# CVD Synthesis of Hierarchical  3D  MWCNT/Carbon-Fiber  Nanostructures

Authors: Toma Susi; Albert G. Nasibulin; Hua Jiang; Esko I. Kauppinen
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Multiwalled carbon nanotubes (MWCNTs) were synthesized by CVD on industrially  manufactured highly crystalline vapor-grown carbon fibers (VGCFs). Two catalyst metals (Ni and Fe) and carbon precursor gases (C2H2 and CO) were studied. The catalysts were deposited on the fibers by sputtering and experiments carried out in two different reactors. Samples were characterized by electron microscopy (SEM and TEM). Iron was completely inactive as catalyst with both C2H2 and CO for reasons discussed in the paper. The combination of Ni and C2H2 was very active for secondary CNT synthesis, without any pretreatment of the fibers. The optimal temperature for CNT
synthesis was 750∘C, with total gas flow of 650 cm3min−1 of C2H2, H2, and Ar in 1.0:6.7:30 ratio.


---

## Body

## 1. Introduction

Carbon nanotubes  (CNTs) have in recent
years   stimulated
intensive scientific research due  to their
exceptional physical and chemical properties  [1, 2]. A few different
techniques, such as carbon arc-discharge  [3, 4], laser ablation  [5, 6],
and substrate supported and aerosol chemical vapor deposition (CVD)  [7–13], have been
successful in producing lab-scale quantities of material.CNT components have been used in a
large range of applications: in light-emitting diodes [14],
transistors [15], filters [16], field emitters [17],
and photovoltaic devices [18]. Most of the proposed applications utilize
the unique high strength, small size, large aspect ratio and surface area, and
the exceptional chirality-determined electrical properties of nanotubes. In
particular, the advantages gained by incorporating CNTs in fuel cells [19–22]
and
supercapacitors [23, 24] rely on the tubes’ characteristic high
surface area, combined with their good electrical conductivity.Vapor-grown carbon fibers (VGCFs)
are a type of carbon fiber produced by the CVD method, similar to nanotube
synthesis  [25]. A specific manufacturing process  [26] commercialized
by Showa Denko yields industrial-scale amounts of high quality material. Many
promising applications for VGCFs have been implemented in, for example, fuel
cells  [27], supercapacitors  [28], and polymer composites  [29].Materials combining structures of
different-size scales can be called hierarchical. The special case considered
in this study is a combination of structures on two different-size scales:
long, large diameter primary (or “first generation”) nanofibers, and shorter,
smaller diameter secondary (or “second generation”) carbon nanotubes, attached
to the primary fibers. Using the nomenclature of Teo and Sun [30], this can be characterized as the hierarchical “1D on 1D”
structure CNT ⊣ VGCF. The secondary tubes
could, theoretically, be either multiwalled or single-walled.In addition to work published
about CNT growth on graphite [31],
carbon cloth or paper [32–34],
and MWCNTs [35, 36], carbon nanotubes have
previously been synthesized on various carbon fibers  [33, 37–42].Most of the above
methods either require complicated procedures for preparing suitable catalyst
particles or extensive pretreatment of the fibers, or produce low yields and
poor-quality tubes. We provide a simple method for synthesizing CNTs on
untreated industrially manufactured highly crystalline Showa Denko VGCFs, and
further study the role of catalyst particles and precursor gases. Two catalyst
metals—nickel and iron—were studied,
with two carbon precursor gases, namely carbon monoxide (CO) and acetylene(C2H2),
used in two different reactors. Iron was found to be totally inactive for
secondary tube growth, while nickel proved very active at suitable conditions.


