Towards the production of large-scale alignedcarbon nanotubes

Charanjeet Singh 1, Milo S.P. Shaffer, Krzysztof K.K. Koziol,Ian A. Kinloch, Alan H. Windle *

Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK

Received 16 February 2003; in final form 25 March 2003

Abstract

A novel method is presented to produce high purity, aligned multi-walled carbon nanotube films grown on thin

quartz flakes by injecting a solution of ferrocene in toluene. After reaction, these quartz flakes support arrays of na-

notubes arranged perpendicular to the surfaces of the substrate. This method is seen to increase the yield of nanotubes

dramatically compared to the conventional injection CVD method. Such a method may offer the possibility of pro-

ducing aligned nanotubes at large scales. In addition, the analysis established the presence of bands of varying iron

concentration within this type of material.

� 2003 Elsevier Science B.V. All rights reserved.

1. Introduction

Since the recognition of multi-walled carbon

nanotubes (MWCNTs) [1], many methods of

production have been reported, including electric

arc-discharge [2], laser evaporation [3], chemical

vapour deposition (CVD) [4], plasma-enhanced

CVD [5] amongst many others. This interest is due

to the great range of promising properties thathave been observed for nanotubes, including un-

ique mechanical and electrical behaviours [6–8],

which are under investigation for a wide range of

practical applications [9]. In order to realise muchof this potential, nanotubes need to be available in

larger quantities, at reasonable prices, with much

greater control over key characteristics such as

length and diameter. So far, the CVD process

seems to be the most promising method for pos-

sible industrial scale-up due to the relatively low

growth temperature, high yields and high purities

that can be achieved.Production of relatively large volumes of un-

aligned MWCNTs has been achieved over the

years by Hyperion Catalysis International and al-

though these materials are generally entangled and

of variable length, they are being successfully used

for commercial purposes [10]. The synthesis of

aligned MWCNTs was first reported by Li et al.

[11] using mesoporous silica containing iron

Chemical Physics Letters 372 (2003) 860–865

www.elsevier.com/locate/cplett

* Corresponding author. Fax : +44-1223-334-567.

E-mail addresses: cs266@cam.ac.uk (C. Singh), ahw1@cam.

ac.uk (A.H. Windle).1 Also corresponding author. Fax : +44-1223-334-567.

0009-2614/03/$ - see front matter � 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0009-2614(03)00531-1

nanoparticles. Since than many reports of aligned

arrays of nanotubes have appeared, mostly in-

volving the use of preformed substrates [12,13].

The floating catalyst method involves pumping

or spraying a catalyst precursor into a suitable

furnace; the precursor is typically a metallocene–hydrocarbon solution, and is most commonly

ferrocene dissolved in benzene or xylene [14–18].

Such a method has been employed to grow high-

purity MWCNTs of controlled architectures (i.e.

length, diameter and crystallinity) [19]. In almost

all cases, the nanotubes are grown on quartz

(SiO2), in the form of either a specific substrate or

the reactor wall. Zhang et al. [17] reported thataligned carbon nanotubes grow only on SiO2

substrates and not silicon.

Results presented here demonstrate how such as

a system can be used to grow large volumes of

aligned nanotubes perpendicular to the surface of

the substrate. By using such a method, the yield of

nanotubes has increased dramatically with the in-

crease in the surface area of the substrate. In somecases, the nanotubes grow perpendicular from

many faces of the quartz flakes resulting in a

structure with its own unique features.

2. Experimental section

The experimental set-up used to synthesise thealigned MWCNTs films has been reported else-

where [19] and is briefly described here. A solution

of ferrocene (between 2 to 9.6 wt%.) in toluene is

injected into a two-stage furnace where ferrocene

decomposes to provide the iron catalyst required

to nucleate the nanotube growth whilst toluene

acts as the carbon feedstock. A mixture of argon

and 10% hydrogen is used as the carrier gas. Thetotal gas flow rate was maintained at 750 ml/min.

The first stage furnace was preheated to about

200 �C to ensure that the solution was vapourised

as it was injected (the vapourisation temperatures

of ferrocene and toluene are 175 and 110 �C, re-spectively). The vapour was then swept into the

second stage furnace, held normally about 700 to

760 �C, where the aligned nanotube films grewfrom both quartz substrates and the surrounding

reaction tube (14 mm inner diameter).

In the first set of experiments, thin quartz flakes

were made by scratching a larger quartz substrate

with a diamond scribe. This process forms thin

shards of quartz with a thickness of not more than

100nmanddiameters of severalmicrons. In the next

stage, the quartz flakes were formed by ball-millinglarger substrates. Here the substrates tend to be

much thicker and larger than the scratchedmaterial.

