C A R B O N 5 0 ( 2 0 1 2 ) 3 6 8 8 – 3 6 9 3

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Carbon nanotube bundles self-assembled in double helixmicrostructures

Felipe Cervantes-Sodi a,*, Juan J. Vilatela b, Jose A. Jimenez-Rodrıguez c,Lucio G. Reyes-Gutierrez c, Samuel Rosas-Melendez a, Agustın Iniguez-Rabago a,Monica Ballesteros-Villarreal a, Eduardo Palacios d, Gerd Reiband c, Mauricio Terrones e,f

a Departamento de Fısica y Matematicas, Universidad Iberoamericana, Prolongacion Paseo de la Reforma 880, Lomas de Santa Fe, DF 01219,

Mexicob IMDEA Materials Institute, E.T.S. de Ingenieros de Caminos, Profesor Aranguren, Madrid 28040, Spainc Ingenierıa Industrial, Grupo JUMEX, Km. 12.5 Ant. Carretera Mexico Pachuca, Xalostoc, Ecatepec de Morelos, Estado de Mexico 55340,

Mexicod Lab. de Microscopıa Electronica de Ultra Alta Resolucion, Instituto Mexicano del Petroleo, Eje Central Lazaro Cardenas Norte 152, San

Bartolo Atepehuacan, DF 07730, Mexicoe Department of Physics, Department of Materials Science and Engineering & Materials Research Institute, The Pennsylvania State University,

University Park, PA 16802-6300, USAf Research Center for Exotic Nanocarbons (JST), Shinshu University, Wakasato 4-17-1, Nagano 380-8553, Japan

A R T I C L E I N F O

Article history:

Received 27 January 2012

Accepted 22 March 2012

Available online 30 March 2012

0008-6223/$ - see front matter � 2012 Elsevihttp://dx.doi.org/10.1016/j.carbon.2012.03.042

* Corresponding author.E-mail address: felipe.cervantes@uia.mx

A B S T R A C T

Double helix microstructures consisting of two parallel strands, each composed of hun-

dreds of multiwalled carbon nanotubes (MWCNTs) are synthesised by chemical vapour

deposition (CVD) of ferrocene/toluene vapours on thermochemically treated metal sub-

strates, such as steel, Cu, Al and W. The thermochemical treatment produces a thin and

brittle layer of SiOx. During the CVD process, carbon nanotubes (CNT) grow adhered to this

layer, and as growth progresses, small SiOx microparticles detach from the substrate,

directing the helical development of the growing MWCNTs double strands. This growth

model for the helical microstructures is compared in the manuscript with models previ-

ously reported for coiled carbon fibres grown in the gas phase. A unique aspect of these

double helices when they are composed of carbon nanotubes is that they grow on top of

a forest of aligned CNTs.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Micro/nanostructures with helical morphology are applicable

as reinforcement in high-strain composites, resonating ele-

ments, electromagnetic wave absorbers [1], microsensors [2]

and in micro/nano-electromechanical systems (MEMS or

NEMS) where energy absorption is sought (e.g. microsprings

[3], damping, and impact resistance systems). While for ten-

sile strength the axial properties of the carbon nanotubes

er Ltd. All rights reserved(F. Cervantes-Sodi).

(CNTs) and nanofibres are best exploited when they are

straight and aligned relative to each other and to the loading

axis [4], helical arrangements are more efficient for these

applications. Among helical structures, coiled carbon fibres

(CCFs), coiled CNTs, coiled nanowires [5–10] or helical nano-

tube arrays [11,12] have been produced either by introducing

curvature directly on the building blocks during the synthesis

process as in the former three cases, or by assembling them

as helical nanotube arrays as in the latter case [11,12]. In

.

C A R B O N 5 0 ( 2 0 1 2 ) 3 6 8 8 – 3 6 9 3 3689

particular, CCFs with micrometre dimensions, have been

studied for over four decades [5,13]. These microcoils are va-

pour-grown carbon fibres (VGCF) [14] produced using metallic,

bimetallic, metal-oxides or metal-carbides catalysts in the

presence of impurities (e.g. S, P, Si, Sn, In, TiC, TiOx, MgOx,

SiOx, AlOx) introduced in the nanoparticle either by means

of the carrier gas, the substrate, or an alternative nanoparti-

cle’s production method in itself [5,15–19].

Here, we report the synthesis and structural characterisa-

tion of one-step self-assembled double-helices of multi-

walled carbon nanotubes (MWCNT) arrays. These helical

MWCNT arrays are catalytically grown by Fe nanoparticles

bound to SiOx micro-particles, a process similar to the one re-

cently reported for single-walled CNT-array helices in Ref.