## 2. Results and Discussion

### 2.1. Acetylene Experiments

The effect of various experimental
parameters was extensively studied in the horizontal reactor used
(Section3.2),
and a successful combination was found. Practically, no secondary nanotubes
were produced at 650°C, confirming the findings of Duesberg et al. [43]. Of the three higher temperatures
studied (700, 750 and, 800°C), the best results were
clearly obtained at 750°C. As for reduction time, the shortest tested period of
2.5 minutes seemed to be too short to form good catalyst particles, as activation (defined as a visual estimation of the density and
length of produced secondary tubes in SEM) was poor. A reduction time of 5
minutes—which has been
successfully utilized for iron [44]
in a different setup—worked best with
nickel. Based on 24 catalyst particles measured from TEM images of the 5-minute
samples, the average diameter of nickel particles after
synthesis on the Showa Denko fibers was
9.5±1.9 nm,
while on the grid surface their average diameter was only 5.5±1.3 nm. After
7.5 minutes of reduction even though activation was in some places good, the
produced secondary tubes were very short. Moreover, many particle sites were
inactive and covered with amorphous carbon. The effect of gas composition was
found to be less important within the range we studied, and the optimum flow
rates were determined to be 30, 200, and 420cm3min−1 for acetylene, hydrogen, and argon, respectively.In order to investigate the
timeline of CNT growth, we carried out experiments for different synthesis
times. The samples were prepared in the same manner as before (Section3.1),
but acetylene and hydrogen flows were stopped at the allotted time instead of
the typical 10 minutes, and the reactor temperature was lowered. SEM
observations showed that after 2.5 minutes of synthesis, growth has just barely
begun, with very little activation on the Showa Denko fibers. After 5 minutes,
activation was already significant, but the lengths of the synthesized
secondary tubes were short. Activation after 15 minutes was significantly
higher than activation after 10 minutes, suggesting that the growth was still
proceeding at some rate after 10 minutes. The difference between 15- and 20-minute
samples was not clear by visual observation. This behavior is in agreement with
the findings of Krestinin et al. for methane pyrolysis [38].In the optimal synthesis
conditions, the VGCFs were well covered with secondary multiwalled tubes
(Figure1). Some secondary tubes had catalyst particles embedded at their ends, as
found by TEM observation (Figure 2(a)). Most of them had between 10–20 walls and
reasonably good wall structure (Figure 2(b)). A statistical survey of 20
MWCNTs observed in TEM found the average outer diameter to be 12.5±1.7 nm,
while the inner was 5.2±2.0 nm.SEM images showing the morphology of produced MWCNTs on the Showa Denko carbon
fibers. The sample was grown for 10 minutes using Ni andC2H2 at 750°C
with optimal gas composition, supported on an SiO2 substrate.
(b) is a close-up of the area indicated in (a) Imaged using Leo Gemini DSM982 SEM.
(a)(b)TEM images of Showa Denko fibers and the produced secondary tubes, grown
with Ni andC2H2 for 20 minutes at 750°C with optimal gas composition: (a)
VGCFs and secondary MWCNTs. The inset is a close-up of the end of a secondary MWCNT,
showing an encapsulated Ni catalyst particle.
(b) Two different secondary MWCNTs. Imaged using Philips CM200 FEG TEM.
(a)(b)Switching to iron catalyst and
using the same gas mixture that had proven successful with nickel, synthesis
was attempted at temperatures of 700, 750, and800°C. The catalyst particles on
the SiO2 substrate activated CNT growth effectively, with activation
increasing with temperature. However, activation of the catalyst particles on
the VGCFs was nonexistent. The good activation on the substrate itself proves
that reactor conditions were perfectly suitable for growing tubes with
acetylene from the iron catalysts. Furthermore, at the synthesis temperature of
800°C, carbon nanotube forests were grown from iron
catalyst on the SiO2 substrate. At many places on the substrate,
forests of several hundred micrometers in height were growing directly adjacent
to the catalyst-containing fibers on which the catalysts were totally inactive
(Figure 3).SEM images of CNT forests grown onSiO2 substrate from Fe catalyst
with C2H2  at 800°C with optimal gas composition: (a) Nanotube forest of
about 300μm in height growing from the substrate right next to the Showa Denko fibers.
(b) Magnified image of the roots of the nanotube forest.
The adjacent VGCFs were inactive for the growth of secondary tubes.
(a)(b)


### 2.2. Carbon Monoxide Experiments

Experiments with carbon monoxide using the vertical reactor (Section3.3)
with iron catalyst were unsuccessful in producing hierarchical structures. The
iron catalyst particles were again completely inactive on the fibers. We
carried out statistical measurements from TEM images of the samples prepared
with 5-minute reduction and 30-minute growth time at 890°C. We measured 72
inactive iron particles: 40 on the Showa Denko fibers and 32 on the grid
surface. The average diameter of iron particles on the fibers was 13.5±2.7 nm, while on the grid surface it was 6.4±1.4 nm.


