Carbon-nanotube metal-matrix composites preparedby electroless plating

Xiaohua Chen a,b,*, Jintong Xia a, Jingcui Peng a, Wenzhu Li b, Sishen Xie c

aDepartment of Applied Physics, Hunan University, Changsha 410082, People's Republic of ChinabDepartment of Physics, Zhejian University, Hangzhou 310027, People's Republic of China

cInstitute of Physics, Chinese Academy of Science, Beijing 100080, People's Republic of China

Received 27 April 1999; received in revised form 29 July 1999; accepted 9 August 1999

Abstract

It has been demonstrated that cobalt could be plated onto the surfaces of carbon nanotubes by electroless plating. In this mannera layer of cobalt is formed as nanoparticles on the suface of the carbon nanotube. It was found that the activation process and low

deposition rate are critical for getting better coating. Furthermore, the heat-treatment of the coated carbon nanotubes was found tobe a very e�ective way of improving the deposited coating layer. The results from this study have demonstrated the technical fes-sibility of electroless plating for the preparation of a one-dimensional nanoscale composite. # 2000 Elsevier Science Ltd. All rightsreserved.

Keywords: B. Magnetic properties; D. SEM; E. CVD; Carbon-nanotube metal-matrix composites; Electroless plating

1. Introduction

Carbon nanotubes are attracting increasing scienti®cand technological interest with their novel propertiesand potential applications [1]. The morphology and sizeof carbon nanotubes suggests that they could be used assupports for heterogeneous catalysis or as templates forcreating small wires or tubular structures. Severalapproaches have been explored for the preparation ofcarbon nanotubes ®lled with metallic elements or com-pounds [2±5]. Carbon nanotubes have also been used toreact with metal oxides to produce nanoscale metal-carbide rods [6,7]. It may be speculated that one-dimensional nanoscale composites can also be preparedby coating the carbon nanotubes with other materials.This work presents the results of a study of the use of

electroless metal plating as a technique for coating car-bon nanotubes. Since many metals can be deposited onalmost any substrate after a previous activation, byelectroless plating, the carbon nanotubes could beencapsulated by a thin layer of metal. However, becausethe carbon nanotubes have low chemical reactivitytypical of highly graphitized carbon aqueous materialsand a large curvature, the nucleation and coating of

nanotubes has been made speci®c and enhanced by thepre-oxidation and pre-activation of the surface and bychoosing the appropriate reaction, as well as by heattreating the reacted carbon nanotubes. The selection ofcobalt is based on the fact that it is a magnetic metal.Cobalt-coated carbon nanotubes should have vastlydi�erent electrical and magnetic properties which maybe of interest for nanoscale magnetic research and high-density recording. Otherwise, carbon nanotubes can beused as nanoscale ®bres in superstrong, lightweightcomposite materials an account of their high sti�nessand ¯exibility, coupled with their low density. However,the interfacial adhesion between the nanotubes and themetal matrix has been a problem. It may be speculatedthat high-strength adhesion between nanotubes and thematrix metal can be achieved by coating the nanotubeswith metallic materials.

2. Experiments

The multi-shell carbon nanotubes used in this workwere produced by the catalytic decomposition of acet-ylene over mesoporous silica containing iron nano-particles embedded in the pores which were prepared bya sol-gel process [8]. A mixture of 9% acetylene innitrogen was introduced into the chamber at a ¯ow rate

0266-3538/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.

