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Effective Condition for Purification of Multi-walled Carbon Nanotubes by NitricAcidS. Mirershadi a; S. Z. Mortazavi b; A. Reyhani b; N. Moniri a; A. J. Novinrooz b

a Department of Physics, Karaj Islamic Azad University, Karaj, Iran b Material Research School, Karaj, Iran

Online Publication Date: 01 July 2009

To cite this Article Mirershadi, S., Mortazavi, S. Z., Reyhani, A., Moniri, N. and Novinrooz, A. J.(2009)'Effective Condition forPurification of Multi-walled Carbon Nanotubes by Nitric Acid',Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-MetalChemistry,39:6,312 — 316

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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 39:312–316, 2009Copyright © Taylor & Francis Group, LLCISSN: 1553-3174 print / 1553-3182 onlineDOI: 10.1080/15533170903094840

Effective Condition for Purification of Multi-walled CarbonNanotubes by Nitric Acid

S. Mirershadi,1 S. Z. Mortazavi,2 A. Reyhani,2 N. Moniri,1 and A. J. Novinrooz2

1Department of Physics, Karaj Islamic Azad University, Karaj, Iran2Material Research School, Karaj, Iran

The multi-walled carbon nanotubes were synthesized on Fe-Nibimetallic catalysts supported by MgO using thermal chemical va-por deposition. A comparative study on the effect of nitric acidmolarity and time duration of acid treatment on purification ofmulti-walled nanotubes was done. The studies revealed that forfinding a suitable condition for purification of multi-walled carbonnanotubes, it is necessary to determine suitable molarity of acid andimmersion time of the carbon nanotubes. It was found that 5 M ofboiling nitric acid at 3 h leads to the highest removal of the impuri-ties with the least destructive effect. Various analytical techniquessuch as: scanning electron microscopy (SEM), high resolution tran-sition electron microscopy (HRTEM), thermal gravimetric analysis(TGA), differential scanning calorimetry (DSC) and Raman spec-troscopy techniques were employed to investigate the morphology,graphitization and quality of the carbon nanotubes.

Keywords multi-walled carbon nanotubes, nitric acid, purification,thermal chemical vapor deposition

1. INTRODUCTIONIn the past few years, carbon nanotubes (CNTs) have at-

tracted a lot of attention on their unique properties; strength andstiffness along with high thermal and electrical conductivity.[1,2]

Several methods for large scale synthesis of carbon nanotubeshave opened the door to studying their physical and chem-ical properties, which were expected to be unique.[3,4] ThusCNTs have a variety of potential applications for example: hy-drogen storage medium, field emitter in display technology,electrodes of rechargeable batteries, one-dimensional quantumwire and resistors and nanoprobes in scanning and atomic forcemicroscopies.[5]

Three principal methods have been used to grow bothsingle-walled and multi-walled carbon nanotubes (SWCNTs

Received 14 February 2009; accepted 28 March 2009.The authors would like to thank the Material Research School

(Karaj, Iran) for financial support. The assistances of Mr. Sh. Lahoutiand Mr. Ahamdi are also acknowledged for their kind cooperation inthe SEM and STA analyses.

Address correspondence to A. Reyhani, Material Research School,P.O. Box 31485-498, Karaj, Iran. E-mail: a.reyhani@yahoo.com

and MWCNTs) including Laser ablation,[6,7] arc-discharge[1]

and chemical vapor deposition (CVD).[8] Both laser ablationand arc discharge methods have the problem that it is difficult tocontrol the structure and is difficult to scale up the productionof CNTs to industrial level.

