purification of multi-walled carbon nanotubes by microwave digestion

method

Introduction

Many kinds of synthetic techniques have been developed, such as laser ablation, plasma-enhanced chemical vapor deposition, arc discharge, pyrolysis and thermal chemical vapor deposition. Transition metal (e.g. Fe, Co, Ni) particles are known to be

catalysts for vapor grown carbon nanotube synthesis, in which hydrocarbons (e.g. CH4, C6H6) are used as gas source. Metal catalysts are generally necessary to activate carbon nanotube growth. Carbon nanotubes may find their limited use for some applications as they contain a small fraction of metal catalyst in the 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 soot. One of the efficient purification methods reported by Tsang et al. 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 adopted thermal annealing and similar thermal oxidation method to purify CNTs but with low yield.

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 that residue in carbon nanotubes. Since Environmental Protection Agency (EPA) recommended the microwave-assisted method with nitric acid, this leaching procedure of metals has been widely applied in sediments soils and sludge.

Nitric acid is strong enough to solubilize

metals from material.

After purification, morphology of carbon nanotubes and degree of purification are evaluated by SEM and TEM. The amount of residual catalyst metals in the samples is estimated by using the thermogravimetric analysis (TGA). A high-yield and no destructive multi-walled carbon nanotubes in high purity are obtained.

Metal content is under 5 wt.%.

Experiments

The Co catalyst nanoparticles were deposited on p-type Si (111) wafer by sputtering method. Co catalyst was 7.5 nm in thickness. Experiments took place by electron cyclotron resonance chemical vapor deposition (ECRCVD). Mixture of CH4 and H2 was used as sources gas. Gas flow rates of CH4 and H2 were 18 sccm and 2 sccm, respectively. Power was set at 800 W. The reaction temperature was 600 °C.

An acidic treatment in microwave digestion system (Milestone Microwave Labstation ETHOSD) was used to dissolve the metal catalysts. In this procedure, raw sample of MWCNTs were placed in a 100 ml Pyrex digestion tube. The first digestion step run at 210 °C for 20 min with a 1:1 mixture of 5 M HNO3 and 5 M HCl. The microwave power was set at 100 W. The second digestion step was carried out at 210

°C for 30 min. After digestion, the suspension was filtered with 0.1 mm PTFE (poly-(tetrafluoroethylene)) membrane in

After purification, the morphology of MWCNTs and the degree of purification were observed by TEM. The amount of residual catalyst metals in the samples were 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 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.

Fig. 1: Low magnification TEM image of raw sample.

Fig. 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 evident embedded in the tip or in tube core of MWCNTs. Many bundles with a diameter ranging from 10 to 30 nm can be observed in the TEM image.

Fig. 2: Low magnification TEM image of MWCNTs after purification by microwave digestion.

Fig. 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

35

HCl is good in solving metal oxide.

Amorphous carbon can be removed by nitric acid because it is a strong oxidant. 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 1 h. In a microwave digestion system without agitation, heat was absorbed rapidly so that metal catalysts could be eliminated from MWCNTs rapidly with no damage.

Chen et al. reported a three-step non-destructive purification of MWCNTs by which the raw material can be purified completely without damage. However, their procedure crude stirred in 3 M nitric acid and refluxed for 24 h at 60 °C, and then was suspended and refluxed in 5 M HCl solution for 6 h at 120 °C. The total acid treatment processing time was above 30 h. Moon et al.

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. 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 3 h at 1600 °C. After the final step, approximately 20% of the 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. Nitric acid treatment for too long a time will break down the CNTs to small pieces.

Fig. 3 shows a TEM image of acid treated MWCNTs. It indicates the open end of MWCNTs, revealing that the cap is etched off and the wall of the graphite structure is not damaged. The diameter of tube is approximately 20 nm. So, lower concentration of acids and acid immersing time are available to completely retain wall of carbon nanotubes.

Fig. 3: HRTEM image of acid treated MWCNTs

In this research, microwave assisted digestion system was used to dissolve metal catalysts. The total acid treatment time of the two-step digestion system was below 1 h.

So, microwave digestion is an effective and fast method to remove metal particles from carbon nanotubes.

The purpose of the combustion of acid treated samples is 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.

Fig. 4: (a) TGA graph of raw samples ranged from 30 to 900 °C at 20 °C/min under 10 sccm air flow rate. (b) TGA graph of the sample after

microwave digestion acid-treatment ranged from 30 to 900 °C at 20 °C/min under 10 sccm air flow rate.

Fig. 4 shows TGA graphs of raw samples and purified MWCNTs. Fig. 4a

shows the TGA of raw samples and indicates that the weight starts to reduce near 410 °C. The MWCNTs are completely evaporated at 730 °C. The remaining materials are metal catalysts, which are approximately 30% of the whole weight.

The TGA graph indicates the existence of three phases in the sample. The differential TGA demonstrates a peak at 520 °C, which is suggested to be amorphous carbon and other small peak at 630 °C indicates that high temperature oxidation damages the MWCNTs.

Fig. 4b is the TGA graph of the sample after microwave digestion and acid purification treatment. After acid treatment, some water remained in the sample. It shows the correspondence between the slow weight loss from 30 to 450 °C and the loss of water and minor loss of amorphous carbon. In the temperature range from 450 to 650 °C, the weight decreases sharply to 5.25 wt.%. The peak at 520 °C in the differential TGA is assumed to be major weight loss of

MWCNTs begins at 600 °C. The curve slope is maintained almost the same in the temperature range between 490 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 approximately 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. reported that the combustion temperature of carbon nanotubes is 785 °C by TGA. In their procedure, carbon nanotubes were synthesized by using laser vaporization method. The combustion temperature of raw carbon nanotubes synthesized by ECRCVD in our purification procedure begins at 520 °C. This conclusion is the same as that of Colomer et al. They reported the burning temperature in air is 500 °C for CNTs synthesized by catalystic chemical vapor deposition method.

Conclusion

Presented in the research are MWCNTs of high yield and no damage by a two-step microwave digestion system with acidic treatment. In the microwave system, HNO3 and HCl rapidly absorb microwave heat and energy and

37

two-step microwave-assisted and acid treated system to dissolve metals in the MWCNTs is below 1 h. After purification, the amounts of residual catalyst metals in the samples were reducing from 30 to 5.15 wt.%. The results show that no damage multi-walled carbon nanotubes with metals approximately 5% are obtained. Conclusion is attained that microwave digestion method is an effective purification procedure for MWCNT.