PI I : S0266-3538(99 )00127-X

Composites Science and Technology 60 (2000) 301±306

* Corresponding author.

of 110 ml/min for 2±5 h at 700�C at 110 torr, and car-bon nanotubes were formed on the substrate by thedeposition of carbon atoms. The nanotubes, with thetypical diameter around 10±30 nm, are relatively pure(Fig. 1). Low- and high-magni®cation TEM images inFig. 1(b),(c) show that the nanotubes are well graphi-tized and typically consist of �10 concentric shells ofcarbon sheets. The inner and outer diameters of thetubes are 4.5 and 14 nm.In order to modify the surface chemistry, the nano-

tubes were subjected to an oxidation treatment beforesurface activation. The nanotubes were suspended in anaqueous solution 0.38 M of K2Cr2O7/4.5 M of H2SO4

and re¯uxed in an oil bath maintained 60�C. It is wellknown that an acid solution of CrO3 readily etches thesurfaces of carbon nanotubes. After this treatment thenanotubes were rinsed with distilled water and pre-acti-vation was accomplished by dispersing them in a solution

of 0.1 M SnCl2/0.1 MHCl for 30 min, followed by rinsingin distilled water. The Sn2+-sensitized nanotubes werefurther activated in a aqueous solution of 0.0014MPdCl2/0.25M HCl for another 30 min. The activatednanotubes were washed with distilled water and thenintroduced into an electroless plating bath. The compo-sition of the plating solution and the reaction conditionsare given in Table 1. As is well known, an electrolesssolution contains, besides a metal salt and reducingagent, other compounds such as a complexing agent (tomaintain the level of metal ions in solution) and bu�ers(to maintain a given pH). There are, in addition, otheradditives, at very low concentrations, to stabilize thesolution or to improve the morphology of the metalcoating. Once the plating was ®nished, the plated nano-tubes were washed with distilled water and dried at100�C. The plated nanotubes were then heat treated in aquartz tube at 600�C in 200 torr of ¯owing 10% H2/N2

Fig. 1. Micrographs of catalytically grown carbon nanotubes before pre-activation: (a) SEM, (b) low-magni®cation TEM, (c) high-magni®cation

TEM. The tubes are well graphitized and consist of about 10 concentric shells of carbon sheets. The inner and outer diameters of the tube are 4.5

and 14 nm, respectively.

302 X. Chen et al. / Composites Science and Technology 60 (2000) 301±306

(100 cm3/min) for 5 h. The morphology and size of thecobalt-coated nanotubes before and after heat-treatmentwere analyzed by means of a JEOL scanning electronmicroscope (SEM), ®tted with an energy dispersive X-ray analytical system for composition analysis (EDX).

3. Results and discussion

Surface activation of the nanotubes with a solution oftin and palladium results in the formation of Pd/Snparticles as activated sites which initiate the depositionof cobalt (Fig. 2). From this ®gure we can see that thePd/Sn particles appear as aggregates on the outer

surfaces of the nanotubes, indicating that activated siteswere formed. In order to increase the number of acti-vated sites, oxidation is necessary to increase the wett-ability of the surface before surface activation. For thisapproach, the carbon nanotubes were oxidized with aCrO3 solution.After activation, the cobalt would be reduced on the

activated sites at appropiate pH values and tempera-ture. The experimental conditions for cobalt platinghave been modi®ed with respect to the conventionalprocedures in order to e�ect the electroless process atlow temperature. Fig. 3(a) and (b) shows the coatingresults carried out at pH=9 and at the temperature of35�C. The results indicate that a layer of deposit is ableto form on the surfaces of the carbon nanotubes,although the surfaces were not completely covered. TheSEM/EDX analyses show that the deposit is Co/P (Fig.4). In Fig. 3 the Co/P deposit is formed as sphericalgrains which appear as very closely packed crystallites.The results indicate that the cobalt tended to aggregateas nanoparticles on the outer surfaces of the nanotubes.Consequently, voids or gaps were formed. It is alsoobserved that the nanotubes are not completely anduniformly coated. The reason for the incomplete cover-ing of the carbon nanotube by cobalt may be due to thefact that the activation process of the nanotube surfacedoes not produce a uniform distribution of palladium.Caturla et al. [9] also found a very irregular distributionof palladium after activation of a polycrystalline gra-phite with Sn/Pd, with large areas of the surface notcovered by palladium. We noticed that the depositedlayer was very sensitive to the pH value of the solution,which in¯uenced the reaction rate. If the pH value ishigher than 10, the deposition rate is very high so thatthe aggregation tendency would be more serious [Fig.3(d) and (e)]. When the pH value is lower than 8, thereaction rate is so slow that the SEM image did notshow any deposit on the outer surfaces of the nano-tubes. Keeping a low deposition rate is, therefore,helpful in obtainining a better coating layer on thenanotubes. However, even with optimum experimentalconditions, spherical grains are still formed on thenanotube surfaces, perhaps because the coating was ona surface of high curvature. On the curved surface, ifthe normal growth rate is higher than the lateralgrowth rate, the coating layer tends to form as sphe-rical grains.As cobalt coating proceeds with increasing reaction

