Materials Letters 93 (2013) 404–407

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Materials Letters

0167-57

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n Corr

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journal homepage: www.elsevier.com/locate/matlet

Facile processing of zeolite based catalyst support for carbonnanotube synthesis

S.W. Pattinson, A.H. Windle, K.K.K. Koziol n

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

a r t i c l e i n f o

Article history:

Received 7 September 2012

Accepted 30 October 2012Available online 6 November 2012

Keywords:

Carbon nanotubes

Chemical vapour deposition

Powder technology

7X/$ - see front matter & 2012 Published by

x.doi.org/10.1016/j.matlet.2012.10.119

esponding author. Tel.: þ44 1223 334356.

ail address: kk292@cam.ac.uk (K.K.K. Koziol).

a b s t r a c t

Before many of the most promising applications of carbon nanotubes can be realized, nanotube

characteristics including chirality, length and diameter must be optimised. One of the most important

aspects of supported carbon nanotube synthesis is the interaction of the catalyst with the substrate. The

highly uniform and dense pore structures of zeolites make them ideally suited for improving carbon

nanotube synthesis, but their powder form renders them incompatible with many commonly used and

highly developed synthesis techniques. Existing methods for making planar substrates from zeolites are

complex and unsuitable for incorporation into carbon nanotube synthesis. We present an investigation

into facile methods for forming zeolites into substrates, demonstrating that these are compatible with

many different CNT synthesis techniques and even able to support the growth of aligned arrays of CNTs

that are hundreds of microns long. These substrates can be easily produced and incorporated into

existing CNT synthesis processes and should therefore aid in the use of zeolites in CNT synthesis

generally. This method is also compatible with any powder based porous material such as activated

alumina or silica gel, allowing the production of substrates with a wide variety of pore size distributions

and surface properties.

& 2012 Published by Elsevier B.V.

1. Introduction

Considerable research has been carried out on the synthesis ofcarbon nanotubes (CNTs) in order to grow them in ways thatallow their exceptional properties to be exploited. The mostimportant CNT properties to improve are chirality, diameter andlength, since many of the potential applications of CNTs dependcritically on these. One of the most important interactions indetermining these structural parameters is that between thecatalyst and the substrate [1]. Many different substrates, includ-ing alumina [1,2], quartz [3], titanium nitride on silicon carbide[4], magnesium oxide [5] and zeolites [6–14], have been usedpreviously in CNT growth. Zeolites, in particular, have the advan-tage of featuring dense surfaces of regularly sized pores, with anumber of potential benefits. The regular pore size can be used totemplate the catalyst aiding CNT diameter control [7,13]. Thetypical diameter of zeolite pores (0.4–1.0 nm) is also such thatnanotubes with a similar size would be restricted to a small set ofpossible chiralities, and superconducting properties in 0.4 nmdiameter CNTs grown in zeolite channels have been shown [10].Zeolites also appear to allow CNT growth at low reaction tem-peratures [14]. Finally, one of the primary factors limiting CNT

Elsevier B.V.

length appears to be termination due to catalyst diffusion acrosssurfaces [15], which decreases catalyst size to the point wheregrowth is terminated [16]. The pores surrounding catalyst may beable to reduce such diffusion and thereby increase catalystlifetime.

These considerations have led to a variety of investigationsinto zeolites in CNT synthesis [6–10]; however, the powder formof zeolites has limited their usefulness by, for instance, renderingthem incompatible with a number of common CNT catalystdeposition techniques such as sputtering, spin coating and dipcoating, which are used to controllably distribute catalyst foruniform CNT growth [17]. Similarly, the powder form of zeoliteshas made it difficult to grow aligned arrays of CNTs from them,though such arrays would be advantageous in applicationsincluding separation membranes and flexible electronics [18].Zeolites can be grown as planar membranes [19] but these arerelatively difficult to make, often fragile, and therefore unsuitablefor CNT synthesis, particularly in a research setting not focused onzeolite growth. We report a method for producing planar zeolitesubstrates that is both easily reproducible and forms substratesthat are durable under reaction conditions, in handling post-synthesis, and are well suited to novel continuous CNT productionprocesses [18]. We demonstrate the compatibility of these sub-strates with a wider variety of carbon nanotube synthesis pro-cesses than is available to zeolite powder. We also show thatthese nanotubes can be well aligned and grown in dense arrays

S.W. Pattinson et al. / Materials Letters 93 (2013) 404–407 405

hundreds of microns long, which we do not believe has yet beenshown on zeolite substrates.

