IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 22 (2011) 435302 (7pp) doi:10.1088/0957-4484/22/43/435302

Hierarchical carbon nanostructure design:ultra-long carbon nanofibers decoratedwith carbon nanotubesA A El Mel1, A Achour1, W Xu2, C H Choi2, E Gautron1,B Angleraud1, A Granier1, L Le Brizoual1, M A Djouadi1 andP Y Tessier1

1 Universite de Nantes, CNRS, Institut des Materiaux Jean Rouxel, UMR 6502,2 rue de la Houssiniere, BP 32229, 44322 Nantes Cedex 3, France2 Department of Mechanical Engineering, Stevens Institute of Technology, Hoboken,NJ 07030, USA

Received 29 June 2011, in final form 1 September 2011Published 4 October 2011Online at stacks.iop.org/Nano/22/435302

AbstractHierarchical carbon nanostructures based on ultra-long carbon nanofibers (CNF) decorated withcarbon nanotubes (CNT) have been prepared using plasma processes. The nickel/carboncomposite nanofibers, used as a support for the growth of CNT, were deposited onnanopatterned silicon substrate by a hybrid plasma process, combining magnetron sputteringand plasma-enhanced chemical vapor deposition (PECVD). Transmission electron microscopyrevealed the presence of spherical nanoparticles randomly dispersed within the carbonnanofibers. The nickel nanoparticles have been used as a catalyst to initiate the growth of CNTby PECVD at 600 ◦C. After the growth of CNT onto the ultra-long CNF, SEM imaging revealedthe formation of hierarchical carbon nanostructures which consist of CNF sheathed with CNTs.Furthermore, we demonstrate that reducing the growth temperature of CNT to less than 500 ◦Cleads to the formation of carbon nanowalls on the CNF instead of CNT. This simple fabricationmethod allows an easy preparation of hierarchical carbon nanostructures over a large surfacearea, as well as a simple manipulation of such material in order to integrate it into nanodevices.

S Online supplementary data available from stacks.iop.org/Nano/22/435302/mmedia

1. Introduction

With the increased developments in nanotechnology, substan-tial efforts have been placed on developing nanodevices basedon carbon nanomaterials (e.g. carbon nanotubes (CNT), carbonnanofibers (CNF) and graphene) [1–7]. Owing to their originalone-dimensional structure, CNT and CNF offer the prospectof both new fundamental science and original industrialapplications. The high aspect ratio and the high surface-to-bulk ratio make these nanomaterials one of the best candidatesfor sensor applications. The decoration of CNF by CNTprovides three-dimensional and multifunctional hierarchicalcarbon nanostructures (CNT/CNF) with an enhanced surface-to-bulk ratio, mechanical stability and electrical conductivity.Recently, these nanostructures were considered as promisingmaterials for applications in fuel cell catalysis, field emissionand sensor devices [4–7]. Several fabrication capabilities ofsuch types of nanostructures have been demonstrated over the

last few years [4–14]. In general, these methods consist inusing a powder of carbon microfibers as a support for thegrowth of CNTs by chemical vapor deposition (CVD) at hightemperature (800–1000 ◦C) [4–10]. The catalytic nanoparticlesused for the synthesis of CNTs can be coated on the uppersurface of the fibers by various deposition processes such aselectrodeposition. However, in some cases silicon dioxideshell encapsulating the microfibers is required for the growthof CNTs [4]. The presence of the insulator shell (i.e. silicondioxide) can block the charge transfer between the CNTand the microfibers which leads to a reduced efficiency ofthese hierarchical nanostructures when used in a nanodevice.Moreover, the fibers used in these techniques were not at thenanoscale (i.e. microfibers) and in some cases the synthesizedCNTs were sparse with poor organization and mechanicalstability [5–11]. In addition, mechanical instabilities, suchas poor adhesion of CNTs to CNF, can be observed whenimmersing these nanostructures in a fluid leading to the

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Nanotechnology 22 (2011) 435302 A A El Mel et al

Figure 1. A schematic illustration of the procedures followed for the fabrication of hierarchical carbon nanostructures over a large surfacearea. The procedure contains three main separated processes: (1)–(3) fabrication of nickel/carbon nanofibers on nanograted substrate.(4) Etching of the sacrificial oxide layer leading to a partial disengagement of the nanofibers from the substrate. (5) Decoration of the CNF byCNT.