time, the thickness of the deposit progressively increa-ses, leading to a su�ciently large grain size to ensurethat the grains may have several points of contact toform continuous ®lms. Fig. 5 shows the morphology ofcoated carbon nanotubes after 30 and 35 min of elec-troless process. The gaps or voids in the coating layerwere eliminated by increasing the thickness of thedeposit so that the cobalt completely isolated the carbon

Table 1

Bath composition and operating conditions of electroless cobalt coat-

ing

Chemical Concentration (g/L)

CoSO4.7H2O 20

NaH2PO2.2H2O 25

(NH4)3C6H5O7 24

Pb(NO3)2 4.5�10-2Bath temperature 35�CpH at 35�C (adjusted using NH4OH) 9.0

Fig. 2. Micrographs of carbon nanotubes after pre-activation: (a)

SEM, (b) TEM. The tin and palladium aggregated on the surfaces of

the carbon nanotube as the active sites.

X. Chen et al. / Composites Science and Technology 60 (2000) 301±306 303

nanotube from the exterior. Longer coating timesimprove the surface of the coating, but increase thediameter of the coated nanotubes (Fig. 5).Electroless plating with cobalt does not isolate the

carbon nanotubes from the exterior unless the thicknessof the deposites increased. The palladium nuclei depos-ited in the activation process are not uniformly dis-tributed on the surfaces of the nanotubes with largecurvature and, consequently, the lateral growth is notenough to form a complete layer covering the wholecarbon nanotube. One way of obtaining a better coatingof the nanotube surface would be to heat treat the

cobalt deposit on the nanotube surface. Fig. 6 shows anSEM image of the cobalt deposit after heat treatment .Itcan be seen that the voids or gaps has been eliminatedor at least reduced by recrystallization of the cobaltduring the heat treatment. The carbon nanotubes havebeen covered completely by a smooth and dense layer ofcobalt on their surfaces. Similar results have beendescribed for graphite covered with copper subjected toheat-treatment [9]. The results indicate that the heat-treatment is an e�ective way of improving the coatinglayer. What is important is that this treatment does notincrease the diameter of the coated carbon nanotubes.

Fig. 3. SEM image of the cobalt-coated carbon nanotubes by electroless plating: (a) and (b) pH=9.0, t=18 min, carbon nanotubes are coated with

a layer of cobalt; (c) pH=9.5, t=18 min, a carbon nanotube was coated with a discontinuous layer of surface of carbon nanotube; (d) pH=10.0,

t=18 min, cobalt deposits are formed by grains on the surface of carbon nanotube; (e) pH=10.5, t=18 min, cobalt formed as ball on the net of

carbon nanotube.

304 X. Chen et al. / Composites Science and Technology 60 (2000) 301±306

There has recently been signi®cant interest and specula-tion about the novel properties and potential applicationsof nanoscale magnetic materials. It was con®rmed [10]that there is enhancement of magnetic coercivity innanoscale magnetic rods, by comparison with bulkmaterials. Thus, it is necessary to carry out magnetiza-tion measurements on carbon nanotubes coated withcobalt. We determined the hysteresis curves at roomtemperature with a vibrating sample magnetometer, andthe result is shown in Fig. 7. From this ®gure we can seethat the coercive force for the coated carbon nanotubesis 1235Oe, which is much higher than the values of 10obtained for cobalt microscale particles. These char-acteristic magnetic properties, resulting from the nan-ometer size e�ect, can be used for high-density magneticrecording.