2. Methods

CNT growth catalyst was deposited onto the zeolite substratesin four different ways: deposition from the vapour phase duringgrowth, sputtering, spin coating, and dip coating. For growthusing catalyst deposited from the vapour phase, the zeolitesubstrates were placed into a quartz furnace, similar to that usedby Singh et al. [3], which was heated to 760 1C in 1 L/min of argonflow. 5 wt% ferrocene in toluene was evaporated in a pre-heater at180 1C and passed into the main furnace by the argon carrier gas.In the case of sputtering, spin coating, and dip coating, growthwas carried out in a cold wall reactor operating at 850 1C in anargon and hydrogen atmosphere using 0.1 L/min ethylene as thecarbon source. Sputter deposition of catalyst was carried outusing an Fe target (99.99% Pi-Kem), to sputter depths of betweenapproximately 1 nm and 15 nm. Spin and dip coating were carriedout using a variety of different catalyst sources, including ironnitrate, iron chloride, cobalt acetate and ferrocene, as well asmany different solvents such as ethanol, acetone, toluene, iso-propanol and water. Typically, 1–5 wt% catalyst source in solventwas used. Spin coating was carried out between 1000 and6000 rpm for a period of 20 s, and 5 drops of dissolved catalystwere dispersed onto the substrate per deposition. The solutionsfor dip coating were similar to those used for spin coating. Dipcoating itself was carried out manually by immersing the sub-strate in the catalyst solution for 5 min, and then taking thesubstrate out of the solution at a rate of approximately 1 mm/sbefore leaving it to dry in an upright position.

Fig. 1. (a) Optical and (b) SEM images of CNT substrate formed by compression (th

dimensions). (c) SEM image of a zeolite substrate after dispersion in binder and dryin

image of a zeolite substrate made using binder that has been polished with colloidal sili

uncovered them. Some colloidal particles are also still visible on the substrate.

3. Results and discussion

Many industrial zeolites are produced in very large quantitiesand are thus easily obtainable and inexpensive. There are, how-ever, two constraints on the zeolites that can be used assubstrates in CNT growth. The high temperatures of CNT synth-esis mean that high silica content zeolites must be used. Addi-tionally, given that the orientation of the particles in the substratecannot be controlled, zeolites with three-dimensional pore struc-tures are best, so as to ensure that pores will always be present onthe topmost surface of the substrate. Based on these criteria, twotypes of zeolite were chosen: ZSM-5 (Acros Organics) and Y (HSZ390 HUA, Tosoh Corp.).

Two methods for forming planar substrates from zeolitepowder were investigated. The first of these was the uniaxialcompression of the powder. To form the zeolite particles intotablets, typically 150 mg of zeolite was compressed at 10 t (tabletdiameter 13 mm). These zeolite tablets appeared as seen in Fig. 1aand b. This high pressure was required to make the tabletsmechanically strong and to allow easy handling, even though itcould lead to delamination if the tablet was too thick.

The second substrate production method involved the use ofan inorganic binder, Ceramabind 634-2 (Aremco), which featuredhigh temperature resistance and was also able to bond wellenough with the zeolite surface such that a free-standing objectwas formed upon drying. The binder was added to the zeolite inratios ranging from 1:1 to 4:1 zeolite:binder by volume, and theresulting mixture was then ultrasonicated up to an hour to ensureeven binder distribution. The mixture was then allowed to dry foran hour, before being heated to 100 1C for 2 h, and then 200 1C for1 h. Following this treatment, the surface of the zeolite particleswas visibly covered with binder (Fig. 1c), thus preventing growth

e tablet has been broken into smaller pieces to suit the CNT synthesis reactor

g, showing that the zeolite particles appear to be covered by the binder. (d) SEM

ca. The zeolite particles are now distinct, suggesting that the polishing process has

Fig. 2. SEM images of CNTs produced on zeolite through catalyst deposited by (a) sputtering, (b) spin coating, (c) vapour injection, and (d) dip coating.

