Synthesis of well-aligned carbon nanotubes on the NH3 pretreatment Ni catalyst films

ISSN 0036�0244, Russian Journal of Physical Chemistry A, 2010, Vol. 84, No. 9, pp. 1560–1565. © Pleiades Publishing, Ltd., 2010.

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INTRODUCTION

Since the first discovery of carbon nanotubes(CNTs) by Iijima in 1991 [1] they have been promisingfor many potential technological applications due totheir extraordinary chemical and physical properties.Synthesis of carbon nanotubes for mass productionhas been achieved by several methods such as laservaporization [2] arc discharge [3, 4] and chemicalvapor deposition (CVD) [5–17]. Synthesis of well�aligned CNTs on a large area is inevitably necessary forfield emission device applications. Arc discharge andlaser vaporization techniques can produce a largeamount of CNTs, but it is very difficult to obtain uni�form alignment on a large area. The CVD process hasbeen developed for growth of CNTs in well�alignedconfigurations. Terrones el al. [18[ fabricated verti�cally aligned carbon nanotubes on pulse�laser depos�ited Co films on silica substrates by means of thermalCVD. Li et al. [19] used a method based on CVD cat�alyzed by Fe nanoparticles embedded in mesoporoussilica and produced large areas of vertically well�aligned long carbon nanotubes. Fan et al. [20] synthe�sized aligned nanobubes on Fe�patterned porous Sisubstrates using thermal CVD. Howerer, detailedstudies of the effects of the structures of metal films onthe growth of nanotubes have not been hitherto per�formed.

In this article, we report that very uniform, high�density nano�sized catalysts were obtained by NH3

etching pretreatment from a homogenous Ni layer.Well�aligned vertically arranged and high density

CNTs can be synthesized by thermal CVD on opti�mally broken catalysts. The objective of this study is toget reproducibly the catalyst nanoparticles not onlywith the appropriate size and distribution but also withthe high density suitable to promote the growth ofdensely�packed vertically aligned CNTs suitable forfuture application.

EXPERIMENTAL

The substrates used in this study are N�type Si(111) wafers coated with SiO2 by thermal oxidation.The SiO2 acts as a diffusion barrier between Ni and Sipreventing the formation of nickel silicide. Ni catalystfilms were deposited by K575X Peltier Cooled HighResolution Sputtering Coater with base pressure<10–5 mbar (Quorum/Emitech, English). The filmthickness was monitored in situ by a quart/balance,and calibrated ex situ by atomic fore microscopy(AFM, Veeco Explorer).

CVD was carried out in a horizontal quartz tube(60 mm in diameter; 1200 mm in length) at atmo�spheric pressure. The substrates were then cut intosmall (20 mm × 30 mm). In each experiment, onesample was placed in the middle of the CVD tube. Thequartz tube was first purged by N2 gas for 15 min toexclude air. A constant flow rate of N2 was maintainedwhile the reaction chamber was then heated up to600°C, which was held constant in H2 instead of N2

and Ni catalyst deoxidized for 60 min. After that, thereaction chamber was continuously heated up to thereaction temperature in hydrogen. Prior to depositionreaction, to form the nanometer�sized catalyst parti�

Synthesis of Well�Aligned Carbon Nanotubes on the NH3 Pretreatment Ni Catalyst Films1

Gang LiLaboratory of Ministry of Education for Conveyance and Equipment,

School of Mechanical and Electrical Engineering, East China Jiaotong University, Nanchang, P.R. Chinae�mail: [email protected]

Received September 5, 2009

Abstract—Well aligned multi�walled carbon nanotubes (CNTs) have been synthesized on large area Ni�deposited SiO2/Si substrates via the pyrolysis of C2H2 using thermal chemical vapor deposition technique at900°C. We concluded that NH3 pretreatment was very crucial to control the surface morphology of catalyticmetals and thus to achieve the vertical alignment of CNTs. With higher density of the Ni particles, betteralignment of the CNTs can be obtained due to steric hindrance effect between neighboring CNTs. The degreeof crystallization of the CNTs enhanced with the increase of the NH3 pretreatment time was investigated byX�ray diffraction and transmission electron microscope studies. Energy dispersive X�ray spectrum analysisrevealed that CNTs grew by a tip growth mechanism.