### 2.3. Discussion

Synthesis of hierarchical nanostructures using iron catalyst was consistently
unsuccessful with both carbon monoxide and acetylene. Variation of the
experimental conditions together with the prolongation of synthesis time up to
2 hours did not result in any significant improvement. However, as noted above,
reactor conditions were obviously appropriate for the CNT growth at least in
the case of acetylene and iron, since CNT forests were successfully synthesized
on the silicon wafer in the close vicinity of inactive fibers
(Figure3).Let us discuss possible reasons
for the catalytic inactivity of iron for the formation of secondary tubes on
the surface of untreated Showa Denko VGCFs, and compare them with successful
experiments with nickel and literature data. It is worth noting that in spite
of the similarities between Fe and Ni, some of their properties are
significantly different. Of main interest here, the solubility of carbon in
bulk iron is much higher than that in nickel [45, 46]. Size effects alter the
properties of Fe  [47, 48] and Ni  [49] nanoparticles significantly from those in
the bulk. However, even for small nanoparticles, crucial differences between
the two metals are preserved, with Amara et al.  [49] explicitly underlining the
importance of limited carbon solubility in Ni. Also, with iron, the higher
solubility can result in the rapid formation of iron carbide, which is—conversely to
nickel carbide—quite stable at
the experimental conditions [50].
The formation of iron carbide has been proposed to poison the catalyst [51, 52], inhibiting CNT growth [9, 53]. This view has been called into
question, however, by recent in situ TEM observations of nanotube growth from
Fe3C nanoparticles   [54].Another reason for different
products can be attributed to the different diffusion of catalyst particles on
the VGCFs surfaces. Iron particles coalesced during the reduction step and CNT
growth initiation period, producing excessively large particles for CNT growth.
As mentioned above, the average diameter of both Fe and Ni catalyst particles
was much larger on the Showa Denko fiber surfaces than on the TEM grid surface after synthesis (13.5 and 9.5 nm on the
fibers and 6.4 and 5.5 nm on the grids for Fe and Ni, resp.). However, it is
further evident that the Fe particles on the fibers were significantly larger
than the Ni particles. Theoretical considerations  [55] would expect particles
of the sizes observed to be solid, especially at 750°C used in the acetylene
experiments.However, as noted in  [55],
observing the size of catalyst particles either before or after synthesis will
not give a realistic picture of their true state during synthesis, as it is
known that they are not static during reduction and nucleation. In situ
measurements would be needed to observe the evolution of the catalyst particle
sizes during reduction and synthesis, and possible implications to the state of
the particles and their inactivity. It is noteworthy that shortening the
reduction period in either the Fe/C2H2 or the Fe/CO
experiments did not enable the production secondary tubes.Importantly, data found in the
literature indicates that pretreatment of the supporting carbon structure is
vital when using iron  [38, 56], as illustrated by the works listed in
Table 1.
The table displays a collection of relevant parameters from experiments
successful in the synthesis of secondary nanotubes from iron catalyst on carbon
fibers  [33, 37–42]. Several
successful special CVD methods are also included.Table 1
Pretreatment conditions and special synthesis methods found in the literature, which have enabled iron-catalyzed secondary
CNT growth on various carbon materials. CF = carbon fibers, CP = carbon paper.
Support
(supplier)Treatment/methodCatalyst
precursorCarbon
sourceT  [C°]Ref.CP
(E-tech)PECVDe-beam FeMethane600[33]CF
(Toray)InjectionFerroceneToluene750[37]CFAcid bathFe(NO3)3Acetylene750[38]CP
(Toray)Injection,H2SFerroceneXylene800[39]CFInjectionFerroceneXylene800[40]CFReduced pressureFerrofluidMethane750[41]CP
(E-TEK)Ohmic
heatingFe(NO3)3Ethylene700[42]As an example, Zhu et al.  [41] have demonstrated that there exists a
limited temperature range where CNTs can grow from iron particles on
noncrystalline carbon fibers, using low pressure methane gas as carbon
precursor. At low temperatures, only a carbon layer is formed on the fiber
surface. At higher temperature, the diffusion rate of iron particles into
carbon fibers is enhanced. Small particles diffuse into the fibers before the
carbon precursor is introduced, while larger ones nucleate only carbon
clusters.The case above and most of the
others listed in Table1 differ from our experiments either by a radically
modified CVD procedure or by the
lower crystallinity of the supporting fiber—a property which
is altered by pretreatment.  A more
crystalline support could affect both proper carbon diffusion and the diffusion
of catalyst particles on the fibers. Interestingly, Li et al.  [31] have found
that when growing CNTs from a stainless steel film  was deposited on a
graphite foil surface, phase segregation happens during the formation of
catalyst particles, and only Fe/Ni alloy particles rich in Ni are responsible
for the CNTs growth.