The product was characterised using a JEOL

6340 field emission gun scanning electron micro-

scope (FEGSEM) and a JEOL 5800 LV, which

were both equipped with EDX detectors for ele-

mental analysis, a JEOL 200CX transmission

electron microscope (TEM), a Perkin–Elmer Series7 thermogravimetric analysis (TGA), a Microm-

eritics ASAP 2010 for surface area measurements,

and a Renishaw 1000 Ramascope spectrometer

using a 514 nm excitation laser.

3. Results and discussion

3.1. Scratched substrate

Fig. 1 shows a SEM and TEM image of na-

notubes grown on the quartz flakes formed by the

scratching process. The nanotube arrays were

grown at 760 �C for 60 min, and at a feed rate 1.2

ml/h of 9.6 wt% ferrocene in toluene concentra-

tion. Aligned arrays of nanotubes can be seengrowing perpendicular from both sides of the thin

layer of quartz, to form wide �bundles�. From

SEM, the thickness of this quartz substrate was

between 50 and 100 nm, with lateral dimensions,

typically in microns. The lengths of the nanotubes

were typically tens of microns in length. Also in-

teresting is the fact that the initial few microns of

growth on the surface the substrate surface have adifferent contrast to the rest of the nanotubes. It

seems likely that this zone is present in other cases

of injection-based, aligned growth of nanotubes on

silica, but it has not been reported previously. In

earlier reports, the nanotubes are usually removed

from the substrate prior to the electron micros-

copy, potentially losing important information

regarding the growth process. We believe that thenanotubes are growing via the base-growth

mechanism, as discussed previously [19]. The

C. Singh et al. / Chemical Physics Letters 372 (2003) 860–865 861

alignment is as a result of overcrowding, as sug-

gested previously in literature [12,13]. The lack of

spread at the edges of the bundle suggests that the

nanotubes are attracted or entangled with eachother. Since the average separation is rather large

compared to van der Waals interactions, some

gentle entanglement or weaving seems the most

likely explanation; a similar argument was recently

proposed to explain the formation of fibres drawn

from continuous films of similar nanotubes [20].

Using TEM analysis, the outer diameters of the

nanotubes were seen to vary from 20 to 80 nm withan average outer diameter of 38� 5 nm. The inner

diameter of the nanotube is typically about 10 nm.

Fig. 1b indicates that the products consist of high-

purity MWCNTs and some encapsulated particles,

as well encapsulated particles attached to the

nanotube walls due to the continuous injection of

the ferrocene. The encapsulated particles within

the nanotubes are typically tens to hundreds ofnanometres in length with a diameter similar or

even larger than the inner diameter of the nano-

tube (see Fig. 1b).

EDX analysis was performed from both the

substrate area and the nanotubes and is shown in

Fig. 2. EDX confirms that the substrate material is

indeed quartz (SiO2). Furthermore, the material

was imaged in SEM (BSEI) mode. Figs. 3a,bshows the nanotubes in secondary (SEI) and back-

scattered (BSEI) electron imaging modes. The

BSEI mode is capable of picking out elements of

different atomic masses present within the nano-

tube ropes. It�s interesting to see streaks/bands

running across the nanotube bundles. These

streaks/bands can be attributed to the encapsu-

lated iron particles present within the nanotubes.

The streaks are believed to be a result of the

heating tape system employed in the initial vapo-

urisation furnace, which had a thermal controllerwith relatively high hysteresis. The number of

streaks seems to coincide with the number of times

the tape switched on during the experiment,

thereby supplying an increased ferrocene concen-

tration. The material was mapped to identify the

elements present within the bundle structures and

the result is shown in Fig. 3. The mapping clearly

identifies the presence of the quartz (SiO2) in thesubstrate and iron in the streaks/bands and also at

the substrate surface.

Fig. 1. SEM and TEM image of nanotube arrays grown at 760 �C for 60 min, 750 ml/min Ar=H2 flow rate and at a feed rate 1.2 ml/h

of 9.6 wt%. ferrocene toluene solution.

Fig. 2. EDX spectrum taken from the support and tube area.

The spectrum indicates that the support is quartz.

862 C. Singh et al. / Chemical Physics Letters 372 (2003) 860–865

Raman spectroscopy was performed on the

nanotubes and is shown in Fig. 4. In carbon based

materials, typically two main first order peaks are

present, the D peak, observed at 1350 cm�1 relates

to the presence of defects whilst the G peak at

1580 cm�1 is associated with the in-plane vibra-

tions of the graphene sheet. Ratios of the D peak

to the G peak have been used an indicator of the

amount of disorder within nanotubes, in particular

[21]. The smaller the ID=IG ratio, the fewer defects

within the nanotubes. For the spectrum given in

Fig. 4, the ratio of the D–G peak was determined

to be about 0.2, indicating reasonable crystalline

quality. The experiment was repeated successfully

to demonstrate that such structures could be re-

produced.