The thermal chemical vapor deposition (TCVD) method hasmany advantages over other methods due to its simplicity andlow cost. In TCVD method, the catalyst materials, usually tran-sition metals such as Ni, Co, Fe or binary alloys of them areused for CNTs growth.[9]

The CNTs as produced by the various synthesis techniquescontain impurities such as graphite nanoparticles, amorphouscarbon, smaller fullerenes, and metal catalyst particles.[6] Theseimpurities have to be separated from the carbon nanotubes be-fore they can be used for applications such as composites, na-noelectronics, etc. Purification techniques have been devised inorder to improve the quality and yield of carbon nanotubes,and these methods include air oxidation, acid treatment, an-nealing, micro filtration, sonication, ferromagnetic separation,functionalization and chromatography techniques.[10] The phys-ical methods are non-destructive and not damaging tubes, butare less effective to purification of carbon nanotubes. On theother hand, the chemical methods generally result in CNTswith higher purity, but cause remarkable damages to nanotubesmorphology.[11] So, finding suitable conditions for purificationof CNTs is a crucial step in order to remove the impurities fromthe as prepared products with lowest damaged carbon nanotubesstructure.[12−15]

In order to approach a final product with the best pu-rification degree and lowest damaging effect on the CNTstructure, it is necessary to combine chemical and physicalmethods.[16] For this purpose, we combine oxidation in O2

gas and sonication processes to acid treatment for MWCNTspurification.

In this paper, the MWCNTs were synthesized on Fe- Nibimetallic catalysts supported by MgO using TCVD. Then,the effect of nitric acid with various molarities and immersingtime on purification of the MWCNTs was systematically stud-ied. The scanning electron microscopy (SEM), high resolutiontransition electron microscopy (HRTEM), thermal gravimetric

312

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MULTI-WALLED CARBON NANOTUBES 313

TABLE 1TGA and DSC results of the pristine MWCNTs, purified MWCNTs by 1M, 3M and 5M boiling nitric acids at the immersing

times of 1, 3 and 6 h

MWCNT samplesInflection

temperature (◦C) Amorphous carbon (%) CNT’s yield (%)

Raw 640 — 46.17Purified at time duration of 1 h by nitric acid 1M 670 — 72.50

3M 676 0.05 82.015M 677 0.09 86.06

Purified by 5M nitric acid at the time duration of 3h 677 0.18 93.006h 678 0.29 86.32

analysis (TGA), differential scanning calorimetry (DSC) andRaman spectroscopy techniques have been used to evaluatethe morphology, graphitization and quality of the purifiedMWCNTs.

2. RESULTS AND DISCUSSIONFigure 1 shows SEM images of the pristine CNTs (without

any purification treatment), and the sample treated at 5M boilingHNO3 acid for 3 h, respectively. It is clear from Figure 1athat pristine CNTs contain many impurities and it is hard totrace nanotubes. In Figure 1b, the effect of acid treatment indissolution of catalyst is quite visible.

TGA and DSC derived from STA were used to determine theyield and graphitization degree of the MWCNTs. It is generallyagreed, however, that amorphous carbon burns first, followed bySWNTs and then multiwall materials (shells, MWNTs), even ifTGA is often unfortunately not able to clearly separate the differ-ent oxidation steps.[17] The weight loss (%) in temperature ragefrom 500 to 800◦C indicated the percentage of CNTs in samples.In addition, the amount of amorphous carbon was achieved bydetermination of the weight loss in a temperature rang from 300to 400◦C.[18] TGA and DSC results of the pristine and purifiedMWCNTs using boiling nitric acid with various molarities andtime duration are shown in Table 1. The inflection temperaturesof the samples listed in Table 1 represent the temperature atthe maximum weight loss. The higher inflection temperature isrelated to better graphitization of MWCNTs.[17] The increasingof the inflection temperature is indicative of the improvementin graphitization properties of the MWCNTs.[19] It is observedthat the inflection temperatures increased by the increasing nitricacid molaritiy and time duration of acid treatment.

From TGA analysis, we found that the percentage of MWC-NTs in the pristine sample was 46.17 and lack of weight lossfrom 300 to 400◦C indicates that no amorphous carbon existedin the pristine sample.

As well, TGA results indicated that the percentages of CNTsin the samples that purified by 1M, 3M and 5M boiling nitricacids at the time duration of 1 h were 72.50, 82.01 and 86.06,respectively. These results indicated that the 5 M nitric acidin 1 h treatment duration was more effective than the others.

