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Influence of Electric Field on Dispersion of Carbon

Nanotubes in LiquidsZhu YueFeng

a, Zhang Chan

a, Wang JingDong

a, Shi Lei

a& Liang Ji

a

aKey Laboratory for Advanced Manufacturing by Materials Processing Technology,

Department of Mechanical Engineering , Tsinghua University , Beijing, P.R. China

Published online: 06 Feb 2007.

To cite this article:Zhu YueFeng , Zhang Chan , Wang JingDong , Shi Lei & Liang Ji (2006) Influence of Electric

Field on Dispersion of Carbon Nanotubes in Liquids, Journal of Dispersion Science and Technology, 27:3, 371-375, DOI:

10.1080/01932690500359582

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Influence of Electric Field on Dispersion of Carbon Nanotubesin Liquids

Zhu Yue-Feng, Zhang Chan, Wang Jing-Dong, Shi Lei, and Liang JiKey Laboratory for Advanced Manufacturing by Materials Processing Technology, Department of Mechanical Engineering,

Tsinghua University, Beijing, P.R. China

Production processes for carbon nanotubes commonly produce mixtures of solid morphologiesthat are mechanically entangled or that self-associate into aggregates. The entangled oraggregated carbon nanotubes often need to be dispersed in corresponding material matricesin order to develop materials that have unique mechanical characteristics or transportproperties. The most effective method for dispersion of carbon nanotubes is to prepare fluidsuspensions of them in liquid media with applications of surfactant or/and ultrasonication.The authors propose an innovative dispersion method for carbon nanotubes by which anelectric field is applied to suspensions of carbon nanotubes in liquids treated by surfactantand ultrasonication. Compared to dispersion without the electric field, the dispersion status

of carbon nanotubes in liquid media is evidently improved with the electric field. The resultsindicate that the electric field conditions are effective for dispersion of carbon nanotubes inliquids and that complex effects of electric field, surfactant, and ultrasonication are beneficialfor improvement of dispersion of carbon nanotubes.

Keywords Carbon nanotubes, dispersion, electric field, ultrasonication, surfactant

INTRODUCTION

Carbon nanotubes (CNTs) have an interesting set of proper-

ties that position them for a wide variety of potential appli-

cations in composites. Their unusual properties include high

moduli of elasticity and strength, high aspect ratios, excellent

thermal and electrical conductivities, and magnetic properties

(Yu et al., 2000; Salvetat et al., 1999; Xie et al., 2000; Wonget al., 1997; Yao et al., 2001; Hone et al., 1999, 2000; Kim

et al., 2001; Berber et al., 2000; Kaneto et al., 1999).

However, a number of current synthesis methods, especially

the methods yielding CNTs in mass production, produce

CNTs that are physically entangled. It is difficult for the

entangled CNTs to be applied in manufacturing or modification

of current practical materials. Important challenges to develop-

ing applications for these unique materials include: (1) purifi-

cation and separation of CNTs by chemistry and

morphology, (2) uniform and reproducible dispersion, and

(3) orientation of these solids in liquid and melt phases.

One objective of dispersion science and technology is to

produce a suspension of independent, separated CNTs in

liquid media that then can be manipulated into preferred

orientations in one-dimensional (fiber), two-dimensional (flat

sheet), or three-dimensional (bulk solid) objects. Up to now,

there have been two different approaches to CNT dispersion:

mechanical (or physical) methods and chemical methods.

Mechanical dispersion methods, such as ultrasonication

(Koshio et al., 2001), high impact mixing (Kim et al., 2002;

Pierard et al., 2001), and high shear mixing (Hilding et al.,2003), separate CNTs from each other, but, meanwhile, can

also fragment the nanotubes, decreasing their aspect ratio

during the processing. Chemical methods use surfactants or

functionalization to change the surface energy of the nano-

tubes, improving their wetting or adhesion characteristics and

reducing their tendency to agglomerate in the continuous

phase solvent. However, aggressive chemical functionaliza-

tion, such as using neat acids at high temperatures, can also

digest the nanotubes. Both mechanical and chemical methods

can alter the aspect ratio distribution of the nanotubes and

result in changes in the properties of their dispersions.

Recently, electric fields have been applied for orientation,

array, filtration, and purification of CNTs. Du et al. (2002)

prepared CNT films by means of electrophoretic deposition

(EPD) with an external direct current (DC) electric field and

investigated primarily the electric properties of the films. Bae

et al. (2002) studied the field-emission property of the EPD

CNT films and found that the higher the graphitization

degree, the lower the resistivity of the films, and then the

lower the critical exciting electric field intensity for the

Received 23 July 2005; Accepted 8 August 2005.Address correspondence to Zhu Yue-Feng, Key Laboratory for

Advanced Manufacturing by Materials Processing Technology,Department of Mechanical Engineering, Tsinghua University,Beijing 100084, P.R. China. E-mail: yfzhu@tsinghua.edu.cn

Journal of Dispersion Science and Technology, 27:371375, 2006

Copyright# Taylor & Francis Group, LLC

ISSN: 0193-2691 print/1532-2351 online

DOI: 10.1080/01932690500359582

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electron emission taking place. This research successfully

realized directional transplantation of CNTs by means of

EPD processes. It should be mentioned that, in order to be

manipulated with EPD processes, it is necessary for CNTs to

be charged by proper functional treatments.

