22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Tuning of the size and shape of metal nanoparticles synthesized by atmosphericpressure micro-plasma
J. Baneton1, C. De Vos1, J. Dille2, S. Godet2 and F. Reniers1
1
CHANI, Université Libre de Bruxelles, Brussels, Belgium
4MAT, Université Libre de Bruxelles, Brussels, Belgium
2
Abstract: Gold and silver cations have been reduced into solid nanoparticles in aqueous
medium using an atmospheric micro-plasma source. Concentration of the metallic salt and
nature of the surfactant are two essential factors to obtain particles with a controlled size, a
well-defined shape and to prevent agglomeration even after several weeks.
Keywords: atmospheric plasma, micro-plasma, nanoparticles synthesis
1. Introduction
Due to their particular properties, nanotechnologies are
used in many fields of application including medical,
energetic and material modification domains. In
particular, silver and gold nanoparticles present a
significant interest for detection and treatment of diseases:
silver has a remarkable antimicrobial activity [1] and gold
can be bound with many kinds of biomolecules and with
thiols as stabilizers to be sent in the bloodstream of the
human body [2]. Many techniques such as Turkevich
method, sonochemistry, micro-emulsions or Ultravioletvisible (UV-vis) irradiation are developed to produce
particles and control their size, shape and aggregation.
2. Experimental
We propose to investigate a promising approach using an
atmospheric micro-scale plasma [3], generated between a
DC hollow capillary (0.6 mm inner diameter) and the
surface of an aqueous solution. A platinum foil is
immerged in the medium to form the mass electrode and
the distance between the nozzle’s head and the liquid
surface is about 1 mm. In this study, we focus on the
synthesis of gold and silver nanoparticles by reduction of
the metal cations from silver nitrate and tetrachloroauric
acid at different concentrations [4,5]. Trisodium citrate or
polyvinyl alcohol (PVA) is added to improve the stability
of the system (Fig.1).
Several analytical techniques are performed to
characterize the particles. Ultraviolet-visible absorbance
spectroscopy showed intense plasmon bands associated to
spherical silver and gold nanoparticles at 415 and 530 nm,
respectively. Variations of size, shape or agglomeration
can be directly observed and mathematically described by
the Mie theory [6]. The morphology and the size are
verified by transmission electron microscopy (TEM) and
the metallic nature is investigated by x-ray photoelectron
spectroscopy (XPS).
3. Results
The concentration of the metallic salt and the nature of the
surfactant are two essential factors to obtain nanoparticles
with a controlled size, a well-defined shape and to prevent
agglomeration.
Decreasing the concentration of metallic cations in the
aqueous solution leads to spherical and unaggregated
particles characterized by a small average size. For
example, a concentration of 0.2 mM of HAuCl 4 leads to a
large amount of Au particles of 8 ± 3 nm as observed on
TEM pictures (Fig. 2).
(a)
100 nm
(b)
Fig. 1. Representation of the experimental device with the
composition of the treated aqueous solution.
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50 nm
Fig. 2. TEM images of (a) (b) Au nanoparticles synthesized from
HAuCl4 0.2 mM (PVA 1 %, i = 2 mA, t = 10 min, gap =1 mm,
flowAr = 25 mLmin-1).
1
(a)
0.7
0.6
Absorbance
On the other hand, an increasing concentration of the
precursor salt leads to a significant broadening of the size
distribution (Fig. 3 (a)). Furthermore, although the
majority of the particles remain spherical, specific new
shapes can appear such as triangular or hexagonal as
shown in Fig. 3 (b) (c) for a concentration of 10 mM of
AgNO 3 . This phenomenon could be an outcome of the
transition between thermodynamic and kinetic growth
control.
