Synthesis of ZnO nanoparticle coatings on carbon nanotube cathodes
for electrical discharge applications
Martin Lennox, Sylvain Coulombe
Department of Chemical Engineering, McGill University, Montréal, QC, Canada
Abstract: The enhanced field emission of carbon nanotube and carbon-nanotubenanoparticle composite materials have the potential to significantly improve the
voltage requirements and reduce cathode heating in plasma generation. An
integrated dusty plasma process for the deposition of zinc nanoparticle films on
carbon nanotube-coated metal electrodes is described and their performance as
cathodes in DC glow discharges is evaluated.
Keywords: zinc oxide, carbon nanotube, nanoparticle, dusty plasma, thin film
1. Introduction
The lifetime of DC electrical discharge-based device
is largely dependent upon the electron emission
characteristics of the cathode material. Thermionic
and thermo-field emission processes are effective
modes of electron emission, but necessitate high
temperatures and consequently significant thermal
loads at the attachment point, which ultimately lead
to erosion by vaporization and liquid volume
ejection1. Nanostructured materials show great
promise as cathode materials, since reports of their
enhanced electron emission2 could mitigate the need
for high temperature-sustained electron emission
modes. The nanostructure plays the dual role of local
electric field enhancer and host for work functionlowering materials. Carbon nanotubes in particular,
are already in use as field electron emitters. Our
recent work demonstrated that under the right
conditions, carbon nanotubes can also be used to
sustain low-power density DC electrical discharges3.
It appeared that thermo-field emission at lowtemperature is possible due to the drastic lowering of
the CNT work function due to the presence of
contaminants in the plasma environment. On the
other hand, pristine CNTs, being single walled or
multi-walled, appear to not be capable of sustaining
electrical discharge conditions. Uncoated and/or
unfunctionalized CNTs have been documented as
susceptible to erosion4 and suffer etching when
exposed to oxidizing plasmas. There is thus an
interest to protect the CNT surface for high
temperature and/or reaction while maintaining their
nanostructure. The present work reports our
preliminary attempt to deposit ZnO nanocrystal
coatings onto CNTs in an attempt to maintain or
enhance their enhanced emission properties while
protecting the CNTs.
Research into the use of CNTs decorated with zinc
oxide nanoparticles as potential electron emission
sources has reported enhanced electron fieldemission current5 as well as an increase in the
nanotube field emission lifetime6, although it
appears that a different morphology of the ZnO
coating can decrease the electron emission properties
of such composite materials. Documented synthesis
methods frequently rely on wet chemistry,
organometallic precursors, or exposure of CNTs to
high temperatures for zinc vapour deposition or
annealing. Most often, these methods result in
“decorated” CNTs, a term used in the literature to
describe the sporadic deposition of nanoparticles on
CNT substrates. In order to improve the deposition
of ZnO coatings on CNTs for their eventual testing
as cathode materials, a dusty plasma process was
developed that makes use of an aerosol flow
condensation (AFC) process for zinc nanoparticle
synthesis.
AFC is a process for the synthesis of micro- and
ultra-fine metal powders previously described in the
literature7. Collection of these particles has been
noted as problematic, and has relied on the use of
thermophoretic sampling methods. Previous success
in our research group with regards to the collection
of metal nanoparticles onto substrates covered in
CNTs8 has led to the use of the surface charging
experienced by the nanoparticles in a dusty plasma
to enhance their deposition as coatings on CNTcoated metal cathodes.
Given the potential for enhanced electron emission
of CNTs and ZnO-CNT composites, it is of interest
to investigate their utility in reducing the operating
temperature requirements of cathode materials
through improved electron emission, as well as their
durability in such processes.
2. Zinc nanoparticle synthesis
Zinc nanoparticles are synthesized by AFC from a
heated crucible at 450-500 °C that contains pure zinc
pellets. Argon gas is passed over the crucible
through a 1 mm x 20 mm rectangular orifice in a
water-cooled nozzle. Simultaneously, a sheath gas of
argon is flowed over the interior diameter of the
reactor from sixteen 1.6 mm diameter holes, spaced
equally around the circumference of the nozzle. This
stream of zinc nanoparticles flows through two
consecutive glass electrical breaks, of which the
isolated portion is used as the live electrode for the
generation of a continuous RF (13.56 MHz) glow
discharge.
