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Electrochemistry Communications 6 (2004) 1042–1044

Metallization of multi-walled carbon nanotubes with copperby an electroless deposition process

Feng Wang a, Susumu Arai a,*, Morinobu Endo b

a Department of Chemistry and Material Engineering, Faculty of Engineering, Shinshu University,

4-17-1 Wakasato, Nagano-shi, Nagano 380-8553, Japanb Department of Electrical and Electronic Engineering, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato,

Nagano-shi, Nagano 380-8553, Japan

Received 2 August 2004; received in revised form 9 August 2004; accepted 9 August 2004

Abstract

An advantageous procedure has been developed, allowing metallization of multi-walled carbon nanotubes (MWCNTs, mean

diameter: 50–80 nm) with Cu layer by an electroless deposition process. Such a procedure for achieving electroless Cu deposition

on MWCNTs involves acid pre-clean, sensitization and activation. Results show that metallizated MWCNTs keep the fiber-like

appearances with uniform distribution. This resulting nanotube-derived material is comprised of a nanocrocrystalline Cu layer

and inner nanotube covered with Cu layer. The results of this work have demonstrated the effectiveness of electroless deposition

on the metallization of MWCNTs.

� 2004 Elsevier B.V. All rights reserved.

Keywords: Metallization; Carbon nanotube; Electroless deposition; Copper layer

1. Introduction

Multi-walled carbon nanotube (MWCNT) is an ideal

raw material for various applications due to its out-

standing mechanical characteristics such as high tensile

strength and high elastic modulus, high thermal conduc-

tivity and electric conductivity [1,2]. Recently, there has

been great interest in the metallization of MWCNTs

[3,4] for creating new metal-matrix-based carbon tubecomposites. This metallization process firstly belongs

to a kind of surface modification of MWCNTs, not only

can increase the surface active sites to improve bonding

between nanotube and resin or ceramic [5], but also can

maintain the superior performance and excellent intrin-

sic properties of MWCNTs in the composites. Further-

1388-2481/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2004.08.007

* Corresponding author. Tel.: +81 26 269 5413; fax: +81 26 269

5432.

E-mail address: araisun@gipwc.shinshu-u.ac.jp (S. Arai).

more, this metallization of MWCNTs has been shown tohave significant potential for the fabrication of new

powder MWCNT-metal composites [6,7], thus extend-

ing the application fields of MWCNTs.

Such metallization of MWCNTs can be achieved via

an electroless deposition process at normal temperature

state. To obtain the MWCNTs covered with a continual

Cu layer, a pre-treatment procedure comprised of acid

pre-clean, sensitization and activation is essential to pur-ify MWCNTs and increase the catalytic sites on

MWCNTs. In this study, nitric acid was used to pre-

clean the MWCNTs, and so-called �two-step� processwas performed to impart the catalyzation effect to the

surface of MWCNTs for the subsequent electroless Cu

deposition [8]. Moreover, MWCNTs tend to aggregate

into packed ropes or entangled networks due to strong

inter-tube van der Waals attraction, which may hinderthe formation of Cu-deposited MWCNTs with homog-

enous distribution. Here, one cationic polymer was used

F. Wang et al. / Electrochemistry Communications 6 (2004) 1042–1044 1043

to disperse MWCNTs in the aqueous solution on each

pre-treatment step and subsequent electroless deposition

process.

The aim of this work was to develop an advantageous

procedure allowing the metallization of the MWCNTs

with copper layers. An effective deposition bath contain-ing glyoxalic acid as reductant was designed, and then

was used to electrolessly deposited Cu layer on

MWCNTs. The microstructures of resulting Cu-

MWCNT composites were characterized with the help

of field emission scanning electron microscope (FE-

SEM), X-ray diffraction (XRD) and transmission elec-

tron microscope (TEM).

Fig. 1. SEM image of Cu-deposited MWCNTs.

2. Experimental

The MWCNTs used were synthesized via catalyst as-

sisted CVD (Showa Denko Co. Ltd) [9]. They were then

heat-treated at 2800 �C under an argon flow for 30 min

to form graphitic layer structure. MWCNTs were typi-

cally 50–80 nm in diameter and 10–20 lm in averagelength.

The electroless deposition bath was composed of 0.03

M CuSO4 Æ 5H2O as the metal ions source, 0.25 M ED-

TA Æ 2Na as the complexing agent and 0.1 M CHO-

COOH Æ H2O as the reductant. All solutions were

prepared using deionized water and reagent grade

chemicals.

MWCNTs were pre-cleaned in 12.8 M concentratedHNO3 for 12 h and rinsed thoroughly with deionized

water. The pre-clean preformed by nitric acid is to pur-

ify the MWCNTs, especially to eliminate the residual

metal catalyst particles from the carbon tube prepara-

tion process by CVD [10] for avoiding their side-effect

in the subsequent electroless deposition process. Moreo-

ver, the acid pre-clean also can slightly modify the sur-

face of MWCNTs, and then can enhance theinterfacial adhesion between MWCNT and metal film

[11].

The pre-cleaned MWCNTs were then sensitized in

the aqueous solution containing 0.0089 M SnCl2 and

0.0024 M HCl for 10 min, and activated in an aqueous

solution containing 0.0012 M PdCl2 and 0.012 M HCl

for 30 min, followed by rinsing thoroughly with deion-

ized water. In the ‘‘two-steps’’ method, the sensitizationis to improve the adsorption of Sn2+ on the MWCNT

surfaces. The activation then can be done to form the

fine Pd particles on MWCNT surfaces through the dis-

placement reaction between Sn2+ and Pd2+. At this step,

Pd particles serve as seeds for catalytic nucleating

centers.

