22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Charge effect on atmospheric-pressure microplasma reduction of silver and gold
salts to colloidal nanoparticles
C. De Vos1, J. Baneton1, R.M. Sankaran2 and F. Reniers1
1
CHANI, Université Libre de Bruxelles, Brussels, Belgium
2
Case Western Reserve University, Cleveland, U.S.A.
Abstract: Aqueous silver and gold salts have been reduced to colloidal metal nanoparticles
by an atmospheric pressure microplasma. This study focuses on the influence of the charge
effect on the reduction and the nanoparticle synthesis rate. The role of species involved in
the reduction process (e.g., H 2 O 2 , electrons) has also been discussed.
Keywords: microplasma, atmospheric pressure plasma, nanoparticle synthesis
1. Introduction
Microscale plasmas or microplasmas are electrical
discharges presenting at least one geometrical dimension
lower than one millimeter. As a consequence, they are
remarkably stable at atmospheric pressure and nonthermal [1], thus facilitating the introduction of a liquid
phase for applications in water treatment, medicine and
material synthesis [2]. This study focuses on the synthesis
of silver and gold nanoparticles (Ag and Au NPs) in
aqueous solution. These nanosized particles have great
potential in the development of new pharmaceutical
drugs. Ag NPs exhibit interesting antimicrobial activities
[3] and Au NPs have numerous applications in the fields
of biosensors, disease diagnosis and gene expression due
to the ease of binding to biomolecules [4]. They equally
present chemical stability, good conductivity, catalytic
activity and interesting optical properties [5].
The microplasma, formed in a flow of argon, is initiated
at the surface of solutions containing silver nitrate or
chloroauric acid. When the active species (i.e. electrons,
radicals, ions) from the discharge interact with the liquid
phase, they initiate electrochemical reactions and lead to
the reduction of aqueous metallic cations. Although the
reduction mechanism is still not fully understood, some
studies have assumed that gas-phase electrons from the
plasma nucleate and grow colloidal nanoparticles [6]. In
the case of aqueous solution, plasma electrons can also
generate hydrogen and hydroxyl radicals, hydrogen gas
and hydrogen peroxide that could be responsible for
reducing the metal cations [7].
Polyvinyl alcohol (PVA) is mixed with the solution to
prevent uncontrolled particle growth.
2. Experimental section
The synthesis of nanoparticles is performed in a glass
cell, schematically depicted in Figure 1. The atmospheric
pressure microplasma is initiated at the surface of the
solution and supplied with an argon flow of 25 sccm set
by a volumetric flowmeter. A platinum foil is immersed
in the solution and served as the anode. The microplasma
is sustained by a DC power supply and the discharge
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current is controlled by different high-power resistors in
series between the capillary and the power supply. The
anodic and cathodic sides are separated by a fritted glass
plug in order to study the salt reduction occurring at the
plasma-liquid interface.
Figure 1: Schematic of electrochemical cell with
microplasma as gaseous cathode
3. Results
The solutions containing the nanoparticles formed after
plasma operation are characterized by several techniques.
X-ray photoelectron spectroscopy (XPS) on films
deposited on Si substrates confirms reduction of the metal
salts to zero valent metal. Ultraviolet-visible (UV-vis)
absorbance spectroscopy shows the presence of intense
plasmon bands, characteristic of spherical Ag and Au
nanoparticles at 415 and 530 nm, respectively. The
morphology and the size of as-grown colloidal metal
nanoparticles are evaluated by transmission electron
microscopy (TEM). Ag and Au NPs are nonagglomerated and spherical as seen in Figure 2. The
average diameter is estimated on more than 100 particles
and is about 10 nm in these conditions.
1
(a)
(b)
50 nm
Faraday’s Law in then used to determine the theoretical
final Ag+ concentration if 100 % of the injected electrons
go toward silver reduction.
50 nm
Figure 2: TEM images of (a) Ag NPs synthesized
from 1 mM AgNO 3 solution and (b) Au NPs
synthesized from 0.2 mM HAuCl 4 solution
The effect of the charge on the reduction process is then
analyzed by increasing the process time and the discharge
current. Figure 3 shows the linear increase in absorbance
with the amount of injected charge in the system for the
silver reduction. Time and current appear to increase
similarly the intensity of the plasmon band and therefore
the nanoparticles synthesis rate. This outcome could be
explained by the definition of the charge that depends on
those two factors:
Eq. 1
Q = i t = neF
where Q is the charge, i the discharge current, t the
process time, n e the number of injected electrons and F
the Faraday constant.