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Figure 1. Schematic of nanoparticle deposition apparatus.
The total argon flow rate through the system is
typically 1-2 SLPM, with a maximum possible total
flow rate of 7.5 SLPM. The pressure inside the
reactor during deposition is 2 Torr and the input
power to the plasma is 20 W.
3. Results
Using a total system flow rate of 2 SLPM Ar,
exposure of the CNT-SS304 samples to the dusty
plasma stream for 1 min resulted in the deposition of
approximately 5 nm diameter particles, in addition to
larger particles in the range of 20 – 50 nm, as shown
in Figure 2. At the maximum argon flow rate,
nanoparticles in the 5 nm range were deposited, as
shown in Figure 3.
The composite materials for testing are formed by
exposing 9/16” diameter stainless steel 304 (SS304)
mesh rounds that are coated with multi-walled
carbon nanotubes (MWCNTs) by a previously
described process9 to the stream issuing from the
dusty plasma. The deposition is performed for the
desired length of time by making use of a linear
motion feed-through, upon which the deposition
substrates are mounted. Oxidation of the zinc
nanoparticles is accomplished by trace amounts of
oxygen in the argon flow, as well as subsequent
exposure of the composite materials to atmospheric
conditions. A schematic diagram of the experimental
apparatus is shown in Figure 1.
Figure 2. TEM image of the CNT coating synthesized at 2
SLPM Ar (total flow rate) and 1 min deposition time.
To generate nanocomposite structures to be
evaluated as the cathodes of DC electrical
discharges, a series of CNT-SS304 substrates were
exposed to the outlet of the dusty plasma for 5 min
to ensure complete coverage of the nanostructured
surface. In this instance, the nanoparticle coating
resulted in a drastic increase of the effective
diameter of the nanostructured surface; untreated,
the diameter of the MWCNTs growing from the
surface of the SS304 mesh were between 30 – 50
nm, whereas the diameter of the coated MWCNTs
was approximately 500 nm, as can be seen in Figure
4.
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Figure 3. STEM image of the CNT coating synthesized at 7.5
SLPM Ar (total flow rate) and 1 min deposition time.
The performance of the synthesized composite
materials was tested in a linear DC glow discharge
operating at 3.5 Torr and 4 mA current. The
nanocomposite cathodes were mounted in a custom
boron nitride ceramic enclosure that ensured only
the material for testing was exposed to the plasma.
To mitigate the effects of cathode heating, argon was
flowed through the system at a rate of 2 SLPM. The
voltage across the generated glow discharge was
monitored as a function of time, for cathodes
composed of the base SS304 mesh and the ZnOCNT nanocomposite. The results of this are shown
in Figure 5, with error bars showing the 95%
confidence interval.
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Figure 5. Performance of tested cathode materials.
There appears to be no significant difference
between the tested SS304 base substrate and the
ZnO-CNT composite material. In fact, the general
trend of Figure 5 indicates that the composites
formed by 5 min of deposition perform more poorly
than the untreated SS304. This may be explained by
the thickness of the ZnO coating, which may have
acted as a barrier to electron emission. Moreover,
composite material may no longer contain sufficient
nanostructured surfaces to meaningfully enhance the
electron emission, as reported in the literature.
Figure 4. CNTs coated by the dusty plasma, 2 SLPM Ar (total
flow rate), 5 min deposition time.
The large variance observed in the voltage
measurements of Figure 5 has been attributed to
instability in the flow of argon across the cathode
surface and thus fluctuations in the cooling rate of
the cathode. The supplied voltage varied accordingly
to maintain the set current of the discharge, which
resulted in the observed variations voltage across the
discharge.
Consequently, tests are in progress to evaluate the
performance of the nanostructured composites
produced for shorter deposition times under more
stable cooling conditions for the cathode. The results
of the ongoing investigations will be reported at the
conference.
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