On the other hand, it is generally impossible to realize EPD

for CNTs without charges on their surfaces. Therefore, it is to

seek for other ways to manipulate CNTs. (Yamamoto et al.

1996, 1998) purified CNTs with and without charges in

liquid media by electrophoresis and dielectrophoresis, respect-

ively. Krupke et al. (2004) successfully separated the metallic

and semiconductive single-walled carbon nanotubes (SWNTs)

by the dielectrophoresis process with an alternating current

(AC) electric field. The SWNTs were electrically dipolarized

under the electric field and then arrayed under sufficiently

high DC or AC voltage between the electrodes. Tang et al

(2005) controlled the lengths and exposures of CNTs in

liquid media and prepared atomic force microscopy probes

by dielectrophoresis under a 2 MHz AC electric field.

The previously mentioned results demonstrated the effect of

electric fields on CNTs suspended in liquid media. Moreover,an electric field would not result in undesired digestion and

demolishing taking place during other mechanical or

chemical processes. In this study, DC electric field together

with other treatments such as ultrasonication and surfactant

were applied to improve the dispersion of CNTs in liquid for

the potential application of CNTs in modification of practical

materials, and the dispersion mechanism of CNTs in liquids

under electric field will be preliminarily analyzed.

EXPERIMENTAL SECTION

CNTs used in this study were prepared by the chemical vapor

deposition (CVD) method. Thereafter, the prepared CNTs were

treated with two different processes. One process is as follows:the prepared CNTs were treated in HF for 24 hours to remove

catalyst particles and impurities, then the conglomerations of

the entangled CNTs were ball milled slightly, to obtain the

so-called original CNTs. The other process is as follows: 10 g

of the CNTs were added into 100 mL of a blend of dense

H2SO4 and HNO3 with a volume ratio 3 : 1 between H2SO4and HNO3, the blend was boiled for 30 min, rinsed continuously

to chemically neutralize with deionized water, and then dried to

obtain the so-called acid-treated CNTs.

Two kinds of suspensions of CNTs in liquid media were

prepared by adding 0.5 g of the original and acid-treated

CNTs, respectively, into 100 mL of deionized water and then

treating by ultrasonication for 30 min. Eight groups of suspen-

sion samples of the two kinds of CNTs in the liquid medium

were prepared from the suspensions as described in Table 1.

Here, specimens No. 14 were made from the suspension of

acid-treated CNTs in liquid, and specimens No. 5 8 were

made from the suspension of original CNTs in liquid.

Figure 1 shows the schematic drawing of CNT dispersion in

the suspension controlled by electric fields. The specimens

were taken from the suspensions with a pipette and dropped

onto a piece of filtering paper for observation.

The dispersion of CNTs in different specimens described inTable 1 was observed by means of scanning electronic

microscopy (SEM).

RESULTS AND DISCUSSION

The dispersion of acid-treated CNTs in liquid media was

observed with SEM as shown in Figure 2. It is seen that

CNTs disperse well with few conglomerations in the suspen-

sion under DC electric field for four hours (see Figure 2(a))

and under complex effect of both a 25 V DC electric field

and ultrasonication for 30 min (see Figure 2(c)), and there

exist a number of conglomerations of CNTs in the suspension

without DC electric field (see Figure 2(b), (d)).

In order to demonstrate the transplantation of CNTs inliquids under an electric field, an electrophoresis experiment

was carried out with the acid-treated CNTs in deionized

water under DC electric field. During the electrophoresis

process, the CNTs transplanted to and deposited on the

anode and formed a layer of film. This is in agreement with

FIG. 1. Schematic drawing of CNT dispersion in the suspensioncontrolled by electric fields.

TABLE 1

Preparation conditions of the suspension specimens

No. Preparation conditions

1 Acid-treated CNTs, 25V DC electric field for

4 hours

2 Acid-treated CNTs, placed statically for 4 hours

3 Acid-treated CNTs, 25V DC electric field andultrasonication for 0.5 hour

4 Acid-treated CNTs, ultrasonication for 0.5 hour

5 Original CNTs, 25 V DC electric field for 4 hours

6 Original CNTs, placed statically for 4 hours

7 Original CNTs, 25 V DC electric field and

ultrasonication for 0.5 hour

8 Original CNTs, ultrasonication for 0.5 hour

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the previous results of other researchers (Sun et al., 2002;Esumi et al., 1996; Thomas et al., 2005) who reported that

the zeta potential of acid-treated CNTs was negative in

neutral solution. Figure 3(a), (b) show the CNT films formed

on the surface of the anodes during electrophoresis carried

out by the authors and others. It indicates that surfaces ofthe acid-treated CNTs were charged negatively in deionized

water.