[11,12]. However, and to the best of our knowledge, the growth

of helical MWCNT double stranded arrays has not yet been re-

ported. We also point out similarities between the synthesis

of CCFs and CNT-array double helices, putting forward a gen-

eral growth model for these structures that relies on the cat-

alyst-substrate interaction.

2. Experimental

The substrates used in these experiments consist of metallic

bars, particularly an AISI type 1018 carbon steel (0.18% C, 0.6–

0.9% Mn, 0.04% max P, 0.05% max S; dimensions of the sub-

strate: 25 cm · 1.7 cm · 0.45 cm) thermochemically treated

as follows. A 2.54 cm diameter and 40 cm height cylindrical

iron vessel is used as reactor. Layers of reactants are placed

at the bottom of the reactor in this order: 4 g of Al2O3 (Sig-

ma–Aldrich 199974), 1 g of SiO2 nanopowder (Sigma–Aldrich

637246), 1 g of NH4F (Sigma–Aldrich 216011) and 4 g of Al2O3.

The metallic substrate is hung from the top of the container,

thus avoiding contact with the reactants. The vessel is filled

with Ar before and then sealed, after which it is heated it

up to 900 �C and kept at that temperature for 7 h, and finally

cooled to room temperature. During the treatment the inter-

nal pressure reaches over 8300 Torr. Other metallic substrates

include AISI 1518 steel, W, Cu, Al.

After the treatment a metallic bar sample was seg-

mented for characterisation and for the subsequent CNT

growth by chemical vapour deposition (CVD). Before the

CVD reaction the metallic sections (acting as substrates)

are placed in a 2.54 cm quartz tube furnace and heated un-

der a 0.2 l/min Ar flow. After reaching 850 �C, micro-droplets

of a ferrocene/toluene 3.5/96.5 wt% solution are supplied as

the Fe and C feedstock at a flow rate of 2.5 l/min. After

40 min of CVD reaction, the system is allowed to cool down

to room temperature.

Observation and analysis of the samples were performed

using a Dual-Beam Scanning Electron Microscope Nova-200

Nanolab, coupled with an X-ray Si (Li) ultrathin window en-

ergy dispersive spectrometer (EDS) for low atomic number

detection. The quantitative estimation of the elemental atom-

ic percent was done with the ZAF method implemented on

the EDAX EDS Genesis software. Graphitic structure of

MWNTs walls were observed on a Transmission Electron

Microscope (TEM) Tecnai G2 F30 S-TWIN.

3. Analysis and results

The CVD process on the thermochemically treated steel sub-

strate produces carbon nanostructures with unusual mor-

phology, consisting mostly of two-strand helices resembling

micron-scale springs. Typical SEM micrographs are presented

in Fig. 1. These double helices are made up of two parallel

strands with the same helical angle and pitch, each being

composed of hundreds of MWNTs. In spite of the curvature

of the strands, higher magnification SEM images (Fig. 1c)

show the nanotubes to be wavy, but without the intrinsic cur-

vature that would arise due to the presence of pentagonal and

heptagonal defects in the hexagonal lattice [20]. The indica-

tion is that the assembly process of the CNT arrays, rather

than CNT individual structure, is responsible for the morphol-

ogy of the helices. Interestingly, these helical structures are

supported on a forest of aligned CNTs [21]. Fig. 1d shows

the side view of a large area of microhelices on a CNT forest,

this layered structure being discussed later in the manuscript.

Inspection of the ends of the microhelices shows the pres-

ence of a small particle joining the ends of the two arrays on

the particle surface (backscattered electron micrographs

shown in Supplementary data). The example in Fig. 2 shows

the arrays’ ends meeting on opposite sides of a �3 lm size

irregular particle. Elemental composition maps of the particle

location (Fig. 2c) obtained by EDS give strong signals for C, O,

Si and Fe. The intensity colour maps for these elements are

presented in Fig. 2d. The intensity distribution on these maps

shows that the anchoring particle is abundant in Si, O and Fe,

but not in C, which suggests that the particle is a SiOx sub-

strate onto which much smaller Fe catalyst particles deposit

during CVD growth of the nanotubes. For reference, the rela-

tive atomic composition in the area in Fig. 2d is 73% C, 5% Si,

20.8% O, and 1.2% Fe, calculated from the EDS spectrum with

a typical information depth of �2 lm, matching the size of

the anchoring particle (Fig. 3).

3.1. Thermochemically treated steel surfaces

The thermochemical treatment of the steel bar is essential to

grow these microhelices, and in general to grow CNTs on the

steel and other metal substrates used in this study; therefore

we first turn to the substrate to analyse the CNT growth and

assembly mechanisms.