Finally the pre-treated MWCNTs were immersed in

the electroless Cu plating bath with magnetic stirreragitation. On each step, MWCNTs were dispersed by

adding 1.25 · 10�6 M poly(diallyldimethylammonium

chloride) solution (mean molecular weight 400000;

PDMA, Aldrich) in the aqueous solution. The plating

cell was a beaker of 200 cm3, and the electroless deposi-

tion temperature was 60 �C. The deposition time was

controlled as 10 min. The deposited MWCNTs were

then filtered and rinsed with deionized water, then driedat room temperature in a vacuum desiccator.

The morphological features of Cu-deposited

MWCNTs were observed with the help of FE-SEM

(S-4100, HITACHI). The phase structures were deter-

mined by XRD (RINT2200V/PC, RIGAKU) and

high-resolution TEM (JEM-2000FE, JEOL).

3. Results and discussion

FE-SEM has been used to observe the morphological

features of Cu-deposited MWCNTs. The Cu-deposited

MWCNTs in the diameter of 130–180 nm are present

clearly in Fig. 1, showing the fiber-like appearance of

composites with a homogenous distribution. This indi-

cates that the above pre-treatment procedure used toachieve the metallization of MWCNTs with Cu layer

by an electroless deposition process is effective. The

resulting nanotube-derived composite is comprised of

polycrystalline Cu layer and inner MWCNT covered

with Cu layer.

Fig. 2 gives the XRD patterns of (a) MWCNTs and

(b) Cu-deposited MWCNTs. In the diffraction pattern

of un-deposited MWCNTs (a), one sharp peak at2h = 26.42� and several weak peaks are observed, and

these peaks correspond to the (0 0 2), (1 0 0), (1 0 1),

(0 0 4) and (1 1 0) planes of graphitized MWCNTs.

For Cu-deposited MWCNTs, new diffraction peaks at

2h = 43.3�, 50.4� and 74.1� are clearly observed other

than the diffraction peaks of graphitized MWCNTs.

These three peaks correspond to the (1 1 1), (2 0 0) and

(2 2 0) planes of face-centric-cubic Cu metal. TheXRD results clearly indicate that Cu layers are newly

Fig. 3. TEM image and selected area diffraction pattern of Cu-

deposited MWCNT.

Fig. 2. XRD patterns of: (a) un-deposited MWCNTs; (b) Cu-

deposited MWCNTs.

1044 F. Wang et al. / Electrochemistry Communications 6 (2004) 1042–1044

introduced on surfaces of MWCNTs by electroless

deposition.

Fig. 3 shows the representative TEM image of Cu-de-

posited MWCNTs and the corresponding selected areadiffraction pattern. The Cu layer and MWCNT are ob-

served simultaneously. It is clear that the Cu layer with

average thickness of 40 nm is composed of nanocrystal-

line grains, and covers the surface of MWCNT. The typ-

ical diffraction pattern shows several continue rings and

some diffraction spots, indicating that the Cu layer is

highly crystalline and have a polycrystalline structure.

Moreover, the graphitized MWCNT is also identified,

which is consistent with the above XRD results.

4. Conclusions

Metallization of MWCNTs with Cu layer has been

achieved through a pre-treatment process and subse-

quent electroless deposition process. Results show Cu

layers with the average thickness of 40 nm have been

deposited on the surfaces of MWCNTs. The resulting

composites with a homogenous distribution are com-prised of nanocrystalline Cu layers and inner MWCNTs

covered with Cu layers. This study provides an effective

means to fabricate powder metal-deposited MWCNT

composites with the considerable applied value.

Acknowledgement

This research was supported by the CLUSTER of the

Ministry of Education, Culture, Sports, Science and

Technology, Japan. We thank Mr. Yoshinari Misaki

of Tokyo Metropolitan University for assisting in

TEM observation.

References

[1] A. Oberlin, M. Endo, T. Koyama, J. Cryst. Growth 32 (1976)

335.

[2] M. Endo, Y.A. Kim, T. Fukai, T. Hayashi, K. Oshisa, M.

Terrones, T. Yanagisawa, S. Higaki, M.S. Dresselhaus, Appl.

Phys. Lett. 80 (2002) 1267.

[3] S. Chand, J. Mater. Sci. 35 (2000) 1303.

[4] Y.B. Li, B.Q. Wei, C.L. Xu, J. Jiang, D.H. Wu, J. Mater. Sci. 34

(1999) 5281.

[5] S.J. Park, Y.S. Jang, J. Kawasaki, Carbon Lett. 3 (2002) 77.

[6] S. Arai, M. Endo, Electrochem. Commun. 5 (2003) 797.

[7] S. Arai, M. Endo, N. Kaneko, Carbon 42 (2004) 641.

[8] K. Yamagishi, S. Yae, N. Okamoto, N. Fukumuro, H. Matsuda,

J. Surf. Finish. Soc. Jpn. 54 (2003) 150.

[9] M. Endo, Chemtech. Am. Chem. Soc. (September) (1998) 568.

[10] Y. Zhang, Z. Shi, Z. Gu, S. Iijima, Carbon 38 (2000) 2055.

[11] K. Esumi, M. Ishigami, A. Nakajima, K. Sawada, H. Honda,

Carbon 39 (2001) 718.