Materials Letters 63 (2009) 1366–1369

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Materials Letters

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Diameter dependent growth mode of carbon nanotubes on nanoporousSiO2 substrates

Chunyan Li a, Hongwei Zhu a,⁎, Kazutomo Suenaga b, Jinquan Wei a, Kunlin Wang a, Dehai Wu a

a Key Laboratory for Advanced Manufacturing by Material Processing Technology, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR Chinab Center for Advanced Carbon Materials, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan

⁎ Corresponding author. Tel.: +86 10 62781065; fax:E-mail address: hongweizhu@tsinghua.edu.cn (H. Zh

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.03.025

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 February 2009Accepted 18 March 2009Available online 24 March 2009

Keywords:Chemical vapour depositionNanomaterials

Two different growth modes of carbon nanotubes (CNTs) are identified in ethylene chemical vapourdeposition (CVD) using SiO2 as support. With a series of electron microscopy observations, we have foundthat small-diameter nanotubes favor a root-growth mechanism on nanoporous SiO2 support, whilenanotubes with larger diameters prefer a tip-growth. The dependence of growth mode on tube diameter isexplained in terms of the porosity of the support and the size distribution of the catalyst. Our results provideclues to control growth of CNTs and obtain well-organized nanotube structures.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

CVD is a usual method to synthesize CNTs with transition metal(e.g. iron, cobalt and nickel) as catalysts [1]. In a CVD process, apartfrom temperature, carbon source and gas partial pressure, the growthof CNTs is significantly determined by two interfaces (or interactions):catalyst/carbon source and catalyst/substrate. These interactionscause the catalytic decomposition of hydrocarbons and the nucleationof a nanotube. The growth mechanism is essentially divided into rootgrowth [2–5] and tip growth [6–8], depending on the strength of theinteraction between the catalyst and the substrate [9,10]. It has beenfound the structural morphology of the substrate surface influencesthe adhesion force of the catalyst particles to the substrate, andconsequently the growth mode of the CNTs [10]. For example, theweaker adhesion of the catalysts on high roughness substrates [11]will generate a poor wetting of the nanoparticles on substrate, leadinga tip-growth of CNTs. Strong bonding between the catalyst and thesubstrate could be established due to the formation of silicate duringthermal annealing, leading a root growth, while the reduction ofsilicate by a plasma treatment induces a tip growth of CNTs [12].

In this paper,we providenewevidences on this regard throughCVDgrowth of CNTs on nanoporous SiO2 and conclude that small-diameternanotubes (mainly single-walled) favor a root growth mechanism,while large-diameter nanotubes prefer a tip growth. The dependenceof growthmode on tube diameter is explained in terms of the porosityof the support and the size distribution of the catalyst.

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2. Experimental

Substrate preparation and CNTs synthesis followed the proceduresdescribed previously [13]. To promote nanotube growth, TEM coppergrid coated with SiO2 film (~30 nm thick) was used as the catalystsupport which is actually nanoporous with a pore size distribution of1–3 nm. A catalytic metal thin film (iron, cobalt or nickel) with athickness of ~1 nm (for small-diameter CNTs) or ~10 nm (for large-diameter CNTs) was sputtered on the top of the SiO2 support. Prior toCNTs growth, the substrate was annealed in a tube furnace for 20 minunder a flow of 600 mL/min argon and 400 mL/min hydrogen todecompose metal thin film into high density nanoparticles (Fig. 1).After the furnace was heated to 700–750 °C, ethylene of 20 mL/minwas flowed in at as a carbon source and held for 30 s for nanotubegrowth. High-resolution transmission electron microscopy (HRTEM,JEOL, JEM-2010F operated at 120 kV) was employed to characterizethe growth modes of CNTs.

3. Results and discussion

Basically, the nanotube diameter is controlled by the size of thecatalyst nanoparticle [6], which is determined by the thickness of thesputtered metal thin films. As shown in Fig. 1b and c, the catalystnanoparticles converted from 1 nm and 10 nm thick catalyst filmshave a size range of 1–3 nm and 8–15 nm, respectively. Correspond-ingly, the diameter distribution of CNTs grown from these catalystsshow a similar film thickness dependence, which has been confirmedby TEM observations in Figs. 2 and 3.

TEM measurements also reveal that CNTs grow with either root ortip growth mode on the nanoporous SiO2 support, depending on thethickness of the pre-sputtered catalyst films. The sample grown from

Fig. 1. (a) Schematics for substrate preparation and two growth modes of CNTs. (b, c) Catalyst nanoparticles obtained from 1 nm and 10 nm-thick iron thin films. Ins show the corresponding size distribution histograms.