(This figure is in colour only in the electronic version)

generation of brittle fractures. Recently, an electrospinningtechnique has been employed for the fabrication of CNF/CNThierarchical nanostructures [13, 14]. In such a method, electro-spun metal/polyacrylonitrile nanofibers were carbonized andthen collected on a highly porous thin sheet. By using themetal nanoparticles loaded in the inner of the nanofibers asa catalyst, the CNT can be grown on CNF by CVD at hightemperature (800–1000 ◦C). The carbon nanotubes as grownon the electrospun carbonized nanofibers form hierarchicalnanostructures. By replacing the carbon microfibers by CNF,the specific surface area increases, leading to an enhancementof the response of the device based on these nanostructures.However, despite the progress in hierarchical carbon nanos-tructure synthesis, the progress towards nanodevice designbased on these nanostructures is still limited. Indeed, onthe one hand, most of the reported methods are based onhigh temperature processes (900–1000 ◦C) and on the otherhand the prepared hierarchical nanostructures are obtainedas powders. Due to the poor mechanical stability of apowder in fluids, the direct integration of the synthesizednanostructures in nanodevices in which the usage of fluids isrequired (e.g. biosensors, electrochemical electrodes) wouldnot be possible. Therefore, more technological proceduressuch as dispersion, transferring, organization and lithographywill be required. In addition, these high temperature methodscannot be applied to a substrate which has a low melting pointsuch as glass. These entire reasons make the development ofnanodevices based on hierarchical carbon nanostructures animportant challenge facing nanotechnology.

Here we implement a novel fabrication strategy ofhierarchical carbon nanostructures which provides an easymanipulation of the prepared nanostructures for nanodevicedevelopment. The formation of hierarchical carbonnanostructures is achieved by decorating CNF with CNT.The detailed fabrication strategy of these hierarchical

nanostructures is schematically illustrated in figure 1. Inthe first stage, organized and aligned nickel/carbon (Ni/C)composite nanofibers were grown on the tips of the siliconlines of a nanograted substrate, which served as a template, bya hybrid plasma process combining physical vapor deposition(PVD, magnetron sputtering) and plasma-enhanced chemicalvapor deposition (PECVD). Then, one of the ends of thenanofibers can be stuck on the surface by a metal layer.The native silicon oxide, present on the surface of thenanograted silicon substrate, was considered as a sacrificiallayer which was removed in hydrofluoric acid after the growthof the nanofibers. This last procedure leads to a partialdisengagement of the nanofibers from their support. In the laststage, the nickel phase present in the nanofibers was used asa catalyst for the growth of CNT by PECVD in an electroncyclotron resonance (ECR) plasma at 600 ◦C.

2. Experimental details

2.1. Nickel/carbon nanofiber fabrication

The preparation method of metal/carbon nanofibers has beenreported in detail previously [15]. Briefly, it consists ofdepositing nickel/carbon material on a nanograted siliconsubstrate. The detailed study of the preparation method ofthe nanograted silicon substrate, which served as a templateto grow well-aligned nickel/carbon nanofibers along the pre-defined line patterns, was also reported elsewhere [16]. Inbrief, it consists of coupling laser interference lithography withdeep reactive ion etching in one process flow, which allowsus to create periodic silicon nanopatterns of good regularity intheir three-dimensional profiles (230 nm in pattern periodicityand 1 µm in height in this study) with uniform coverage overa large sample area (1 cm2 in this study). The depositionof the nanofibers was performed in a hybrid PVD/PECVD

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Figure 2. Low (A) and high magnification (B) top SEM images of aligned Ni/C nanofibers after deposition; cross-section SEM image of theNi/C nanofibers as-deposited on the nanograted substrate (C). Low (D) and high magnification (E) SEM images of disengaged Ni/Cnanofibers after removing the sacrificial oxide layer in hydrofluoric acid.

system which was described in detail elsewhere [15]. A radio-frequency (RF) generator at 13.56 MHz was connected to anunbalanced magnetron sputtering source via a matching box.A nickel disc (99.999% pure and 50 mm in diameter) wasused as a target. The distance between the nickel target andthe substrate was 80 mm. Another RF power supply wasconnected to a one-turn stainless coil, located at equal distancefrom the target and the substrate, to generate additional plasma.The RF powers coupled to the coil and to the magnetronwere both fixed at 150 W. In pure argon atmosphere, thereactor is operated in PVD mode and pure nickel can bedeposited. The addition of CH4 leads to the deposition ofhydrocarbon species formed in the additional plasma createdby the coil. Thus, when the reactor is operated in PVD/PECVDmode a simultaneous deposition of nickel and hydrogenatedcarbon species takes place, leading to the formation of an Ni/Cnanostructured material. By adjusting the methane fractionin the plasma during the deposition, the carbon concentrationpresent in the nanostructures can be controlled. The methanefraction in the plasma was maintained at 30% during thedeposition. In such a deposition condition the nanofibers haveshown a good mechanical stability during their manipulationand they were considered as the most suitable structures forthe growth of CNT (see supporting information available atstacks.iop.org/Nano/22/435302/mmedia). We have to mentionthat, for a higher methane fraction (not presented here), thenanofibers have a bad mechanical stability and they easily getbroken during the manipulation; this fragility is probably dueto the presence of a high content of amorphous carbon withinsuch nanofibers. Since we are aiming to fabricate hierarchical