DOI: 10.1134/S0036024410090219

PHYSICAL CHEMISTRY OF NANOCLUSTERS AND NANOMATERIALS

1 The article is published in the original.

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 84 No. 9 2010

SYNTHESIS OF WELL�ALIGNED CARBON NANOTUBES 1561

cles required for well�aligned CNTs growth, the Nicatalyst film was pretreated by NH3 gas, with a flowrate of 100 cm3/min for 10–60 min at a temperaturerange of 800–950°C. After the NH3 treatment, C2H2

gas was introduced into the reaction chamber at a flowof 30 cm3/min at the same temperature as the NH3

pretreatment. The growth time was varied from 5 to20 min. Finally, the chamber was slowly cooled downto the room temperature under N2 ambient after thegrowth. As�pretreated Ni catalyst films were charac�terized by AFM in tapping mode at ambient condi�tions. The morphology of as�grown CNTs were exam�ined by field emission scanning electron microscope(FESEM, JEOL, JSM�7001F). Transmission electron

microscope (TEM; JEOL, JEM�2100) was used toinvestigate the wall structure of an individual CNT andthe crystallinity of the CNTs. To prepare the TEMsamples, carbon nanotubes were peeled off by tweezersfrom the SiO2/Si substrates and ultrasonicated in eth�anol for 45 min to form a well�dispersed solutionwhich was then dropped onto a 3 mm Cu grid coatedwith a layer of holey carbon. TEM test was performedafter the grid was dried in an oven. The chemicalcompositional analysis of the as�deposited CNTswas carried out by energy dispersive X�ray (EDX,Oxford, Inca Energy 350) spectrum system. X�raydiffraction studies (XRD, r�Ray Diffractometer,D/max 2500 PC, CuK

α radiation) were performed to

10

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120.22

0

0 105

μm

0 105μm

μmμm (b)

0

221.29nm

nm

(a)

Fig. 1. AFM images for the surface morphology of the Ni catalytic nanoparticles formed on SiO2/Si substrate from 20 nm Ni filmafter the heat and NH3 pretreatment for at 900°C for (a) 20 and (b) 60 min.

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identify the products and determine their degree ofcrystallization.

RESULTS AND DISCUSSION

AFM images of surface morphology of Ni nano�particles transported from the Ni films indicate theeffect of the pretreatment conditions. Figure 1 is theAFM image in contact mode, which shows the surfacemorphologies of the 20 nm thickness Ni films pre�pared at different NH3 etching treatment conditions.In these cases, the etching treatment time werechanged while fixing the flow rate of NH3 with100 cm3/min and the etching temperature at 900°C. Itis seen that the Ni films breaks into small islands andparticles due to surface tension as well as the compres�sive stress resulting from the mismatch of the thermalexpansion coefficients of Ni and SiO2. The Ni particlesize�decreases with the increase of the etching treat�ment time, whereas the particle density increased.

Root�mean�square (RMS) roughness was measuredover the whole area. According to AFM analysis wecan calculate the RMS roughness of Figs. 1a and 1b,which are 7.449 and 3.126 nm, respectively. The NH3

etches metal film surface and decreases the surfaceroughness with the increasing of NH3 pretreatmenttime. The Ni particles are uniformly distributed overthe whole surface of the substrate as shown in Fig. 1bcompared to that shown in Fig. 1a, which is also con�firmed by the RMS height in Fig. 1. It is concludedthat the size and area density of these nanoparticlesdepend on the NH3 etching treatment conditions andthe appropriate size distribution and high density ofparticles can be obtained by prolonging the NH3 pre�treatment time.

Figures 2a, 2b are SEM micrographs of the CNTsgrown on 20 nm thick Ni films with different etchingpretreatment time. The C2H2 gas for CNTs growth isfixed to the flow rate of 30 cm3/min, for 10 min at900°C, but the NH3 pretreatment condition is

10 μm(а) 100 nm

100 nm 10 μm(b)

Fig. 2. SEM morphology of the CNTs grown by thermalCVD process on the SiO2/Si substrates after the NH3etching pretreatment of (a) 20 and (b) 60 min. The insetsshow high magnification cross section views of (a) and (b),respectively.

100 nm

50 nm

(а)

(b)

Fig. 3. TEM images of the CNTs grown by thermal CVDprocess on the SiO2/Si substrates after the NH3 etchingpretreatment of (a) 20 and (b) 60 min.



changed to control the size and number density of theNi catalytic particles. The edges are peeled off usingtweezers to observe and analyze the orientation andphysical properties. For the case of NH3 pretreatmentwith 20 min, because of a lower particle density, thenanotube density is also lower and the nanotubes arenot strictly oriented vertically as shown in Fig. 2a.When NH3 pretreatment time prolonged up to 60 min,the nanotubes are oriented perpendicular to the sur�face of the substrate due to high density of Ni nano�partilces. A single nanotubes has a curly shape asshown in the insets of Fig. 2b. From Figs. 2a and 2b,we can also deduce that the length of CNTs increasefrom about 4 to 9 µm. The insets show that the rangesof the diameter of the two kinds of CNTs decreasefrom 60–250 to 20–40 nm with increasing NH3 pre�treatment time.