## 2.1. Acetylene Experiments

The effect of various experimental
parameters was extensively studied in the horizontal reactor used
(Section3.2),
and a successful combination was found. Practically, no secondary nanotubes
were produced at 650°C, confirming the findings of Duesberg et al. [43]. Of the three higher temperatures
studied (700, 750 and, 800°C), the best results were
clearly obtained at 750°C. As for reduction time, the shortest tested period of
2.5 minutes seemed to be too short to form good catalyst particles, as activation (defined as a visual estimation of the density and
length of produced secondary tubes in SEM) was poor. A reduction time of 5
minutes—which has been
successfully utilized for iron [44]
in a different setup—worked best with
nickel. Based on 24 catalyst particles measured from TEM images of the 5-minute
samples, the average diameter of nickel particles after
synthesis on the Showa Denko fibers was
9.5±1.9 nm,
while on the grid surface their average diameter was only 5.5±1.3 nm. After
7.5 minutes of reduction even though activation was in some places good, the
produced secondary tubes were very short. Moreover, many particle sites were
inactive and covered with amorphous carbon. The effect of gas composition was
found to be less important within the range we studied, and the optimum flow
rates were determined to be 30, 200, and 420cm3min−1 for acetylene, hydrogen, and argon, respectively.In order to investigate the
timeline of CNT growth, we carried out experiments for different synthesis
times. The samples were prepared in the same manner as before (Section3.1),
but acetylene and hydrogen flows were stopped at the allotted time instead of
the typical 10 minutes, and the reactor temperature was lowered. SEM
observations showed that after 2.5 minutes of synthesis, growth has just barely
begun, with very little activation on the Showa Denko fibers. After 5 minutes,
activation was already significant, but the lengths of the synthesized
secondary tubes were short. Activation after 15 minutes was significantly
higher than activation after 10 minutes, suggesting that the growth was still
proceeding at some rate after 10 minutes. The difference between 15- and 20-minute
samples was not clear by visual observation. This behavior is in agreement with
the findings of Krestinin et al. for methane pyrolysis [38].In the optimal synthesis
conditions, the VGCFs were well covered with secondary multiwalled tubes
(Figure1). Some secondary tubes had catalyst particles embedded at their ends, as
found by TEM observation (Figure 2(a)). Most of them had between 10–20 walls and
reasonably good wall structure (Figure 2(b)). A statistical survey of 20
MWCNTs observed in TEM found the average outer diameter to be 12.5±1.7 nm,
while the inner was 5.2±2.0 nm.SEM images showing the morphology of produced MWCNTs on the Showa Denko carbon
fibers. The sample was grown for 10 minutes using Ni andC2H2 at 750°C
with optimal gas composition, supported on an SiO2 substrate.
(b) is a close-up of the area indicated in (a) Imaged using Leo Gemini DSM982 SEM.
(a)(b)TEM images of Showa Denko fibers and the produced secondary tubes, grown
with Ni andC2H2 for 20 minutes at 750°C with optimal gas composition: (a)
VGCFs and secondary MWCNTs. The inset is a close-up of the end of a secondary MWCNT,
showing an encapsulated Ni catalyst particle.
(b) Two different secondary MWCNTs. Imaged using Philips CM200 FEG TEM.
(a)(b)Switching to iron catalyst and
using the same gas mixture that had proven successful with nickel, synthesis
was attempted at temperatures of 700, 750, and800°C. The catalyst particles on
the SiO2 substrate activated CNT growth effectively, with activation
increasing with temperature. However, activation of the catalyst particles on
the VGCFs was nonexistent. The good activation on the substrate itself proves
that reactor conditions were perfectly suitable for growing tubes with
acetylene from the iron catalysts. Furthermore, at the synthesis temperature of
800°C, carbon nanotube forests were grown from iron
catalyst on the SiO2 substrate. At many places on the substrate,
forests of several hundred micrometers in height were growing directly adjacent
to the catalyst-containing fibers on which the catalysts were totally inactive
(Figure 3).SEM images of CNT forests grown onSiO2 substrate from Fe catalyst
with C2H2  at 800°C with optimal gas composition: (a) Nanotube forest of
about 300μm in height growing from the substrate right next to the Showa Denko fibers.
(b) Magnified image of the roots of the nanotube forest.
The adjacent VGCFs were inactive for the growth of secondary tubes.
(a)(b)