carbon nanostructures with a high length and strength, thiswas one of the main reasons for which we have selected theNi/C nanofibers grown at 30% of methane for the fabricationof hierarchical carbon nanostructures. The deposition timewas fixed at 2 min. The pressure during the deposition wasmaintained at 0.67 Pa. No substrate heating or bias wasapplied to the substrate during the deposition, but an increaseof temperature was observed due to the plasma heating.However, this temperature remains lower than 120 ◦C duringthe synthesis procedure of the nanofibers. The total argon andmethane gas flow was fixed at 12 sccm. In such conditions, thenickel and carbon atomic concentrations in the nanofibers asdetermined by x-ray photoelectron spectroscopy (not presentedhere) was found to be 39 and 61 at.%, respectively.

2.2. Synthesis of CNT by PECVD

The synthesis of CNT was performed using an ECR-PECVDprocess, previously described in detail [17]. Such a process isoften employed to reduce the growth temperature of CNT [18].For CNT synthesis the CNF were annealed up to the growthtemperature (600 ◦C). An acetylene plasma diluted in ammonia(C2H2/NH3 = 0.4; deposition pressure = 0.3 Pa) was thengenerated at the top of the chamber by the resonant transfer ofthe microwave power (2.45 GHz; 125 W) to electrons trappedby an additional magnetic field (permanent magnets). An RFbias voltage of −30 V was applied to the heating substrateholder in order to control the plasma diffusion down to thesubstrate as well as the ion energy. The temperature was heldat 600 ◦C during the whole growth process (60 min).

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Figure 3. TEM micrographs of the prepared Ni/C nanofibers showing their high length (A), the homogeneous dispersion of the nickelnanoparticles in the nanofibers (B) and the spherical shape of the nickel nanoparticles of an average diameter of 5 nm (C).

Figure 4. SEM image ((A) and (B)) and high magnification SEM images ((C) and (D)) of CNT/CNF hierarchical nanostructures recoveringthe surface of the nanograted substrate.

2.3. Material characterization

Scanning electron microscopy (SEM) imaging was performedon a JEOL JSM 7600 F microscope operating at 5 kV.Transmission electron microscopy (TEM) imaging wasperformed on a Hitachi HNAR9000 microscope (LaB6filament, 300 kV, Scherzer resolution: 0.18 nm).

3. Results and discussion

3.1. CNT/CNF hierarchical nanostructures

In figures 2(A)–(C) are presented SEM images of the Ni/Cnanofibers deposited along the nanograted template structures.It can be clearly seen that the nanofibers are well aligned tothe template patterns with high fidelity (figures 2(A) and (B)).The key point of the process is due to the high aspect ratio

(depth/width) of the silicon trenches between two lines andto the low directionality of the deposition technique used: thesputtered atoms cannot penetrate between two silicon lineswhich are separated by a trench with a width of very lowdimension (figure 2(C)); during the deposition, a lip at theedges of the lines appears and this effect amplifies the growthof the nanofibers on the line to the detriment of the depositioninside the trench [15]. The prepared nanofibers have anaverage diameter of about 200 nm and they were spaced by50 nm from each other. After etching of the sacrificial oxidelayer, the nanofibers were partially disconnected from theirsupport and randomly oriented over the nanograted substrate(figures 2(D) and (E)). The section of the nanofibers has arectangular-like shape as can be seen on the high magnificationSEM image presented in figure 2(E). This rectangular shapecan be correlated to two main parameters which may have

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Figure 5. TEM micrographs of the prepared CNT/CNF hierarchical nanostructures with a high (A) and low diameter CNT (B).

a direct impact on the dimensions of the nanofibers. Thefirst factor is the width of the silicon lines which define thewidth of the nanofiber (∼150 nm), whereas the second one isthe deposition time which defines their height (2 min for theresults shown in figure 2). Moreover, the TEM imaging of thenanofiber positioned on the copper grid (figure 3(A)) revealedthe presence of two phases (figures 3(B) and (C)). The blackspherical particles can be identified to be nickel-rich, whereasthe bright thin layer can be attributed to amorphous carbon.Homogeneous distribution of the nickel nanoparticles alongthe nanofibers can also be remarked. The average size of thenanoparticles was about 5 nm (figure 3(C)). The presence of theamorphous carbon phase surrounding the nickel nanoparticlesensures a mechanical stability and protection against oxidation.After the partial disengaging of nanofibers from their support,the sample was used in order to synthesize CNT by ECR-PECVD at 600 ◦C. After the growth of CNT, hierarchicalcarbon nanostructures were obtained as demonstrated by SEMimaging (figures 4(A)–(D)). It can be seen in figures 4(C)and (D) that the synthesized CNT onto the CNF have variousdiameters. This variation in CNT diameters can be related to