Since the particle size of catalyst determines thediameter of CNTs, the smaller size of catalytic parti�cles results in a smaller diameter of CNTs. At the sametime, when the density of catalytic metal particleincreases, the vertical alignment degree of CNTs is sig�nificantly enhanced. When the density of CNTsreaches certain high level, the growth in non�verticaldirection is prohibited due to the steric hindrancefrom adjacent CNTs. The vertically aligned CNTs canbundled together by van dé Wäls force. However whenthe nanoparticles density is not high enough, the lackof van dé Wäls interactions between neighboringCNTs results in entangled CNTs, which are observedin Fig. 2a. It strongly suggests that the formationmechanism of the vertically aligned CNTs is the sterichindrance between nanotubes, which is invoked by thehigh number density of Ni particles. The appropriatedensity of nanoparticles on substrate is crucial ingrowing vertically aligned CNTs. Without NH3 pre�treatment, the CNTs can grow on the substrate, butnot in vertical direction. So a prior NH3 treatment is

necessary to obtain high density of nucleation sites forCNT growth.

Figures 3a, 3b, respectively, show the TEM imagesof as�grown CNTs that appeared in the SEM images(Figs. 2a, 2b). Figure 3a shows that the CNTs grownon the nanoparticles pretreated by NH3 for 20 minhave a multi�walled structure with a hollow inside andno compartment layer in the tube. But they also havesome carbon particles in the walls and in the tube. Thegraphene sheets indicate waving structure in the shortrange, revealing poor crystallinity. Figure 3b showsthat the CNTs grown on the nanoparticles pretreatedby NH3 for 60 min, which have a graphite wall thick�ness of 10 nm and a hollow inside of about 10 nm andno compartment layer. The walls and the inside arevery clean. Most graphite sheets have straight fringes,revealing a high degree of crystallinity.

Figure 4 shows XRD pattern of as�grown CNTs,corresponding to the pretreatment duration of 20 and60 min. The sharp peaks at 44° and 52° come from theoxidization of the silicon substrates and Ni filmsdeposited on substrates, and the other peaks lying at26° and 76° attribute to G(002), G(110), respectively.From Fig. 4 we can see the strong SiO2 and Ni peaksof the two kinds CNTs were significantly reduced fromthe NH3 pretreatment of 20 to 60 min. Thus it may beresulted from the lower number density of the formercompared to that of the latter. The G(002) peakenhances and the G(110) peak decreases with the NH3

pretreatment time. And the G(002) peak correspondsto the graphite structure and d002 spacing of 0.337 nm.The appearance of G(110) peak indicates fewerdefects in the graphene sheets. The inlayer distance isslightly shorter and slightly larger than that of the stan�dard graphite structure. This result implies that the as�grown films are composed mainly of layered graphitestructure, and the degree of crystallization of the

20 40 60 802θ, deg

G(0

02)

SiO

2

Ni

G(1

10)

(a)

(b)

Fig. 4. XRD of the CNTs grown on the substrates after the NH3 etched at 900°C for (a) 20 and (b) 60 min.

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CNTs increases with the NH3 pretreatment time. Thisis consistent with TEM analysis.

Figure 5 shows a high resolution EDX spectrum ofthe various sites of aligned CNTs, confirming thechemical composition of the well�aligned CNTs(Fig. 2b). The EDX spectrum in Fig. 5a reveals thatthe top ends of the well�aligned CNTs were composedmainly of carbon and a very small amount of Ni metal,oxygen and Si. Note that in the spectrum, the siliconpeak comes from the substrate, the oxygen peak comesfrom the oxidization of the silicon substrates, and theNi peak comes from the pre�deposited catalyst Nifilms on the substrates. Whereas the roots of the CNTscontained no Ni elements other than carbon, oxygenand silicon as shown in Fig. 5b. This result of EDXanalysis indicates that the aligned CNTs grew by a tipgrowth mechanism, i.e. the seeded metal particleswere lifted up as the CNTs grew. This may suggest thatthe NH3 etching pretreatment weakens the contactforce between the catalyst and substrate. Metal nano�particles are lifted by the carbon source moleculebecause they are loosely bonded to the substrate. Con�sequently, the catalyst nanoparticals remains at the tipof the developing nanotube. The result is in a goodagreement with FESEM observation. The EDX inves�tigations along the side face of the CNT bundles alsoindicate that no Ni elements other than carbon, oxy�gen and silicon are detected.

CONCLUSIONS

We have demonstrated successfully a simple andefficient method for the growth of well�aligned CNTfilms. It was shown that the homogeneous Ni films

were broken up into nanopaticles during the NH3 pre�treatment. Our result indicates that the alignment anddensity of CNTs can be controlled by adjusting thedensity of Ni nanoparticles. The size distribution andarea density of Ni nanoparticles can be controlled bythe NH3 etching pretreatment conditions. With higherdensity of the Ni particles, better alignment of theCNTs can be obtained because of steric hindranceeffect between neighboring CNTs. TEM and XRDshowed that the degree of crystallization of the as�grown CNTs increases with the NH3 pretreatmenttime. The EDX spectrum analysis revealed that CNTsgrew by a tip growth mechanism.

ACKNOWLEDGMENTS

This work is supported by the State Key Program ofNational Natural Science of China under grantno. 30800666.

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200

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E, keV

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Synthesis of well-aligned carbon nanotubes on the NH3 pretreatment Ni catalyst films