## 2.2. Carbon Monoxide Experiments

Experiments with carbon monoxide using the vertical reactor (Section3.3)
with iron catalyst were unsuccessful in producing hierarchical structures. The
iron catalyst particles were again completely inactive on the fibers. We
carried out statistical measurements from TEM images of the samples prepared
with 5-minute reduction and 30-minute growth time at 890°C. We measured 72
inactive iron particles: 40 on the Showa Denko fibers and 32 on the grid
surface. The average diameter of iron particles on the fibers was 13.5±2.7 nm, while on the grid surface it was 6.4±1.4 nm.


## 2.3. Discussion

Synthesis of hierarchical nanostructures using iron catalyst was consistently
unsuccessful with both carbon monoxide and acetylene. Variation of the
experimental conditions together with the prolongation of synthesis time up to
2 hours did not result in any significant improvement. However, as noted above,
reactor conditions were obviously appropriate for the CNT growth at least in
the case of acetylene and iron, since CNT forests were successfully synthesized
on the silicon wafer in the close vicinity of inactive fibers
(Figure3).Let us discuss possible reasons
for the catalytic inactivity of iron for the formation of secondary tubes on
the surface of untreated Showa Denko VGCFs, and compare them with successful
experiments with nickel and literature data. It is worth noting that in spite
of the similarities between Fe and Ni, some of their properties are
significantly different. Of main interest here, the solubility of carbon in
bulk iron is much higher than that in nickel [45, 46]. Size effects alter the
properties of Fe  [47, 48] and Ni  [49] nanoparticles significantly from those in
the bulk. However, even for small nanoparticles, crucial differences between
the two metals are preserved, with Amara et al.  [49] explicitly underlining the
importance of limited carbon solubility in Ni. Also, with iron, the higher
solubility can result in the rapid formation of iron carbide, which is—conversely to
nickel carbide—quite stable at
the experimental conditions [50].
The formation of iron carbide has been proposed to poison the catalyst [51, 52], inhibiting CNT growth [9, 53]. This view has been called into
question, however, by recent in situ TEM observations of nanotube growth from
Fe3C nanoparticles   [54].Another reason for different
products can be attributed to the different diffusion of catalyst particles on
the VGCFs surfaces. Iron particles coalesced during the reduction step and CNT
growth initiation period, producing excessively large particles for CNT growth.
As mentioned above, the average diameter of both Fe and Ni catalyst particles
was much larger on the Showa Denko fiber surfaces than on the TEM grid surface after synthesis (13.5 and 9.5 nm on the
fibers and 6.4 and 5.5 nm on the grids for Fe and Ni, resp.). However, it is
further evident that the Fe particles on the fibers were significantly larger
than the Ni particles. Theoretical considerations  [55] would expect particles
of the sizes observed to be solid, especially at 750°C used in the acetylene
experiments.However, as noted in  [55],
observing the size of catalyst particles either before or after synthesis will
not give a realistic picture of their true state during synthesis, as it is
known that they are not static during reduction and nucleation. In situ
measurements would be needed to observe the evolution of the catalyst particle
sizes during reduction and synthesis, and possible implications to the state of
the particles and their inactivity. It is noteworthy that shortening the
reduction period in either the Fe/C2H2 or the Fe/CO
experiments did not enable the production secondary tubes.Importantly, data found in the
literature indicates that pretreatment of the supporting carbon structure is
vital when using iron  [38, 56], as illustrated by the works listed in
Table 1.
The table displays a collection of relevant parameters from experiments
successful in the synthesis of secondary nanotubes from iron catalyst on carbon
fibers  [33, 37–42]. Several
successful special CVD methods are also included.Table 1
Pretreatment conditions and special synthesis methods found in the literature, which have enabled iron-catalyzed secondary
CNT growth on various carbon materials. CF = carbon fibers, CP = carbon paper.
Support
(supplier)Treatment/methodCatalyst
precursorCarbon
sourceT  [C°]Ref.CP
(E-tech)PECVDe-beam FeMethane600[33]CF
(Toray)InjectionFerroceneToluene750[37]CFAcid bathFe(NO3)3Acetylene750[38]CP
(Toray)Injection,H2SFerroceneXylene800[39]CFInjectionFerroceneXylene800[40]CFReduced pressureFerrofluidMethane750[41]CP
(E-TEK)Ohmic
heatingFe(NO3)3Ethylene700[42]As an example, Zhu et al.  [41] have demonstrated that there exists a
limited temperature range where CNTs can grow from iron particles on
noncrystalline carbon fibers, using low pressure methane gas as carbon
precursor. At low temperatures, only a carbon layer is formed on the fiber
surface. At higher temperature, the diffusion rate of iron particles into
carbon fibers is enhanced. Small particles diffuse into the fibers before the
carbon precursor is introduced, while larger ones nucleate only carbon
clusters.The case above and most of the
others listed in Table1 differ from our experiments either by a radically
modified CVD procedure or by the
lower crystallinity of the supporting fiber—a property which
is altered by pretreatment.  A more
crystalline support could affect both proper carbon diffusion and the diffusion
of catalyst particles on the fibers. Interestingly, Li et al.  [31] have found
that when growing CNTs from a stainless steel film  was deposited on a
graphite foil surface, phase segregation happens during the formation of
catalyst particles, and only Fe/Ni alloy particles rich in Ni are responsible
for the CNTs growth.