For more investigation, the MWCNTs were purified by 5Mnitric acids at 3 and 6 h. Figure 2 shows the weight loss vs.temperature for the pristine MWCNTs and purified MWCNTsby 5M boiling nitric acids at immersing times of 1, 3 and 6 h.TGA results showed that the percentages of CNTs in the samples

FIG. 1. SEM images of a) the pristine MWCNTs, and b) the purifiedMWCNTs by 5M boiling nitric acid at the immersing time of 3 h.

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314 S. MIRERSHADI ET AL.

0

0.2

0.4

0.6

0.8

1

1.2

0 200 400 600 800 1000

Temperature(ºC)

No

rmal

ized

Wei

gh

t

Pristine

1h

3h

6h

a

b d c

FIG. 2. TGA results of weight loss vs. temperature of the a) pristineMWCNTs, and purified MWCNTs by 5M boiling nitric acids at the immersingtime of b) 1, c) 3 and d) 6 h.

purified at the time duration of 3 and 6 h were 93 and 86.32,respectively. These results showed that the percentage of CNTsin the purified sample by 5 M nitric acid at 6 h was decreased.This decreasing is related to destructive effect of 5 M nitric acidat 6 h. This judgment was confirmed by amount of amorphouscarbon that exists in the sample. Existences of amorphous carbonrelated to corrosive effect of nitric acid on the outer walls ofthe MWCNTs. The percentages of amorphous carbons in thesamples that purified by 5M boiling nitric acids at the immersingtimes of 3 and 6 h were 0.18 and 0.29, respectively. As mentionedabove, at immersing time of 6 h, the highest amorphous carbonforms in the sample that relate to destructive effect. Therefore,purified MWCNTs by 5M boiling nitric acid at the time durationof 3 h was more suitable among the others.

A typical Raman spectrum of the MWCNTs shows that thereare two strong peaks at around 1250–1450 cm−1 (D-band) and1500–1605 cm−1 (G-band), which are associated with the vi-brations of carbon atoms with dangling bonds for the in-planeterminations of disordered graphite and the vibration in all sp2

bonded carbon atoms in a two-dimensional hexagonal lattice,respectively. Hence, intensity of the G-band to intensity of theD-band (IG/ID), are used to estimate the quality of CNTs.[20,22]

Figure 3 shows the Raman spectra of the pristine MWCNTs(Figure 3a) and purified MWCNTs by 5M boiling nitric acidsat the immersing times of 1, 3 and 6 h (Figure 3 b, c and d,respectively). As shown in this figure, the IG/ID ratio for thepristine sample was 2.12 and for the purified MWCNTs by 5Mnitric acid at 1, 3 and 6 h were 3.2, 3.57 and 2.7, respectively.The increasing of IG/ ID ratio after acid treatment indicates thatsome of the carbon impurities or defective structures of MWC-NTs have been removed from the pristine sample. On the otherhand, the acid treatment might have introduced few defects tothe nanotubes as seen from the IG/ID ratio of purified MWCNTsby 5M nitric acid at time duration of 6 h. This observation isin good agreement with our TGA results that showed purifiedMWCNTs by 5 M nitric acids at the immersing time of 6 hhad the highest weight loss (%) in a temperature rang from

1000 1200 1400 1600 1800 2000

DG

IG/ID: 2.12

IG/ID: 3.2

IG/ID: 3.57

IG/ID: 2.7

a

b

c

d

Raman shift (cm-1)

Inte

nsit

y (a

rb.u

nit)

FIG. 3. Raman spectra of the a) pristine MWCNTs, and purified MWCNTsby 5M boiling nitric acids at the immersing time of b) 1, c) 3 and C) 6 h.

300 to 400◦C related to amorphous carbon. So, from Ramanspectroscopy results, the MWCNTs purified by 5 M nitric acidsat the immersing time of 3 h had the best quality among theothers.

High resolution TEM was employed to explore the modifi-cations in the structure of the MWCNTs because of purificationtreatment. Figure 4 shows the TEM analyses of the pristineMWCNTs (Figure 4a) and purified MWCNTs by 5 M nitricacids at the immersing time of 3 h (Figure 4b). It is observedthat the pristine sample has impurity and various structure ofcarbon. The effectiveness of 5 M nitric acid at the immersingtime of 3 h in the removal of the metallic catalysts is evident inthe TEM image illustrated in Figure 4b.

