Download Microplasmas as short length and time scale reactors for controlled fabrication of nanomaterials

22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Microplasmas as short length and time scale reactors for controlled fabrication
of nanomaterials
R.M. Sankaran
Department of Chemical Engineering, Case Western Reserve University, Cleveland, Ohio, U.S.A.
Abstract: This invited talk will present recent progress on the development of
microplasma-based schemes for the synthesis of nanomaterials, primarily nanoparticles. In
the first scheme, the microplasma dissociates vapour precursors to nucleate aerosol
nanoparticles in the gas phase. In the second scheme, the microplasma is employed as an
electrochemical cathode and reduces metal ions in water or dispersed in polymer films.
Spatial and temporal scales are critical in both schemes to controlling particle nucleation
and growth and ultimately producing well-defined nanomaterials.
Keywords: microplasma, nanomaterials, nanoparticle, plasma-liquid, nanodiamond
1. Introduction
Plasma processing is a well-established top-down
process to fabricate microscale patterns in thin films.
There has been growing interest in implementing plasmas
in bottom up schemes to produce nanomaterials that are
smaller in size than possible by lithography. However,
nucleation and growth must be precisely controlled to
produce well-defined nanomaterials. Here, two schemes
are presented based on microplasmas to synthesize
nanomaterials.
Microplasmas are confined to
sub-millimetre
dimensions,
leading
to
unique
spatiotemporal effects that help control precursor
dissociation, particle nucleation and growth, and
agglomeration.
2. Experimental Details
Fig. 1 schematically illustrates the two schemes that
have been developed to synthesize nanomaterials based
on microplasmas. The schemes bear similarities to
chemical vapour deposition (CVD) and electrodeposition,
two well-known techniques for thin film fabrication. In
the first scheme, vapour precursors are dissociated in the
microplasma, similar to CVD, but the dissociated
precursor species homogeneously nucleate particles, free
of a substrate. In the second scheme, the microplasma is
formed at the surface of an aqueous solution and metal
cations either generated from oxidation of the metal
anode, or dissolved from a metal salt precursor, are
reduced at the cathode, similar to electrodeposition, but
nucleate and colloidally disperse in solution, free of a
substrate.
3. Results and Discussion
3.1. Gas-phase synthesis of nanodiamonds
We have recently applied the aerosol synthesis of
nanoparticles by microplasma dissociation of vapour
precursors to the formation of nanodiamonds.
Nanodiamonds are carbon nanoparticles that exhibit
diamond phase and properties. Theory has shown than
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Fig. 1.
Two general schemes for synthesizing
nanomaterials (nanoparticles) using microplasmas: (a)
dissociation of vapour precursors to nucleate aerosol
nanoparticles, and (b) cathodic reduction of aqueous
metal cations to nucleate colloidal nanoparticles.
while graphite is the thermodynamically stable form of
bulk carbon at normal conditions, at the nanoscale,
tetrahedral forms of carbon which are the precursor to
diamond are more stable than hexagonal forms of carbon
which are the precursor to graphite. In support of these
predictions, we have carried out experiments with a
carbon precursor, ethanol, and produced nanometre-sized
particles by the microplasma process [1]. Extensive
materials characterization of the carbon nanoparticles
confirms that diamond-phase carbon is produced with
sizes less than 5 nm and structures corresponding to cubic
diamond, lonsdaleite, and n-diamond. The importance of
the non-equilibrium chemistry on the synthesized material
will also be discussed.
3.2. Microplasma reduction of Ag and Au precursors in
baths, water jets, and polymeric films
Using a microplasma as a cathodic electrochemical
electrode, we have studied the reduction of various metal
salts in aqueous solutions. In a static bath without any
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stirring, nanoparticles are formed, but the nucleation and
growth depends on the bath volume since the
microplasma is formed at the surface and the metal
cations must diffuse to the interfacial region. To
introduce forced convection and control the reaction zone,
we have recently formed a microplasma at the surface of a
liquid water jet [2]. Metal cations are transported in the
water jet at a controlled flow rate, through the plasmaliquid interface, to nucleate and grow nanoparticles. This
approach allows metal nanoparticles to be continuously
produced, and the residence time to be controlled so that
it is possible to synthesize well-defined nanoparticles.
Alternatively, we have dispersed metal salts with
polymers, cast films, and exposed the films to a scanning
microplasma to reduce metal cations on a substrate. This
direct-write method leads to the fabrication of flexible,
microscale patterns of metal nanoparticles.
Under
appropriate conditions, the patterns exhibit high electrical
conductivity, enabling applications as flexible electrodes
[3]. Characterization of the reduced films suggests that
the reduction process involves a diffusion/reaction
mechanism whereby the metal cations in the film bulk are
electrophoretically drawn to the film surface to be
reduced by the microplasma.
4. Conclusions
Microplasmas offer a unique platform for synthesizing
well-defined
nanomaterials
by
spatiotemporally
controlling nucleation and growth. In a continuous flow
geometry, particle nucleation and growth are controlled
by the confined plasma volume and the flow rate of the
gas or liquid which carries the reactant into and out of the
reactor. Alternatively, microplasmas can be scanned
across a film surface to control nucleation and growth of
supported nanomaterials. In addition to the unique nonequilibrium chemistry, these approaches open up
possibilities to produce novel materials for a broad range
of applications.
5. References
[1] A. Kumar, P.A. Lin, A. Xue, B. Hao, Y.K. Yap and
R.M. Sankaran. “Formation of nanodiamonds at
near ambient conditions via microplasma
dissociation of ethanol vapor”. Nature Comm., 4,
2618 (2013)
[2] S. Ghosh, B. Bishop, I. Morrison, R. Akolkar,
D. Scherson and R. M. Sankaran. “Generation of a
direct-current, atmospheric-pressure microplasma at
the surface of a liquid water microjet for continuous
plasma-liquid processing”. J. Vac. Sci. Technol. A,
in press (2015)
[3] S. Ghosh, R. Yang, M. Kaumeyer, C.A. Zorman,
S.J. Rowan, P.X.-L. Feng and R.M. Sankaran.
“Fabrication of electrically-conductive metal
patterns at the surface of polymer films by
microplasma-based direct writing”. ACS Appl.
Mater. Interfaces, 6, 3099-3104 (2014)
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