## 3. Experimental

### 3.1. Sample Preparation

For our supported CVD synthesis of hierarchical
carbon nanostructures, industrially manufactured Showa Denko VGCFs were used as
the primary level of the hierarchical structure. The tubes were delivered as a
powder of very crystalline and pure nanofibers with an average diameter of
around 150 nm. The fibers were dispersed in (1,2)-dichloroethane (DCE) in a
low-power ultrasonic bath at40–50°C, with
concentrations ranging from 0.5 to 1.5mg/mL−1.For
acetylene experiments, silicon wafers were used to support the VGCFs. The sizes
of the substrates cut from the wafers were about 0.5cm2. The pieces
were cut from either a (111) SiO2/Si wafer with
a thermally grown 260 nm SiO2 layer, or from a (100) Si wafer
with a native oxide layer. Droplets of DCE solution of VGCF material were
deposited on the substrate surfaces by drop-drying. After synthesis, all
samples were observed by field emission scanning electron microscope (Leo
Gemini DSM982). Selected samples were also observed by field emission
transmission electron microscope (Philips CM200 FEG) after transferring the
fabricated material onto TEM grids (SPI, 100 Ni SiO2/SiO). This was
done by wetting the silicon substrate surfaces with DCE and wiping them with
TEM grids. For carbon monoxide experiments, the samples were grown directly on
TEM grids, which were inserted into a vertical reactor using a specially
designed holder.Sputtering
was used to deposit the catalyst particles needed for the CVD synthesis. For
theC2H2 experiments, the sputtering targets used were
nickel and iron, while iron was used for synthesis with CO. A magnetron sputter
coater (AGAR Auto Sputter Coater 108A) was used to produce nanoparticles on the
carbon fibers deposited on the substrates. A sputtering current of 20 mA
maintained for 15 seconds was used for both targets, as this has been found to
be optimal for CNT production with the reactors used [57].