Figure 3: Maximum of absorbance of AgNO 3
solution processed at different charges by varying
operation time and discharge current
The increase in absorbance can evidence an
enhancement in nanoparticles density but the intensity of
the plasmon band can also be modified by nanoparticles
characteristics such as sphericity and aggregation. At
longer plasma time, a slight decrease in absorbance is
detected due to the appearance of some aggregates and
some bigger particles with new and different geometric
shapes (i.e. hexagonal or trigonal).
In order to get more information on the nanoparticles
synthesis rate, the concentration in silver ions after
different plasma conditions is determined by measuring
the potential of the solution and using the Nernst
equation:
𝑅𝑅
1
E = E° - ( ) ln
.
Eq. 2
𝐹
2
y = - 0.26 x + 1.03
y = - 0.52 x + 1.00
Figure 4: Decrease in Ag+ concentration after plasma
treatments at different process times and discharge
currents
The remaining Ag+ concentration decreases linearly
with the charge injected in the system showing that the
silver reduction is directly connected to the number of
electrons coming from the discharge. However, one could
notice that the slope is twice lower than the one
corresponding to the yield of 100%. It seems that only
one electron out of two go toward the reduction of silver.
This could be due to the loss of electrons or the use of
electrons to other reactions such as the hydrogen peroxide
formation.
Similarly, for the formation of gold nanoparticles, the
absorption resonant peak increases in intensity as the
amount of nanoparticles rises in the solution. Gold
reduction seems to be also dependent on the amount of
electrons injected in the solution but the consumption of
the gold complex is 4.5 times lower than in the case of
silver as seen in Figure 5. The final concentration of the
gold complex has been quantified from its absorbance at
220 nm. The different rates between the two reactions can
be partly explained by the stoichiometry, as three
electrons are needed to reduce the gold complex and only
one for silver. However it also seems to highlight
differences in kinetics and reduction mechanisms for the
reduction of the two studied metal.
[𝐴𝐴+]
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by electrons. But on the other hand, the formation of
H 2 O 2 is required in the reduction mechanism for the gold
ions.
4. Acknowledgements
This work is supported by PSI-IAP 7 (plasma surface
interactions) from the Belgian Federal Government
BELSPO agency. I would like to thank the Federation
Wallonia-Brussels for the Travel CFWB grant and the
Université Libre de Bruxelles for the BRIC grant.
y = - 0.17 x + 1.00
Figure 5: Decrease in [AuCl 4 ]- concentration after
plasma treatments at different discharge currents
Indeed a main difference between the mechanisms with
gold and the silver is observed. The behavior of the
absorption resonant band, which continues to rise after the
plasma treatment in the case of AuCl 4 - reduction,
suggests that either Au NPs aggregate or either AuCl 4 - is
still reduced after the plasma process. This increase is
correlated to a decrease in the concentration of the gold
complex suggesting that the reduction still occurs after the
plasma is turned off.
5. References
[1] D. Mariotti, Appl. Phys. Lett. 92 (2008), 151505
[2] D. Mariotti, R. M. Sankaran, J. of Phys. D: Appl.
Phys. 44 (2011), 174023
[3] M. Guzman, J. Dille, S. Godet, Nanomedicine 8
(2012), 37
[4] M-C. Daniel, D. Astruc, Chem. Rev. 104 (2004), 293
[5] M. Haruta, Nature 437 (2005), 1098
[6] C. Richmonds, R. M. Sankaran, Appl. Phys. Lett. 93
(2008), 131501
[7] D. Mariotti, J. Patel, V. Švrček, P. Maguire, Plasma
Process. Polym. 9 (2012) 1074
[8] B. Locke, K. Shih, Plasma Sources Science and
Technology 20 (2011) 034006
Figure 6: Decrease in gold complex concentration
after the plasma treatment
Electrons can drive to the direct reduction of metallic
salts but can also induce the formation of other species
such as the formation of the OH radical [8]:
Eq. 3
e- + H 2 O → H− + OH
OH radicals can then react to produce hydrogen peroxide,
known to reduce the gold complex.
To validate the hydrogen peroxide reduction, the
synthesis has been performed in ethanol. Ethanol is
known to consume OH radicals, the precursor of
hydrogen peroxide and thus preventing its formation, the
consequence was that only silver nanoparticles could be
synthesized in presence of ethanol, no gold nanoparticles
were produced. It confirms that the reduction mechanism
is different according to the nature of the particles formed.
On the one hand, silver ions seem to be directly reduced
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