Inspection of the steel substrate before the CVD reaction

shows that the treatment produces a thin coating of SiOx par-

ticles with cubic shape (Fig. 4). Their size is �0.91 lm (Fig. 5a),

whereas the average diameter of a single helix array is

�42 lm (Fig. 5b), implying that an aggregate of several of these

SiOx particles is required for the growth of two helical arrays

sharing the same microparticle base (Fig. 2).

According to EDS analysis on the treated steel surfaces,

the elemental composition is 58.7% O, 14.8% Si, 0.9% Mn

and 24.6% Fe. The Mn and Fe signals correspond to the steel

surface. The atomic ratio of Si:O in the substrate particles is

(4.05) virtually the same as when they are attached to the

microhelix ends (4.2), thus indicating that the CVD reaction

does not affect their composition, but only perhaps their mor-

phology. We note that thermochemical treatment of other

Fig. 1 – (a–c) Double helices composed of two parallel strands of carbon nanotubes array. Colour code is added in (b) and (d) to

identify individual strands of the coiled two-strand systems. (d) CNT forest-coils double layer. (e) TEM image of individual

MWCNTwith a diameter of �53 nm. (f) Graphitic structure of MWNTwith interwall distance of�0.34 nm. (For interpretation of

the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2 – (a–c) SEM images of a double helix microstructure tip and (d) elementary mapping of the tip area.

3690 C A R B O N 5 0 ( 2 0 1 2 ) 3 6 8 8 – 3 6 9 3

Fig. 3 – Energy-dispersive X-ray spectroscopy spectrum of

the area shown in Fig. 2d.

Fig. 5 – (a) Lateral size of cubes on the SiOx film and (b)

diameter of single helix arrays. Giddings fittings (curves)

with peak position at �0.8 and �35 lm respectively.

C A R B O N 5 0 ( 2 0 1 2 ) 3 6 8 8 – 3 6 9 3 3691

metallic substrates, such as 1518 steel, aluminium, copper

and tungsten, also produces growth of CNTs that assemble

into double helices (see electron micrographs in Supplemen-

tary data), suggesting that the steel substrate (or the other

metallic substrates) acts mainly as an anchoring base for

the formation and deposition of SiOx particles. These parti-

cles would first reach the substrate carried by NH4F gases

formed during the treatment at 900 �C; they would then de-

posit and aggregate to form a thin layer of SiOx particles on

the metallic substrate. We note that when using plain metal

substrates without any treatment, no double-helices of

MWNT growth is observed.

This process is in fact similar to the gas-phase growth of

carbon nanocoils (CNCs) on stainless steel substrates treated

through oxidation of a previously spun 0.0025/0.25 M

(C2H3O2)2 Sn/ethanol solution [18]. In the growth of CNCs

the formation of catalyst base microparticles, arising from

the fragmentation of the steel surface, is also seen as a requi-

site for the synthesis of coiled structures in this process [18].

4. Growth mechanism

The growth of individual CNTs/CNFs has been the subject of a

wide range of theoretical and experimental works leading to a

general agreement on the so called VLS and VSS-like mecha-

nisms (vapour–liquid–solid or vapour–solid–solid mecha-

nisms); including all their known variations. Of special

interest has been the role of the catalyst particle and its inter-

action with the substrate and the growing nanostructure

[9,22–24]. However, in the present study the morphology of

the microhelices derives from their assembly rather than

from the particle-mediated growth of individual helical CNTs,

Fig. 4 – Thin SiOx layer covering the steel su

and in fact the models on the growth of CCFs, amply studied

several decades ago [5], are particularly relevant here. CCFs

are grown by CVD and present a micron-size catalyst particle

joining both ends of two parallel CCFs with the same helicity,

as is the case of the CNT double helices in this study. CCF

growth has also been reported for various substrates [5,25].

The most accepted growth mechanism for CCFs postulates

that their curvature is due to anisotropy of the microcatalyst

particle’s surfaces and differences in the concentration of

purposely-added impurities [5]. As a consequence, the parti-

cle would have different rates of carbon dissolution that

would produce a gradient in the fibre extrusion velocity, caus-

bstrate after thermochemical treatment.

Fig. 6 – Schematic of CNT double-helix and forest growth model. (a) A crust of SiOx microparticles is deposited on the steel

substrate. During CVD, Fe/C-containing vapours deposit hundreds of Fe nanoparticles on the microparticles (b) that detach

from the substrate due to the catalytic growth of two opposing MWNT strands (c) anchored on the substrate (d). With the

continuation of the growth, the microparticle is lifted off by the parallel growing nanotube strands (d). The helical shape arises

from the torque induced during unstable parallel growth (e). A simultaneous growth of a CNT forest results in a double layer (f).