Fig. 3. Nanotubes rope imaged using (a) SEI and (b) BSEI mode. The length of the rope (in one direction) is about 80 lm and the

substrate is approximately 50 lm across. Below, mapping of elements present within the sample. The mapping clearly indicates the

presence of the quartz (SiO2) as well as the encapsulated iron.

C. Singh et al. / Chemical Physics Letters 372 (2003) 860–865 863

A separate attempt was made to grow nanotu-

bes onto fumed silica. Fumed silica of a high-sur-face area ð350 m2=gÞ was placed into a boat and

the reaction was carried out at 760 �C for 60 min

with 9.6 wt% ferrocene in toluene concentration.

SEM analysis indicates that the nanotubes grew in

random directions and were entangled. Therefore,

it seems that a flat surface morphology is crucial

for the growth of aligned nanotubes.

3.2. Ball-milled substrates

Ball-milling was used to scale-up the initial

promising results. After the ball-milling process

(typically a two hour treatment produced a pow-

der of several microns in diameter), the resulting

powder was dried to remove any moisture and

then either sprinkled over a quartz slide or placed

in a crucible. The surface area of the ball-milled

silica was approximately 1 m2=g. Growth was

performed using a 2 wt% ferrocene toluene solu-

tion at 700 �C for 90 min. The 2 wt% ferrocenetoluene solution was chosen since previous results

indicate that the nanotubes grown using this so-

lution have smaller outer diameters and less en-

capsulated material formation compared to the

9.6 wt% solution [19]. An optical micrograph of the

as-grown product is shown in Fig. 5a. SEM images

of these products are shown in Fig. 5b. There is a

large apparent volume increase which can be at-tributed to the separation of the substrate particles

by the growth pressure of the nanotubes pushing

against each other. TEM indicates that the material

consists of high-purity MWCNTs aligned on

quartz particles with some encapsulated nanopar-

ticles. Typical lengths of these structures ranged

from tens to hundreds of microns. The average

outer diameter of the MWCNTs was 27:0� 2:7nm. The smaller average outer diameter, compared

to the scratching experiments, is due to the lower

ferrocene concentration and lower growth tem-

perature, and agrees with our previous results [19].

TGA was performed on the products and the

weight loss averaged over five separate measure-

ments was �39:7� 8:7 wt%, corresponding to a

yield of 66.0 wt%. The yield was calculated on thebasis of the following formula: yield ð%Þ ¼½ðm1 � m0Þ=m0� 100%; where m1 is the mass of the

Fig. 4. Raman spectrum taken from nanotubes grown on thin

quartz slides. The ratio of the D–G peak ðID=IGÞ was 0.2.

Fig. 5. (a) Optical micrograph of the as grown-product obtained after the reaction, and (b) SEM image of aligned MWCNTs grown on

quartz substrates at �700 �C for 90 min, 700:50 ml/min Ar:H2 flow rate and at a feed rate 1.2 ml/h of 2 wt%. ferrocene solution.

864 C. Singh et al. / Chemical Physics Letters 372 (2003) 860–865

carbon deposit gain and substrate after growth,

whilst m0 is the mass of the substrate after TGA.

The yield is lower than anticipated visually because

the substrates are rather large and dense compared

to the nanotube bundles as observed during SEM.

However, previously, macroscopic large flat sub-strates [19] resulted in yields not exceeding 0.62

wt% under optimised conditions. The ratio of the

D–G peak was determined to be about 0.43, higher

than the previous runs due to the lower ferrocene

concentration and lower growth temperature [19].

4. Conclusions

The growth of aligned carbon nanotubes has

been demonstrated on thin flakes of quartz sub-

strates. The nanotube product consists of high

yield and purity, of aligned MWCNTs which have

relatively low defect concentrations as measured by

Raman spectroscopy. However, this study has es-

tablished the presence of bands of varying ironcomposition within the product, which have not

been previously reported although they are likely

to be present in other materials grown using the

injection CVD technique. The formation of small

and thin quartz substrates contributed to high-

surface area for the growth of the nanotubes and

increased the yield of nanotubes. Such nanostruc-

tures may be of interest for large-scale synthesis ofaligned carbon nanotubes. In particular, the pro-

duction of large volumes of mutually aligned na-

notubes is of interest in composite applications

where initial experiments have shown that aligned

arrays grown on flat surfaces disperse much more

readily than entangled material [22]. In addition,

nanotube-modified silica particles may prove to be

interesting composite fillers in their own right. It isworth noting that nanotube-modified conventional

fibres have already demonstrated very large in-

creases in interfacial shear strength [23].

