Microplasma synthesis of metal nanoparticles for gas-phase studies of catalyzed carbon nanotube growth

Article abstract of DOI:10.1063/1.2786835

Catalytic properties of metal nanoparticles toward gas-phase carbon nanotube (CNT) growth are presented. Narrow dispersions of iron (Fe) and nickel (Ni) nanoparticles are prepared in a direct current microplasma reactor and subsequently introduced with acetylene (C2 H2) and hydrogen (H2) into a heated flow furnace to catalyze CNT growth. Aerosol size classification and high-resolution transmission electron microscopy show that CNT growth occurs on Ni particles at lower temperatures than that for similarly produced Fe nanoparticles. Activation energies of 117 and 73 kJmol are found for Fe and Ni catalyst particles, respectively, suggesting that CNT growth occurs by carbon surface diffusion.

Full text of DOI:10.1063/1.2786835

Microplasma synthesis of metal nanoparticles for gas-phase studies of catalyzed
carbon nanotube growth
Wei-Hung Chiang and R. Mohan Sankaran
Citation: Applied Physics Letters 91, 121503 (2007); doi: 10.1063/1.2786835
View online: http://dx.doi.org/10.1063/1.2786835
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APPLIED PHYSICS LETTERS 91, 121503 ͑2007͒
Microplasma synthesis of metal nanoparticles for gas-phase studies
of catalyzed carbon nanotube growth
Wei-Hung Chiang and R. Mohan Sankarana͒
Department of Chemical Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7217
͑Received 27 July 2007; accepted 30 August 2007; published online 20 September 2007͒
Catalytic properties of metal nanoparticles toward gas-phase carbon nanotube ͑CNT͒ growth are
presented. Narrow dispersions of iron ͑Fe͒ and nickel ͑Ni͒ nanoparticles are prepared in a direct
current microplasma reactor and subsequently introduced with acetylene ͑C2H2͒ and hydrogen ͑H2͒
into a heated flow furnace to catalyze CNT growth. Aerosol size classification and high-resolution
transmission electron microscopy show that CNT growth occurs on Ni particles at lower
temperatures than that for similarly produced Fe nanoparticles. Activation energies of 117 and
73 kJ/mol are found for Fe and Ni catalyst particles, respectively, suggesting that CNT growth
occurs by carbon surface diffusion. © 2007 American Institute of Physics.
͓DOI: 10.1063/1.2786835͔
The unique physical and chemical properties of carbon
nanotubes ͑CNTs͒ are defined by their atomic structure. Car-
bon nanostructures are commonly synthesized using transi-
tion metal nanoparticles as catalysts.1,2 The structure of
CNTs has been found to be determined by the size and
chemical composition of the metal catalysts.3,4 However, at
present, the diameter, length, and chirality of CNTs have not
been controlled sufficiently in a single process due to incom-
plete understanding of the role of the catalyst in nanotube
nucleation and growth. In conventional thermal and plasma-
enhanced chemical vapor deposition ͑CVD͒ processes, CNT
growth is performed on a substrate making it difficult to
separately control the properties of catalytic particles.5 In
addition, CNT nucleation and growth are difficult to monitor
in situ which necessitate postgrowth characterization.6,7 Re-
cently, CNTs have been synthesized in a flowing gas stream
using metal particles that catalyze decomposition of carbon
precursors in a substrate-free reactor.8,9 Elimination of the
substrate offers the advantage of revealing the true catalytic
properties of metal nanoparticles toward CNT nucleation and
growth. Gas-phase synthesis of CNTs can be coupled with
aerosol instrumentation to monitor growth in real time.10,11
Therefore, kinetic studies of CNT growth can be easily per-
formed giving valuable insight into the growth mechanism
on various catalysts.12
In this letter, we report on a synthesis route based on a
microplasma reactor that produces metal particles less than
3 nm in diameter for catalyzed, gas-phase CNT growth. Pre-
vious reports of CNT growth using aerosol catalyst particles
have suffered from broad size distributions which required
size selection.12,13 Microplasmas are miniaturized glow dis-
charges that contain energetic electrons which permit non-
thermal decomposition of gaseous precursors to nucleate par-
ticles. Because of the small spatial scale of the reaction zone,
narrowly dispersed nanoparticles are produced in a single
step.14 We have extended this gas-phase process to the prepa-
ration of iron ͑Fe͒ and nickel ͑Ni͒ catalyst particles from
ferrocene and nickelocene, respectively. CNT growth is
achieved by adding acetylene ͑C2H2͒ and hydrogen ͑H2͒
gases to the particle-laden flow in a tubular flow furnace.