Fig. 5. SEM micrographs of coated carbon nanotubes after longer

coating times. (a) 30min; (b) 35 min.

Fig. 6. SEM image of coated carbon nanotubes after heat-treatment

corresponding to Fig. 3(a).

Fig. 7. M-H curves for cobalt-coated carbon nanotubes correspond-

ing to Fig. 6.

Fig. 4. EDX pattern of the cobalt-coated carbon nanotubes corre-

sponding to Fig. 3(a).

X. Chen et al. / Composites Science and Technology 60 (2000) 301±306 305

4. Conclusions

The main conclusions from this work may be sum-marized as follows:

1. a method of preparing one-dimensional nanoscalecomposites based on the coating of carbon nanotubeswith metal is proposed and demonstrated. Thismethod is simple, inexpensive, highly reproduci-ble,and might be use to prepare a wide variety ofone-dimensional nanocomposites by using carbonnanotubes as templates;

2. surface activation of the carbon nanotubes is anecessary step before the electroless deposition ofcobalt. However, owing to the low chemical reac-tivity and the high curvature of the carbon nano-tubes, the cobalt tended to form as particles on theouter surfaces of the nanotubes and, consequently,the nanotubes were not completely covered by thedeposit. The fraction of voids or gaps not coveredby cobalt decreases with increasing thickness ofthe deposit, but this does not mean an e�ectiveimprovement of the coating since the diameter ofthe coated carbon nanotube is then too large. Inorder to obtain a better coating, an e�ective way wasfound to heat treat the coated carbon nanotubes.

3. carbon nanotubes coated with a magnetic metalmay be useful for microscopic-scale magnetism

research and high-density magnetic recording anaccount of their higher magnetic coercivity.

References

[1] Robert FS. Superstrong nanotubes show they are smart too. Sci-

ence 1998;281:940±2.

[2] Ajayan PM, Iijima S. Capillarity-induced ®lling of carbon nano-

tubes. Nature 1993;361:333±4.

[3] Ajayan PM, Stephan D, Redlich Ph, Colliex C. Carbon nano-

tubes as removable templates for metal oxide nanocomposites

and nanostructures. Nature 1995;357:564±7.

[4] Sloan J, Cook J, Heesom JR, Green MLH, Hutchison JL. The

encapsulation and in situ rearrangement of polycrystalline SnO

inside carbon nanotubes. J Crystal Growth 1997;173:81±7.

[5] Setlur AA, Lauerhaas JM, Dai JY, Chang RPH. A method for

synthesizing large quantities of carbon nanotubes and encapsu-

lated copper nanowire. Appl Phys Lett 1996;69:345±7.

[6] Satishkumar BC, Govindaraj A, Erasmus M. Oxide nanotubes

prepared using carbon nanotubes as templates. J Mater Res

1997;12:604±6.

[7] Dai H, Wong EW, Lu YZ, Fan S. Synthesis and characterization

of carbide nanorods. Nature 1995;375:769±72.

[8] Li WZ, Xie SS, Qing LX, et al. Large-scale synthesis of aligned

carbon nanotubes. Science 1996;274:1701.

[9] Caturla F, Molina F, Moline-Sabio M, Rodriguez-Reinoso F.

Electroless plating of graphite with copper and nickel. J Electro-

chem Soc 1995;142:4084±90.

[10] Whitney TM, Jiang JS, Searson PC, Chien CL. Fabrication and

magnetic properties of arrays of metallic nanowires. Science

1993;261:1316±9.

306 X. Chen et al. / Composites Science and Technology 60 (2000) 301±306