Acknowledgements

The author (CS) would like thank SIRIMBerhad, the Cambridge Commonwealth Trust and

Selwyn College, Thomas Swan & Co. Ltd. for fi-

nancial support. The authors would also like to

acknowledge the help of Dr. Francis Tailoka and

Dr. Simon Roberts and David Vowels for their

helpful discussions [24].

References

[1] S. Iijima, Nature 354 (1991) 56.

[2] T.W. Ebbesen, P.M. Ajayan, Nature 358 (1992) 220.

[3] A. Thess, R. Lee, P. Nikolaev, H.J. Dai, P. Petit, J. Robert,

C.H. Xu, Y.H. Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert,

G.E. Scuseria, D. Tomanek, J.E. Fischer, R.E. Smalley,

Science 273 (1996) 483.

[4] M. Joseyacaman, M. Mikiyoshida, L. Rendon, J.G.

Santiesteban, Appl. Phys. Lett. 62 (1993) 657.

[5] Z.F. Ren, Z.P. Huang, J.W. Xu, J.H. Wang, P. Bush, M.P.

Siegal, P.N. Provencio, Science 282 (1998) 1105.

[6] E.W. Wong, P.E. Sheehan, C.M. Lieber, Science 277

(1997) 1971.

[7] S. Frank, P. Poncharal, Z.L. Wang, W.A. de Heer, Science

280 (1998) 1744.

[8] M.F. Yu, O. Lourie, M.J. Dyer, K. Moloni, T.F. Kelly,

R.S. Ruoff, Science 287 (2000) 637.

[9] R.H. Baughman, A.A. Zakhidov, W.A. de Heer, Science

297 (2002) 787.

[10] H.G. Tennent, J.J. Barber, R. Hoch, US Patent 5578543

(1996).

[11] W.Z. Li, S.S. Xie, L.X. Qian, B.H. Chang, B.S. Zou, W.Y.

Zhou, R.A. Zhao, G. Wang, Science 274 (1996) 1701.

[12] M. Terrones, N. Grobert, J. Olivares, J.P. Zhang, H.

Terrones, K. Kordatos, W.K. Hsu, J.P. Hare, P.D.

Townsend, K. Prassides, A.K. Cheetham, H.W. Kroto,

D.R.M. Walton, Nature 388 (1997) 52.

[13] S.S. Fan, M.G. Chapline, N.R. Franklin, T.W. Tombler,

A.M. Cassell, H.J. Dai, Science 283 (1999) 512.

[14] R. Sen, A. Govindaraj, C.N.R. Rao, Chem. Mater. 9

(1997) 2078.

[15] R. Andrews, D. Jacques, A.M. Rao, F. Derbyshire, D.

Qian, X. Fan, E.C. Dickey, J. Chen, Chem. Phys. Lett. 303

(1999) 467.

[16] R. Kamalakaran, M. Terrones, T. Seeger, P. Kohler-

Redlich, M. Ruhle, Y.A. Kim, T. Hayashi, M. Endo, Appl.

Phys. Lett. 77 (2000) 3385.

[17] Z.J. Zhang, B.Q. Wei, G. Ramanath, P.M. Ajayan, Appl.

Phys. Lett. 77 (2000) 3764.

[18] H.W. Zhu, C.L. Xu, D.H. Wu, B.Q. Wei, R. Vajtai, P.M.

Ajayan, Science 296 (2002) 884.

[19] C. Singh, M.S. Shaffer, A.H. Windle, Carbon 41 (2003)

359.

[20] K.L. Jiang, Q.Q. Li, S.S. Fan, Nature 419 (2002) 801.

[21] P.H. Tan, S.L. Zhang, K.T. Yue, F.M. Huang, Z.J. Shi,

X.H. Zhou, Z.N. Gu, J. Raman Spectrosc. 28 (1997) 369.

[22] J.K.W. Sandler, J.E. Kirk, I.A. Kinloch, M.S.P. Shaffer,

A.H. Windle, Polymer, submitted (2003).

[23] W.B. Downs, R.T.K. Baker, J. Mater. Res. 10 (1995) 625.

[24] The research presented in this paper has been patented,

with UK patent application No. 0226590.8.

C. Singh et al. / Chemical Physics Letters 372 (2003) 860–865 865