Au
0.5
0.4
0.3
0.1
(a)
Ag
0
350
3
400
450
500
λ / nm
550
600
650
10
20
30
40
50
60
(b)
2.5
Absorbance
Quantity / %
0.2
100
90
80
70
60
50
40
30
20
10
0
Size / nm
2
1.5
1
0.5
(b)
0
0
Number of days after plasma treatment
200 nm
(c)
200 nm
Fig 4. (a) UV-Visible spectra of AgNO3 1 mM (dots) and
HAuCl4 1 mM (line) solutions 3 days after addition of H2O2 30%
solution and (b) time evolution of absorbance at maximum
wavelength of HAuCl4 0,2 mM (line) and 1 mM (dots) solutions
(PVA 1 % , i = 2 mA, t = 10 min, gap =1 mm, flowAr = 25
mLmin-1).
We also studied the influence of the nature of the
surfactant. Two types of stabilization have been
highlighted: an electrostatic stabilization which concerns
interactions between similar charges and a steric
stabilization which uses the length of carbon chains to
avoid the contact between particles (Fig. 5).
Fig. 3. (a) Size distribution of Ag nanoparticles synthesized from
AgNO3 1 mM (black) and 10 mM (grey) solutions and TEM
images of (b) (c) Ag nanoparticles synthesized from AgNO3 10
mM (PVA 1 % , i = 2 mA, t = 10 min, gap =1 mm, flowAr = 25
mLmin-1).
The optimal conditions of concentration are different for
silver or gold nanoparticles and are dependent on the
reactional pathways. The formation of hydrogen peroxide
in an aqueous solution treated by atmospheric microplasma is well known [7] but it seems to reduce only the
gold precursor (Fig. 4 (a)). Indeed, a peak around 540 nm
appears when the HAuCl 4 and H 2 O 2 are mixed in
contrast to the mixture containing AgNO 3 where no peak
is visible around 410 nm. Therefore, to prevent
aggregation, the concentration of HAuCl 4 has to be
limited to 0,2 mM unlike AgNO 3 was maintained to 1
mM (Fig. 4 (b)).
2
Fig. 5. Electrostatic (a) and steric (b) stabilization of particles.
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In the case of Ag system, using an electrostatic agent as
trisodium citrate leads to a spectra with a characteristic
peak at 409 nm and an absorbance of 0,64. On the other
hand, using a steric agent as polyvinyl alcohol (PVA)
leads to a signal at 401 nm with an intensity of 1,23.
These spectroscopic differences imply that the PVA
system contains a larger amount of non-aggregated
particles with an average size of less than 10 nm (Fig. 6).
1.6
1.4
Absorbance
1.2
1
0.8
0.6
0.4
0.2
0
300
350
400
450
λ / nm
500
550
600
Fig 6. UV-Visible spectra of AgNO3 1 mM solutions with
trisodium citrate 1 % just after the plasma treatment (grey, line)
and 4 weeks later (grey, dots) and with PVA 1 % just after the
plasma treatment (black, line) and 4 weeks later (black, dots)
(i = 2 mA, t = 10 min, gap =1 mm, flowAr = 25 mLmin-1)
Moreover, after 4 weeks, the signal for silver
nanoparticles stabilized by trisodium citrate tends to
disappear whereas with PVA it slightly increases.
It is assumed that the charge transfers induced by the
trisodium citrate are responsible for the instability of the
system leading to the formation of large agglomerates. In
the case of PVA, the evolution of absorbance is directly
linked to the polymer flocculation phenomenon.
4. References
[1] V.K. Sharma, R.A. Yngard, Y. Lin. Adv. Colloid
Interface Sci., 145, 83-96 (2009)
[2] M.C. Daniel, D. Astruc, Chem.Rev., 104, 293-346
(2004)
[3] K.H. Becker, K.H. Schoenbach, J.G. Eden, J. Phys. D:
Appl. Phys., 39, R55-R70 (2006)
[4] W. Chiang, C. Richmonds, R.M. Sankaran, Plasma
Sources Sci. Technol., 19, 1-8 (2010)
[5] J. Patel, L. Nemcova, P. Maguire, W.G. Graham, D.
Mariotti, Nanotechnology, 24, 1-11 (2013)
[6] G. Mie, Ann. Phys., 3, 377-445 (1908)
[7] D. Mariotti, J. Patel, V. Svrcek, P. Maguire, Plasma
Process. Polym., 9, 1074-1085 (2012)
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