Aerosol instrumentation is used for in situ measurement of
CNTs, allowing the length of CNTs to be obtained directly.
The temperature dependence of CNT growth rate shows that
the activation energies for Fe and Ni particles are 117 and
73 kJ/mol, respectively, which are significantly lower than
values associated with other thermal-CVD processes.7,15,16
The experimental setup schematically illustrated in Fig.
1 consisted of a microplasma reactor for synthesis of metal
catalyst particles, tubular flow furnace for CNT growth, and
aerosol size classifier. Metal nanoparticles were grown in a
direct current ͑dc͒, atmospheric-pressure microplasma reac-
tor. The microplasma was formed in a flow geometry using a
stainless-steel capillary tube ͓180 ␮m inside diameter ͑i.d.͔͒
as the cathode and a stainless-steel mesh as the anode.17 The
electrodes were separated by a gap of 2 mm and pressure
sealed inside a quartz tube. To nucleate metal particles, either
ferrocene or nickelocene vapor was introduced into the reac-
tor by subliming the solid powder at room temperature in an
argon flow. Typical discharge voltages and currents used
were 320 V and 3 mA, respectively, for ferrocene and 380 V
and 4 mA, for nickelocene. The particle size and distribution
of metal catalysts were determined by aerosol size classifi-
cation using a cylindrical differential mobility analyzer
͑DMA͒ ͑model 3080N, TSI, Inc.͒ and ultrafine condensation
particle counter ͑CPC͒ ͑model 3776, TSI, Inc.͒. Details of
this particle sizing method have been described elsewhere.18
To prevent aggregation, size distributions of the metal par-
ticles were determined immediately downstream from the
microplasma reactor. For both precursors, the mean particle
size and standard deviation of the distribution increased
with concentration. Here, we present size distributions
of Fe and Ni particles acquired with the DMA-
FIG. 1. Schematic diagram of microplasma reactor and tubular flow furnace
used for catalytic, gas-phase growth of carbon nanotubes. As-grown nano-
structures are continuously monitored by a DMA and CPC.
a͒
Author to whom correspondence should be addressed. Electronic mail:
mohan@case.edu
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0003-6951/2007/91͑12͒/121503/3/$23.00 91, 121503-1 © 2007 American Institute of Physics
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121503-2
W.-H. Chiang and R. M. Sankaran
Appl. Phys. Lett. 91, 121503 ͑2007͒
FIG. 3. HRTEM images of carbon nanostructures grown in a tube furnace
on ͑a͒ Fe particles at 500 °C, ͑b͒ Fe particles at 600 °C, ͑c͒ Ni particles at
500 °C, and ͑d͒ Ni particles at 600 °C ͑experimental conditions: 0.5 SCCM
C2H2 +50 SCCM H2͒.
further increased, the mean mobility diameters increased sig-
nificantly for both Fe and Ne catalyst particles ͓see Figs. 2͑a͒
and 2͑b͔͒, indicating the onset of CNT growth. Comparing
the size distribution of Fe- and Ni-catalyzed growths at
600 °C, we found that the particle counts were much higher
for Ni than for Fe, implying that Ni nanoparticles catalyze a
larger number of CNTs. Between 400 and 700 C, no particle
counts were observed when the plasma or metallocene flow
was shut off, confirming that the metal particles produced in
the microplasma reactor catalyzed CNT growth. However,
above 700 °C, counts were observed in the absence of nano-
particles. This is consistent with previous studies which have
reported that self-pyrolysis of C2H2 occurs at these tempera-
tures leading to nucleation of carbon nanoparticles.12 There-
fore, below 700 °C, the size distributions are representative
of catalytic decomposition and allow kinetic studies of CNT
growth to be performed.
FIG. 2. ͑Color online͒ Temperature-dependent aerosol size distributions of
carbon nanostructures grown on ͑a͒ Fe and ͑b͒ Ni catalyst particles. Log-
normal distribution fits are shown. The insets show the size distributions of
the nanoparticles ͑NPs͒ synthesized in the microplasma reactor ͑experimen-
tal conditions: 10 SCCM ͑SCCM denotes cubic centimeter per minute at
STP͒ ferrocene+90 SCCM Ar for Fe NPs and 20 SCCM nickelocene
+80 SCCM Ar for Ni NPs͒.