### 3.2. Acetylene Reactor

The synthesis of carbon nanotubes withC2H2 was carried out in a horizontal
reactor. It consisted of a 40 cm long quartz tube with an inner diameter of 1.2 cm, set inside a ceramic tube with an inner diameter of 3.0 cm. The tube was
further placed inside a 34 cm long furnace, which was heated to a high
temperature needed for the synthesis. Temperature calibration was done with a
digital thermometer  (Fluke 52 K/J) attached to a K-type metal wire
thermocouple. The temperature inside the reactor at growth conditions was
measured to be constant for a distance of several centimeters around the sample
position.For
the synthesis, the samples were put into a ceramic boat, one end of the tube
opened, and the boat pushed to the high temperature zone of the reactor with a
steel rod. The reactor was then sealed. Before the introduction of the carbon
source gas, deposited Ni or Fe catalysts were reduced at the selected synthesis
temperature in a hydrogen-containing atmosphere, typically for 5 minutes. After
reduction, acetylene was introduced and kept at a constant flow for the desired
period of time (typically 10 minutes). The flows were controlled using a mass
flow controller (Aalborg GFC17) or critical orifices calibrated with a flow
calibrator (Gilian Gilibrator-2).After
the synthesis period, acetylene and hydrogen flows were stopped and the reactor
temperature was lowered. Argon flow rate was increased to 700cm3min−1 to hasten the cooling.
After a cooling period of about 1.5 hours, at which point the sample
temperatures were well below 200°C, the ends of the quartz tube were opened and
the boat pushed out of the quartz tube with the steel rod.The
main parameters varied in the acetylene experiments were synthesis temperature
(650–800°C), reduction
time (2.5–7.5 minutes),
acetylene flow rate (10–50 cm3min−1, corresponding to
concentrations of 1.6–13.0% depending
on the gas mix), and synthesis time (2.5–20 minutes).
Furthermore, in some experiments the flow rates of H2 (0, 130, and
200 cm3min−1)
and Ar (0, 270, 420, and 700 cm3min−1) were varied to
study the possible effects of mixing and gas composition.


### 3.3. Carbon Monoxide Reactor

The synthesis of CNTs with CO was
attempted in a vertical laminar flow reactor. It consisted of a ceramic tube
with an inner diameter of 2.2 cm inside a 44 cm long furnace. A stainless steel
rod of 6 mm in diameter with a TEM grid holder at one end was used for
inserting the samples. The reactor design and procedure for the CO experiments
were identical to previous work  [57].Briefly,
a temperature of890°C had been found to be optimal for iron-catalyzed growth
on SiO2 wafers, with a reduction time of 5 minutes with H2 and Ar flows (200 and 400 cm3min−1) passed into the
reactor. Ar was then replaced with CO (400 cm3min−1),
while the H2 flow remained constant. A small amount of CO2 (8 cm3min−1) was introduced into the reactor for the
whole duration of the experiment, as this had been found to have a critical
effect on nanotube growth  [58]. In this study, typical synthesis
times were 30 minutes, and the samples were characterized by SEM and TEM.


## 3.1. Sample Preparation

For our supported CVD synthesis of hierarchical
carbon nanostructures, industrially manufactured Showa Denko VGCFs were used as
the primary level of the hierarchical structure. The tubes were delivered as a
powder of very crystalline and pure nanofibers with an average diameter of
around 150 nm. The fibers were dispersed in (1,2)-dichloroethane (DCE) in a
low-power ultrasonic bath at40–50°C, with
concentrations ranging from 0.5 to 1.5mg/mL−1.For
acetylene experiments, silicon wafers were used to support the VGCFs. The sizes
of the substrates cut from the wafers were about 0.5cm2. The pieces
were cut from either a (111) SiO2/Si wafer with
a thermally grown 260 nm SiO2 layer, or from a (100) Si wafer
with a native oxide layer. Droplets of DCE solution of VGCF material were
deposited on the substrate surfaces by drop-drying. After synthesis, all
samples were observed by field emission scanning electron microscope (Leo
Gemini DSM982). Selected samples were also observed by field emission
transmission electron microscope (Philips CM200 FEG) after transferring the
fabricated material onto TEM grids (SPI, 100 Ni SiO2/SiO). This was
done by wetting the silicon substrate surfaces with DCE and wiping them with
TEM grids. For carbon monoxide experiments, the samples were grown directly on
TEM grids, which were inserted into a vertical reactor using a specially
designed holder.Sputtering
was used to deposit the catalyst particles needed for the CVD synthesis. For
theC2H2 experiments, the sputtering targets used were
nickel and iron, while iron was used for synthesis with CO. A magnetron sputter
coater (AGAR Auto Sputter Coater 108A) was used to produce nanoparticles on the
carbon fibers deposited on the substrates. A sputtering current of 20 mA
maintained for 15 seconds was used for both targets, as this has been found to
be optimal for CNT production with the reactors used [57].