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Fig. 2. Root growth of small-diameter CNTs. (a–c) SWNTs, (d) a DWNT.

1368 C. Li et al. / Materials Letters 63 (2009) 1366–1369

1 nm thick catalyst thin film followed a root growth mechanism. Fig. 2shows TEM images of three typical single-walled CNTs (SWNTs) withvaried length grown at different stage and a double-walled CNT(DWNT) of small diameter (b4 nm). In another comparison experi-ment, larger metal nanoparticles obtained from 10 nm thick filmwereused as catalysts. After growth, most of these nanoparticles can beidentified at the tips of nanotubes (N4 nm, mostly multi-walledtubes), implying that the CNTs grow in this case based on a tip growthmechanism (Fig. 3). From these images, one can also see that the wallnumbers of the CNTs are gradually decreased while no obviouschanges with their inner diameters. One typical CNT is shown inFig. 3e with its wall number changed continuously from seven to one,finally resulting in a SWNT of large diameter (~6 nm) with a catalyst

Fig. 3. Tip growth of lar

particle on the tip. The decrease of the wall number of a growing CNTis probably due to the different carbon precipitation rates in the innerand outer layers during growth.

The dependence of growth mode on tube diameter (or filmthickness) can be explained by following the model given in Fig. 1a.The key factor influencing the growth mode of CNTs is the adhesionforce of the catalyst nanoparticles to the substrate. In our experiment,the catalyst nanoparticles converted from 1 nm-thick catalyst thinfilm have a diameter distribution of 1–3 nm, which exactly matchesthe pore size of nanoporous SiO2. Hence, they would be trapped intothe pores of the SiO2 support due to the strong adhesion force andanchored at the root of the CNTs during the whole nucleation andgrowth process, leading a typical root growth of CNTs with small

ge-diameter CNTs.

Fig. 4. (a) Tip growth of CNTs on Si3N4. (b) Size distribution histogram of ironnanoparticles. (c) A CNT located at the edge of the support.

1369C. Li et al. / Materials Letters 63 (2009) 1366–1369

diameters. For the catalyst obtained from 10 nm-thick film, thediscrepancy between the diameters of the nanoparticles and the poresize of SiO2 support induced a loose interface between the catalyst andthe substrate with a weak adhesion force. Therefore, the catalystnanoparticles tend to lift off from the substrate, promoting a tipgrowth of CNTs. It is worth noting that the CNTs grown from iron,cobalt and nickel show the same diameter dependent growth modes.This can be attributed to the similarity of the interactions betweenthese metals and the nanoporous SiO2.

Besides the above explanation regarding the catalyst/substrateinteraction, our results are also consistent with the existing evidence[14] indicating two different pathways for carbon diffusion andinteractions between carbon clusters and the catalyst are contributingto the catalyst size dependent growth. It has been found that the smallercatalyst particles are more reactive with carbon clusters. As a con-sequence, a root growth mode arises from the strong catalyst/carboninteraction which favors the formation of a graphene cap on the top ofthe catalyst. In contrast, a tip growth is driven by the precipitation ofcarbon at the catalyst/substrate interface due to the weak catalyst/carbon interaction.

Finally, we would like to point out that though the size of thecatalyst nanoparticle is basically controlled by the thickness of the

sputtered metal thin films, the correlation is somewhat empirical andit is also related to the nature of the substrate surface. As mentionedabove, the nanoporous SiO2 can trap the catalyst particles into thepores of the substrate and thus prevent them agglomerating. We havebeen trying to deposit catalyst nanoparticles of small enough size on aSi3N4 support in order to grow CNTs with desired diameters. However,in spite of the thickness of the sputtered catalyst film (1–10 nm), theobtained particles show a wide size distribution ranging from 3 to15 nm. As shown in Fig. 4, most of the CNTs grow on Si3N4 based on atip growth mode regardless of the thickness of the catalyst film or thetype of the metal. This might be due to the metal films could not makea good contact with Si3N4 support whose surface porosity is generallyin the submicron range [15], thus inducing agglomeration of catalyst.

4. Conclusions

In summary, two different growth modes (root and tip growth) inCNTs synthesis can result based on the interaction of the catalyst withits support. Small-diameter nanotubes are found to favor a rootgrowth mechanism, while nanotubes of large diameters prefer a tipgrowth on nanoporous SiO2 support. The growth mode could be fairlytuned by controlling the surface morphology and the size of thecatalyst. Our results provide clues to control growth of CNTs andobtain well-organized nanotube structures.

Acknowledgements

This work was financially supported by Tsinghua University TalentSupport Foundation (China) and the NEDO Nano-Carbon TechnologyProject (Japan).

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