presence of Co catalysts.Therefore,carbon tubes with the same length and diameter as that of the AAO tem-plate can be synthesized,though the crystallinity of them is generally not high.

On the other hand,Jeong[17]and Iwasaki[18]have reported that carbon tubes are not con?ned in the pores and can overgrow from the pores of an AAO template in the pyrolysis of C2H2with added H2.Transmission electron microscopy(TEM)study reveals that these carbon tubes are mainly composed of concentric gra-phitic walls,which is the main characteristic of multi-walled carbon nanotubes(MWNTs).

In this paper,the e?ect of H2on the synthesis of carbon tubes in the pores of an AAO template is closely investigated and a uni?ed mechanism of CNT synthesis is proposed.

2.Experimental

2.1.Preparation of AAO template

In the present studies,high purity Al sheet(99.999%, 1mm thickness,Nilaco Ltd,Japan)was used as starting materials.Beside phosphoric acid solution(85%,J.T. Baker),all chemicals were purchased from Sigma-Al-drich;Perchloric acid(70%,ACS reagent),ethanol (95%,ACS reagent),oxalic acid(99%),chromium oxide (minimum99.5%,ACS reagent),boric acid(minimum 99.5%,ACS reagent),and cobalt(II)heptahydrate (minimum99%).

High purity Al sheet was used as a starting material. Prior to anodization,the Al sheet was degreased by ultrasonication in acetone and electropolished in a per-chloric acid–ethanol solution.Two-step anodization was employed to produce a hexagonal array of pores[19].A constant voltage was applied by a computer-interfaced power supply(Hobang,HBPS-1A200V,Korea).The ?rst anodization step was carried out at a constant voltage of30–40V for12in a0.3M oxalic acid solution at17°C,then the oxide layer was stripped out in a mixture of chromic acid and phosphoric acid at65°C.

The second anodization step was carried out under identical conditions until the desired thickness of the AAO?lm was obtained.After anodization,the pores of the AAO template were widened by dipping the sample in a0.1M phosphoric acid solution at30°C for50min.

A di?erent type of AAO template,which has small pores in the upper part connected to large pores in the lower part,was also prepared by varying anodization condi-tions.The small pores in the upper part were formed during the anodization in an oxalic acid solution at40V for3min,while the large pores in the lower part were formed after switching to a phosphoric acid solution at 120V in10wt.%phosphoric acid solution for20min.

To facilitate the electrochemical deposition of Co catalyst,barrier layers in the samples were thinned by voltage dropping to16V.Co particles were then elec-trochemically deposited at the bottom of the pores of the AAO in an electrolyte solution consisting of5wt.% CoSO4?7H2O and2wt.%H3BO3at room temperature by applying alternating current[17].

http://www.360docs.net/doc/9fb535bf02d276a200292ec1.htmlT growth with AAO templates

Acetylene(high purity,>99.95%),argon(ultrahigh purity,>99.99%),hydrogen(ultrahigh purity,>99.99%), and carbon monoxide(high purity,>99.95%)were supplied by BOC Gases Korea Co.Ltd.The AAO templates were reduced in a gas mixture of10%H2and 90%Ar at500°C for1h.After the reduction,the temperature was raised to650°C under?owing Ar atmosphere.Then,CNTs were grown by introducing the reaction gas for2h and20min in a tube reactor (DAEPOONG,TF,Korea).The reaction gas for growing CNTs was a20%C2H2in Ar carrier gas with or without10%H2.Pure CO was also used to grow CNTs. The total?ow rate was200sccm.After the CNT growth,the temperature was cooled to room tempera-ture in Ar atmosphere.

2.3.Sample characterization

The morphology and microstructure of CNTs were examined using FE-SEM(Hitachi,S4200,Japan)and TEM(JEOL,JEM-2010F,Japan).Electron di?raction patterns were obtained using the same instrument. Raman(Renishaw,Raman System3000,UK)spectra were obtained using an Ar-ion laser of514.5nm as an excitation source.The integrated intensity of each peak was calculated from the product of the peak height and half the peak width.

3.Results and discussion

3.1.Morphology of CNTs

The growth of carbon tubes in the pores of an AAO template is greatly a?ected by the composition of a gas mixture.When a gas mixture of10%C2H2and90%Ar was used,carbon tubes were synthesized only in the pores of an AAO template,even with Co catalysts.Fig. 1(a)is the SEM photograph of the carbon tubes after removing the AAO template by immersing the sample in a10%HF solution to observe individual carbon tubes. To facilitate the dissolution of the AAO template in the HF solution,carbon deposit on the top surface of the AAO template was removed by O2plasma treatment. The diameter and the length of carbon tubes,which are 50nm and8l m respectively,correspond to those of the

2074S.-H.Jeong et al./Carbon42(2004)2073–2080