The morphology of carbon nanostructures produced by
microplasma synthesis of metal catalysts and free-flow CNT
growth was analyzed by high-resolution transmission elec-
tron microscopy ͑HRTEM͒. Thin films of material were col-
lected for 1 h by depositing the aerosol onto carbon-coated
copper grids using an electrostatic precipitator ͑ESP͒ ͑model
3089, TSI, Inc.͒. HRTEM microcharacterization was per-
formed with a Philips Tecnai F30 operated at 300 kV. Figure
3 shows TEM images of carbon nanostructures grown using
Fe and Ni particles at two different temperatures. At a
growth temperature of 500 °C, the Fe particles appear to be
coated by onionlike graphitic layers ͓see Fig. 3͑a͔͒. The inner
and outer diameters of coated particles are 2 and 5 nm, re-
spectively, with a narrow size distribution which is consistent
with obtained DMA size distributions ͓see inset of Fig. 2͑a͔͒.
The observed carbon structures bear similarity to in situ
TEM observations20 and molecular dynamic simulations21 of
CNTs produced catalytically at low temperatures, which
have shown that graphitic encapsulation of Fe nanoparticles
occurs when the growth temperature is between 227 and
527 °C. At 600 °C, Fe particles were found to catalyze CNT
growth ͓see Fig. 3͑b͔͒. HRTEM shows that the CNTs are
highly crystalline and multiwalled with an outer diameter of
10 nm ͓see Fig. 3͑b͔͒. In Figs. 3͑c͒ and 3͑d͒, TEM images of
CNTs grown at 500 and 600 °C using Ni catalysts are given
CPC system at a fixed precursor concentration ͓see insets of
Figs. 2͑a͒ and 2͑b͒, respectively͔. Experimental data was fit-
ted to a log-normal distribution in order to obtain the geo-
metric mean particle diameter ͑Dg͒ and standard deviation
͑␴ ͒. As-grown Fe and Ni particles were found to have a Dg
g
of 2.87 and 3.11 nm, respectively, with a ␴ of 1.12 and
g
1.16, respectively. The observed ␴ values reflect a much
g
narrower distribution than measured by other aerosol meth-
ods without size selection.19
The metal nanoparticles generated in the microplasma
reactor were rapidly introduced into a tubular furnace with
C2H2 and H2 to synthesize CNTs. The CNTs were activated
and grown to a well-defined length by controlling the tem-
perature and residence time in the tube furnace ͑1.8 cm i.d.
ϫ30 cm length͒. For all experiments discussed here, the
residence time was fixed at 30 s. CNT growth was continu-
ously monitored at the exit of the furnace as a function of
temperature with the DMA-CPC system. At 400 °C, no ap-
preciable change was observed in the particle size distribu-
tion for Fe particles from that of unreacted Fe. In compari-
son, at the same temperature, the size distribution for Ni
particles exhibited a shift from a mean mobility diameters of
3.11 to 5.35 nm, indicating that carbon deposition was oc-
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curring ͓see Fig. 2͑b͔͒. As the furnace temperature was
that show multiwalled nanotubes are grown at both low and
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121503-3
W.-H. Chiang and R. M. Sankaran
Appl. Phys. Lett. 91, 121503 ͑2007͒
be due to the absence of mass-transfer effects,12 the gas-
phase process should be identical for both Fe and Ni par-
ticles. Because of the lower temperatures used here than the
other thermal-CVD processes and the large difference in ac-
tivation energies for Fe and Ni particles, we suggest that
surface diffusion of carbon is responsible for the material
dependent catalytic properties.
In summary, we have demonstrated the controlled syn-
thesis of Fe and Ni catalyst particles by an atmospheric-
pressure, dc microplasma for gas-phase growth of CNTs. Our
study shows that Ni particles allow the growth of CNTs at
lower temperature than Fe where surface diffusion is the
rate-limiting step. The synthesis methodology is also appli-
cable to studies of alloyed, nontransition metal,27 and semi-
conductor28 catalysts for CNT synthesis.
FIG. 4. Arrhenius plots for growth rates of CNTs on Fe and Ni catalyst
particles.
high temperatures. The TEM analysis verifies that Ni-
catalyzed CNT growth occurred at lower temperatures than
in the case of Fe-catalyzed growth, which is in accordance
with our aerosol measurements.
The authors thank Amir Avishai for TEM technical as-
sistance and useful discussions.
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