various sizes of nickel nanoparticles formed on the surfaceof the nanofibers during the sample annealing at 600 ◦C.The TEM analysis of the prepared hierarchical nanostructuresrevealed the presence of two different types of multi-walledCNT (figure 5). The first type has a diameter of about100 nm and a length of 300 nm with a tip-growth mode(figure 5(A)). The nickel phase present in the nanofibers wasalmost completely consumed by the CNT which lead to thepresence of elongated nickel nanoparticles, with an averagesize of 100 nm, on the tip of the CNT. The second type of CNThas a smaller diameter (∼50 nm) and a length of about 500 nmwith a tip-growth mode (figure 5(B)). The nickel nanoparticleswere spherical with a diameter of 20 nm. The nickel phasepresent in the nanofibers was not completely consumed forthe growth of CNT and a high amount of nickel (dark color)was still present on their upper surface. We attribute thisnonhomogeneity to the annealing temperature of the nanofibersduring the growth of the CNT. Indeed, during the PECVDprocess, the nanograted substrate was annealed using a heatingplate. Thus, if the nanofiber is close to the surface of thesubstrate, the temperature will be high enough to catalytically

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Figure 6. SEM image at low (A) and high magnification (B)–(D) showing the CNW/CNF hierarchical nanostructures observed on the edgesof the sample.

Figure 7. TEM micrographs of the CNW/CNF hierarchical nanostructures (A) and high resolution TEM micrograph demonstrating thegraphitic-like structure of the CNW decorating the nanofiber (B).

activate the whole nickel phase present in the nanofibers; incontrast, if the nanofiber is far from the surface of the substratethe temperature will be lower and only a low amount of nickelwill be catalytically activated.

3.2. CNW/CNF hierarchical nanostructures

Meanwhile, we compared the fabricated structures at the centerof the sample where the heating plate ensures a temperature of600 ◦C to the hierarchical nanostructures obtained on the edgesof the sample where the temperature is estimated to be less than

500 ◦C. As can be seen in figure 6, the SEM images revealedthe presence of carbon nanowalls (CNW), instead of CNT,grown onto the CNF. The observed carbon nanowalls can bedescribed as high sp2 carbon sheets with an average thicknessof a few nanometers (figure 6(D)) [19]. The TEM analysisof these hierarchical nanostructures, presented in figure 7, isgood evidence for the modification of the microstructure of thenanofibers. Nickel nanoparticles were observed on the surfaceof the nanofibers (figure 7(A)). In addition, the presence ofCNW decorating the nanofibers was also confirmed by TEM(figure 7(B)). The formation of the nickel nanoparticles on

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the surface of the nanofibers can be related to the annealingduring the synthesis. Indeed, during the nanofiber annealing,the nickel diffuses through the carbon phase leading to itsaggregation on the fiber surface which leads to the formationof nickel nanoparticles with different sizes [20–22]. Moreover,since the growth of CNW does not require a high temperatureor a catalyst [23, 24], as opposed to the case for the growthof CNT, the nickel nanoparticles were still present on thesurface of the nanofibers but were not consumed by theCNW. Thus, we can conclude that for a temperature less than500 ◦C it would be possible to prepare CNW/CNF hierarchicalnanostructures which can be considered of huge interest forsensors [24] and field emission applications [25].

4. Conclusions

In summary, a route was developed for the synthesisof hierarchical carbon nanostructures at relatively lowtemperature by using nanostructured template surfaces.PVD/PECVD was used as a simple and effective methodfor the deposition of organized nickel/carbon nanofibers ona large surface of a nanopatterned substrate. It was shownthat, by using the ECR-PECVD process, it was possible toreduce the synthesis temperature of CNT down to 600 ◦C,compared to the typical high temperate (e.g. 900 ◦C) reportedin most literature. The CNW/CNF nanostructures grown atlower temperatures than 500 ◦C also suggest that the proposedfabrication process is tunable such that various types ofhierarchical nanostructures can be obtained by regulating thesynthesis temperature. According to the aimed application,these hybrid nanostructures can be simply functionalized.Nanodevices based on these hierarchical nanostructures will beof great interest for new scientific and engineering applications,such as sensors and lithium battery electrodes, where thegrafting of CNF on silicon is still a challenge.

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