Also, the diameter of pristine MWCNTs is 120 nm but afteracid treatment is 80 nm. The decreasing of diameter confirmsremoval outer tube’s wall that in accordance with TGA andRaman results.

3. EXPERIMENTAL

3.1. Preparation of CatalystBimetallic catalysts supported by MgO with proportions of

Fe: Ni: MgO (40:40:20 wt. %) was prepared by a wet chemicalimpregnation method. The appropriate amount of Fe (NO3)3.9H2O, Ni (N03)2. 6H2O and MgO were mixed in an aqueoussolution and then kept in ultrasonic bath for about 60 min. inorder to obtain a homogenous suspension. The prepared im-pregnate dried in an oven at 100◦C for 24 h in air. Finally, theresulting yield was milled to obtain fine powder.[12]

3.2. Growth of Carbon NanotubesThe MWCNTs were grown on the Fe-Ni catalysts supported

by MgO with TCVD. The details of growth condition werereported elsewhere.[12] In summary, the catalyst were placed on

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MULTI-WALLED CARBON NANOTUBES 315

FIG. 4. TEM images of the a) pristine MWCNTs and b) purified MWCNTsby 5M boiling nitric acid at immersing time of 3 h.

quartz tubular furnace and heated up to 700◦C under Ar gasflow at the rate of 80 sccm. Then, a mixture of Ar (20 sccm) andH2 (60 sccm) flow was fed into the furnace and it was heatedup to 900◦C in 15 min and after the temperature stabilized at900◦C, a flow of NH3 (80 sccm) introduced instead of Ar andH2 for 40 min. For CNTs growth, high purity CH4 and H2 (withratio of 1:3) were introduced into the furnace with a flow rateof 80 sccm for 40 min at 940◦C. After the growth of CNTs, thefurnace was cooled to the room temperature under flowing Argas.

3.3. Purification of Carbon NanotubesIn order to remove amorphous carbon, the as-grown CNTs

were oxidized at 400◦C for 60 min in O2 ambient. Then, removalof the MgO support materials and Fe-Ni metal catalysts bythe various treatments were tested to determine the suitablecondition for purification of CNTs.

After oxidation of MWCNTs in O2 ambient, 200 mg of theoxidized CNTs was immersed in 50 ml of boiling nitric acid withdifferent molarities 1M, 3M and 5M for the duration of each 1,3 and 6 h and sonicated. Then, the purified CNTs were collectedfrom the suspension by centrifugation and were washed severaltimes with deionized water to remove any trapped acid until thepH value of solution became neutral and finally dried overnightat 130◦C.

3.4. Sample CharacterizationThe surface morphology and average diameter of the CNTs

was examined using a scanning electron microscope SEM(Philips XL30) and TEM (HRTEM, Philips CM 200 FEG).

TGA and DSC were employed to investigate the CNT’s yieldand graphitization degree using Rometric simultaneous thermalanalysis (STA). The heating program was considered in a tem-perature range of 30 to 850◦C with the heating rate of 10◦C/minin air ambient. A micro- Raman spectrometer using 532 nmexcitation light (HR-800, Jobin- Yvon) was used to determinethe quality of CNTs.

4. CONCLUSIONThe MWCNTs were synthesized on Fe-Ni bimetallic catalyst

supported by MgO using TCVD. Our results revealed that mo-larity of nitric acid and immersion time of the MWCNTs in theacid solution are effective factors to obtain suitable conditionfor purification of MWCNTs. From TGA and DSC results, wefound that the acid treatment could improve oxidation stabilityof the MWCNTs. Our results indicated that in spite of effective-ness of the immersing time, there was a limitation for it becauseof its destructive effect on the MWCNTs. From Raman spectra,we found that the IG/ID ratio increase after acid treatment dueto removing of amorphous carbons and defective structures bynitric acid. It was found that 3M boiling nitric acid at 3h timeduration leads to the highest removal of the impurities with theleast destructive effect. The SEM and TEM images showed thatthe impurities were eliminated from the MWCNT matrix afterpurification.

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