## 3.2. Acetylene Reactor

The synthesis of carbon nanotubes withC2H2 was carried out in a horizontal
reactor. It consisted of a 40 cm long quartz tube with an inner diameter of 1.2 cm, set inside a ceramic tube with an inner diameter of 3.0 cm. The tube was
further placed inside a 34 cm long furnace, which was heated to a high
temperature needed for the synthesis. Temperature calibration was done with a
digital thermometer  (Fluke 52 K/J) attached to a K-type metal wire
thermocouple. The temperature inside the reactor at growth conditions was
measured to be constant for a distance of several centimeters around the sample
position.For
the synthesis, the samples were put into a ceramic boat, one end of the tube
opened, and the boat pushed to the high temperature zone of the reactor with a
steel rod. The reactor was then sealed. Before the introduction of the carbon
source gas, deposited Ni or Fe catalysts were reduced at the selected synthesis
temperature in a hydrogen-containing atmosphere, typically for 5 minutes. After
reduction, acetylene was introduced and kept at a constant flow for the desired
period of time (typically 10 minutes). The flows were controlled using a mass
flow controller (Aalborg GFC17) or critical orifices calibrated with a flow
calibrator (Gilian Gilibrator-2).After
the synthesis period, acetylene and hydrogen flows were stopped and the reactor
temperature was lowered. Argon flow rate was increased to 700cm3min−1 to hasten the cooling.
After a cooling period of about 1.5 hours, at which point the sample
temperatures were well below 200°C, the ends of the quartz tube were opened and
the boat pushed out of the quartz tube with the steel rod.The
main parameters varied in the acetylene experiments were synthesis temperature
(650–800°C), reduction
time (2.5–7.5 minutes),
acetylene flow rate (10–50 cm3min−1, corresponding to
concentrations of 1.6–13.0% depending
on the gas mix), and synthesis time (2.5–20 minutes).
Furthermore, in some experiments the flow rates of H2 (0, 130, and
200 cm3min−1)
and Ar (0, 270, 420, and 700 cm3min−1) were varied to
study the possible effects of mixing and gas composition.


## 3.3. Carbon Monoxide Reactor

The synthesis of CNTs with CO was
attempted in a vertical laminar flow reactor. It consisted of a ceramic tube
with an inner diameter of 2.2 cm inside a 44 cm long furnace. A stainless steel
rod of 6 mm in diameter with a TEM grid holder at one end was used for
inserting the samples. The reactor design and procedure for the CO experiments
were identical to previous work  [57].Briefly,
a temperature of890°C had been found to be optimal for iron-catalyzed growth
on SiO2 wafers, with a reduction time of 5 minutes with H2 and Ar flows (200 and 400 cm3min−1) passed into the
reactor. Ar was then replaced with CO (400 cm3min−1),
while the H2 flow remained constant. A small amount of CO2 (8 cm3min−1) was introduced into the reactor for the
whole duration of the experiment, as this had been found to have a critical
effect on nanotube growth  [58]. In this study, typical synthesis
times were 30 minutes, and the samples were characterized by SEM and TEM.


## 4. Conclusion

Two different carbon precursor gases,
carbon monoxide and acetylene, were studied for the synthesis of secondary
carbon nanotubes on the surface of highly crystalline Showa Denko vapor-grown
carbon fibers. Two different catalyst metals were examined, namely nickel and
iron with acetylene, and iron with carbon monoxide. Of these, the combination
of nickel and acetylene proved to be the only working solution.Using
the presented simple and straightforward nickel-catalyzed synthesis on
untreated VGCF surfaces, the achieved optimal secondary tube yield was high,
with good activation and coverage on the carbon fibers. With view of
applications, either replacing the Showa Denko fibers with multiwalled carbon
nanotubes grown directly to a conducting substrate to act as the primary
structure, or replacing the VGCFs used in current applications by our material
would be logical next steps. Potential applications for this nanomaterial would
include catalyst supports in fuel cells, powerful supercapasitors, and possibly
gas sensors. One further interesting application could be the use of hierarchical
nanostructures as reinforcement material. The secondary tubes could act as
“anchors” to prevent the slipping of MWCNT or fiber composites, thus improving
the mechanical properties significantly.



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*Source: 425195-2008-10-26.xml*