3692 C A R B O N 5 0 ( 2 0 1 2 ) 3 6 8 8 – 3 6 9 3

ing torsion on the growing fibre and producing its coiled mor-

phology [5,16,18]. However, in the present case, where the

microhelices are made up of hundreds of MWCNTs, this mod-

el would not hold, since it would require a coordinated cata-

lytic activity gradient among each one of the hundreds of

catalytic nanoparticles attached to the SiOx microparticles.

On the other hand, in recent works by Zhang et al. [11] and

Wei et al. [12] layered double hydroxide (LDH) or oxide (LDO)

flakes with plate-like shape were observed to produce growth

of microhelices of SWCNT. The authors showed that SWCNTs

grow from multiple growing centres that cover the LDH/LDO

particle and when SWCNT tips meet a space resistance, the

as-grown SWCNT bundles twist and self organise into a dou-

ble-helix structure [11,12]. Thus, similar to the LDH/LDO

flakes, the SiOx particles described here act as substrate and

catalyst carrier on which CNTs grow. As CNT bundles grow

simultaneously on opposites sides of the catalyst base (i.e.

the SiOx particle in this study) any lateral force on the base re-

sults in a torque on the two strand system, causing the

strands to twist and form a microhelix (Fig. 6). For this process

to occur, the strands need to be firmly attached to the sub-

strate to withstand the torque imposed by the spiraling CNTs

[11,12,26]. We propose that this mechanism would in fact also

apply to the growth of double- and multi-stranded CCFs. Fur-

thermore, the presence of predominantly two strands, in-

stead of another number of strands, is due to the plate-like

shape of the SiOx particles, which causes most CNTs to grow

on their flat faces. The SEM images in Figs. 1 and 2 and S2

would support this view.

Another aspect of the microhelices in this study is their

location on top of a forest of aligned CNTs, as schematically

shown in Fig 6f. Whereas each double-strand grows indepen-

dent from the others, the forest alignment implies that some

level of uniform parallel growth must be present. The growth

of the CNT forest lift off the base on which the double-strands

are supported, with indication that these two growth pro-

cesses occur simultaneously (Fig. 6d–f). In fact, a similar bi-

layer system has been recently observed in the growth of

small diameter single CCFs produced using spin coated Fe–

Sn–O catalyst particles [25]. These CCFs grow on top of a thick

amorphous carbon deposit at the end of a forest of aligned

CNTs. In our case the two-strand microhelices grow on a SiOx

deposit (Fig. 6d–f) that is also at one end of the CNT forest.

Finally, in relation to the type of nanotubes, the CVD con-

ditions used in this work where chosen to maximise yield by

producing large Fe catalytic particles during the growth pro-

cess and which result in of the synthesis of MWNTs. So far,

there is no indication to suggest that SWNTs could not be

grown on metal substrates using the process described here,

provided that the CVD parameters are modified accordingly,

e.g. controlling the size of the Fe catalyst particles.

5. Summary

Double-helices of CNTs were synthesised in a one-step CVD

process on different metal substrates, including a standard

carbon steel. The metal substrates were subjected to a ther-

mochemical treatment to deposit small silica particles which

act as substrates for the iron catalyst particles. As the CNTs

grow, they tend to align and form strands on opposite sides

of the silica particle aggregate. During growth, these strands

coil and form a double helix with same-helicity strands. This

C A R B O N 5 0 ( 2 0 1 2 ) 3 6 8 8 – 3 6 9 3 3693

mechanism would apply to double stranded CNT/CCFs and

other structures grown by catalytic CVD. Our CNTarray helices

grow on top of CNT forests, suggesting that improvements on

the synthesis process are required to increase the yield of heli-

ces over aligned CNTs. Better control of the size and aggrega-

tion of the silica particles and their adhesion to the metal

substrate should be addressed to produce more uniform

CNT-array helices. Finally, it remains to exploit the potential

of these nanostructures on different scales, by integrating

them in hierarchical composites or MEMS/NEMS devices.

Acknowledgements

The authors acknowledge CID for access to SEM facilities. FCS

acknowledges fruitful discussions with MSc Carlos San Este-

ban and funding from Direccion de Investigacion, Universidad

Iberoamericana and SNI-CONACYT. JJV acknowledges finan-

cial support from CONACYT (Mexico) and MICINN thorough

its Juan de la Cierva Programme. MT acknowledges the sup-

port from the Research Center for Exotic Nanocarbons, Japan

regional Innovation Strategy Program by the Excellence, JST.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at http://dx.doi.org/10.1016/j.carbon.

2012.03.042.

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