Fig. 3. (a) SEM image of the base of a CNT array grown by vapour deposited catalyst on a planar zeolite substrate. Shown is the flat interface of the CNT array with the

substrate. (b) SEM image of �0.5 mm high aligned CNT array grown on planar zeolite substrate. (c) image of edge of CNT array grown by vapour deposited catalyst on

powder zeolite. The lack of rigid, planar support for the array decreases alignment, particularly during movement of the sample post-synthesis. (d) SEM image of the

macroscopic CNT array grown on powder zeolite by vapour deposited catalyst.

S.W. Pattinson et al. / Materials Letters 93 (2013) 404–407406

0

200

400

600

800

1000

1200

0 500 1000 1500 2000 2500 3000

Wavenumber (cm-1)

Inte

nsity

Fig. 4. (a) Raman spectrum of CNTs grown on compressed zeolite substrate by vapour deposited catalyst. (b) Bright field TEM image of these CNTs.

S.W. Pattinson et al. / Materials Letters 93 (2013) 404–407 407

from taking place on the zeolite surface. To uncover the zeolite,the surface of the substrate was polished using colloidal silica(60 nm average particle size) solution for 2 min and then subse-quently washed with water and dried using pressurised air toremove the silica particles from the surface (Fig. 1d).

In practice, CNT yields using zeolite Y were consistently low,most likely due to its lesser thermal stability [8], and thereforethe images in figures are all taken from growth on ZSM-5substrates. Similarly, the zeolite substrates made using bindertended to feature substantially lower CNT yields than those madeby compression, and therefore the latter were primarily used inexperiments, and all images of CNTs on planar substrates are oncompressed ZSM-5.

We were able to grow CNTs using all of the depositiontechniques described above: spin coating, dip coating, sputtering,and vapour phase deposition, as shown in Fig. 2. The nanotubesgrown using continuous injection of catalyst are particularly longand form an aligned array that can subsequently be removed fromthe reaction chamber as shown in Fig. 3. Whilst aligned CNTgrowth is possible using zeolite powders (Fig. 3c, d) the arrays areless ordered due to the lack of a rigid support, particularly whenmoving the sample post-synthesis, which makes subsequentprocessing difficult. As can be seen from the Raman D/G ratio ofapproximately 0.4, as well as the TEM image in Fig. 4, these CNTsare crystalline for multi-walled nanotubes.

The facile processing of zeolite powders to form planar sub-strates for the growth of carbon nanotubes allows for the use ofzeolites as substrates in an increased variety of CNT synthesisprocesses rather than simply the impregnation processes usedpreviously. The simple processing methods used to form zeolitesubstrates should allow them to be easily adopted and integratedinto many CNT growth techniques, thereby opening the door tofurther development of CNT synthesis by making use of theadvantages of zeolites, for instance in novel continuous produc-tion processes [18]. In particular, the regularly sized pores couldbe useful in chirality control of CNTs given that the diameter ofthe pores would significantly restrict the chiralities available toCNTs growing in them. The potential to restrict CNT diametercould also be used for applications making use of CNT diameterdirectly such as membrane separation. It should also be empha-sised that the methods for turning zeolite powder into a substrateare compatible with other powders such as activated alumina orsilica gel, and could therefore be used to make substrates withdifferent pore size distributions and surface properties.

4. Conclusion

We have outlined the advantages of the use of zeolites in CNTsynthesis, and shown how they could easily be formed into planarsubstrates, demonstrating that they are compatible with many morecommon synthesis processes than the existing powder form of thematerial. Additionally, we have shown that the planar zeolitesubstrates can be used for the growth of exceptionally long, dense,and aligned arrays of CNTs. The processing steps outlined shouldfacilitate further research into the use of zeolites and other porouspowders, with their many associated advantages, in CNT synthesis.

Acknowledgements

SWP would like to thank the EPSRC for funding. KK would liketo thank the Royal Society and ERC for funding.

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