Based on infrared ray spectrum analysis results (Jia et al.,

1999; Shaffer et al., 1998; Li et al., 2002), some functional

FIG. 2. Dispersion morphology of the acid-treated CNTs in liquid media under different conditions (SEM); note that the background is the fibers of thefiltering paper.

FIG. 3. CNT films deposited on the electric pole during the electrophoresis process (a) prepared by the authors; (b) prepared by Thomas et al. (2005).

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groups such as hydroxyl (OH), carboxyl (COOH), and car-

bonyl (.C55O) were loaded on surfaces of the acid-treated

CNTs. In the infrared absorption spectrum (Li et al., 2002),

there evidently existed peaks of carbonyl and hydroxyl func-

tional groups corresponding to wave lengths 1750 cm21 and

3500 cm21, respectively. After a saponification reaction

boiling a blend of acid-treated CNTs and NaOH, the existence

of carboxyl functional group was also proved by a translation

of the carbonyl peak in the infrared spectrum (Jia, 1999).

Hydrogen ions were produced by ionization from the

carboxyl bonded on surfaces of the acid-treated CNTs, which

made them negatively charged. The charged CNTs in the sus-

pension were affected by the external DC electric field and

other charged particles in the liquid beside the body forces

such as gravity and buoyancy. Their translation velocities

were different due to the different numbers of the charges on

individual CNTs and masses. So the entangled CNTs might

be detached. Therefore, the effect of DC electric field on sus-

pensions of acid-treated CNTs in liquids could efficiently

improve the dispersion of CNTs in liquids.

Specimen No. 3 was prepared under the complex effect

of both a 25 V DC electric field and ultrasonication for

30 min. By comparison between Figure 2(a) and Figure 2(c),

it is found that, accompanied by ultrasonication, the effect of

DC electric field for only 0.5 hour on the dispersion of acid-

treated CNTs in liquids was approximately similar to that

without sonication for 4 hours. In order to prove that the fine

dispersion effect of CNTs in specimen No. 3 was mainly attrib-

uted to the external electric field, No. 4 specimen was prepared

under ultrasonication without electric field. Comparing

Figure 2(d) with Figure 2(c), it is seen that the dispersion of

CNTs in liquids under effects of both external electric field

and ultrasonication (see Figure 2(c)) is much better than that

with only ultrasonication (see Figure 2(d)). Therefore, it can

be concluded that the complex effect of both the external

electric field and ultrasonication might be an efficient method

to disperse acid-treated CNTs in liquid media.

Dispersion status of the original CNTs in liquids shown in

Figure 4. By comparing Figure 4(a) with Figure 4(b), it is

found that the dispersion of the original CNTs in liquids

FIG. 4. Dispersion morphology of the original CNTs in liquid media under different conditions (SEM); note that the background is the fibers of the filteringpaper.

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under a 25 V DC electric field for four hours is not impro-

ved, and evident conglomerations still exist in specimen

No. 5. An electrophoresis experiment was carried out with

the original CNTs in deionized water, and it was found

that there was a layer of CNT film deposited on neither the

anode nor the cathode during the electrophoresis process.

The electrophoresis results indicated no charge on surfaces

of the original CNTs in deionized water. In a static electric

field, there is no surplus electric charge inside the

entangled CNT conglomerations as conductors in balance in

charge. All of the induced charges distribute on the external

surfaces of the conductor conglomerations. It is difficult for

an external electric field to act on the individual nanotubes in

CNT conglomerations. Therefore, the effect of the external

static electric field on the suspension could have little

improvement on the dispersion of the original CNTs in

water.

Because the external static electric field demonstrates

little effect on the entangled original CNTs without surplus

electric charges, the complex effects of the static electric

field and ultrasonication could not be brought into play asthey are on the acid-treated CNTs but demonstrate only the

effect of simple ultrasonication. Seriously entangled CNT

conglomerations exist in Figure 4(c), (d), which agrees, with

that described above.

CONCLUSIONS

The suspensions of CNTs in deionized water were prepared

with different CNTs, that is, the original and acid-treated

CNTs, respectively. An external DC electric field was

exerted on the suspensions to improve the dispersion of

CNTs in liquids. The results indicated that the electric field

noticeably improved the dispersion of the acid-treated CNTsin deionized water. Moreover, the companion effect of an

electric field with ultrasonication was effective in improving

the dispersion of acid-treated CNTs in liquids. The improve-

ment of dispersion of acid-treated CNTs resulted from the

functional groups loaded on the surfaces of the CNTs

after being treated with acids. On the other hand, the electric

field improved little the dispersion of the original CNTs in

deionized water, so did as electric field companied by the

ultrasonication because of no charge on surfaces of the

original CNTs.

ACKNOWLEDGMENTSThe project was sponsored by the Foundation of National

Natural Science, China (Grant No.10332020) and the Inno-

vation Fund for Outstanding Scholar of Henan Province,

China.

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