19 international congress on acoustics madrid, 2

19th INTERNATIONAL CONGRESS ON ACOUSTICS
MADRID, 2-7 SEPTEMBER 2007
SONOELECTROCHEMICAL PRODUCTION OF GOLD AND SILVER
NANOPARTICLES
PACS: 43.35.Vz
1
2
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1
1
Dabalà Manuele , Cojocaru Paula , Vicenzo Antonello , Zin Valentina , Brunelli Katya ,
2,
Cavallotti Pietro Luigi
1
Università di Padova, Dipartimento di Innovazione Meccanica e Gestionale, via Marzolo, 9,
35131 Padova, Italy; [email protected]
2
Politecnico di Milano, Dipartimento di Chimica, Materiali e Ingegneria Chimica “G.Natta”, Via
Mancinelli, 7, 20131 Milano, Italy
ABSTRACT
In this work the development and the characterization of a sonoelectrochemical process for the
synthesis of gold and silver nanoparticles from sulfite based electrolytes was studied. The
solution chemistry was chosen in alternative to electrolyte composition of chemical deposition
processes and selected because of its low environmental impact and chemical stability.
Electrochemical characterization of metal particles deposition was performed by cyclic
voltammetry; two aspects of the process were studied: (i) potential region of the deposition
process at changing metal ions concentration; (ii) effects of the supporting electrolyte.
Pulsed current electrodepositions were realized on titanium plates in order to optimize the
sono-electrochemical parameters and to obtain small and numerous metal nuclei. It seemed
that silver makes it possible to produce slightly smaller nuclei than gold.
After these preliminary studies gold and silver nanoparticles were synthesized with the pulsed
sonoelectrochemical technique, which couples electrodeposition of metals with the employment
of high power ultrasound (20 kHz).
Produced nanoparticles were analysed both morphologically and dimensionally by SEM, TEM
and light scattering. Particles appeared agglomerated in clusters and formed three-dimensional
structures constituted by units with mean size of ˜ 20 nm. Silver particles were quite smaller than
gold ones as observed in the single current pulse tests.
INTRODUCTION
In the last decade, the production of metallic nanoparticles has been extensively investigated as
they offer high surface-to-volume ratios and may be employed in various areas [1-3]. Methods
of producing metallic nanosized materials are numerous but an alternative, simplest and costeffective method is to use Sonoelectrochemistry where an ultrasonic horn is also used as the
working electrode [4-6].
Gold and silver nanoparticles apply to a wide range of likely applications, e. g. fuel cells
catalysts, drug delivery, bioanalysis, biological electron microscopy, enzyme electrodes,
chemical sensors, Surfaced-enhanced Raman spectroscopy (SERS) and nanoscopic
electrodes.Using nanoparticles in such applications offers several potential advantages. The
most interesting aspect of using nanoparticles is that properties of a material change as the
particle size approaches molecular dimensions and it is a very interesting property of the
nanomaterial that make it useful for a particular application In this work the development and
the characterization of a sonoelectrochemical process for the synthesis of gold and silver
nanoparticles from sulfite based electrolytes was investigated.
EXPERIMENTAL
The electrochemical deposition of silver nanoparticles was carried out from an electrolyte of the
–3
following base composition: AgNO3 in the range from 10 to 0.1 M; NaNO3 in the range from
0.05 to 0.5 M; Na2SO3 0.25 M, at pH 8.60. By slowly adding dilute silver nitrate solution to a
stirred solution of sodium sulfite and sodium nitrate, precipitation of Ag2S was avoided. The
electrochemical deposition of gold nanoparticles was carried out from an electrolyte with the
following base composition: (NH4)3Au(SO3)2 and ethylendiamine 0.01 M; Na2SO3 0.12 M at pH
6.5. Electrochemical experiments were carried out in a conventional three electrode cell using
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as working electrode either an amorphous carbon disc, apparent surface area 0.2826 cm , or a
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titanium sheet, on which an apparent surface area of 1 cm was defined by an insulating tape. A
saturated calomel electrode (SCE) served as a reference electrode. The counter electrode was
a platinum wire with a much larger surface area than the working electrode. The
electrochemical characterization of metal particles deposition was carried out by cyclic
voltammetry using a model 273A EG&G PAR potentiostat. The scan rate during voltammetric
–1
cycling was changed in the range from 50 to 300 mV s .
The system used for the production of nanoparticles consisted of a titanium alloy horn acting
both as the cathode and the ultrasounic emitter [6], described therein as the sonoelectrode,
linked to a AMEL 7060 potentiostat and a 20kHz ultrasonic generator as shown in Figure 1.
Figure 1. - Schematic of the sonoelectrochemical deposition setup
The chemical composition of the solution used to produce gold and silver nanoparticles were
summarized in the table 1.
Au
Ag
(NH4)3Au(SO3)2
10 mM
AgNO3
20 mM
Ethylendiamine
10 mM
NaNO3
0.1 M
Na2SO3
0.12 M
Na2SO3
0.25 M
T = 328°K
T = 298°K
pH = 6.5
pH = 8.5
Table 1. – Chemical composition of solutions used for nanoparticles production
RESULTS AND DISCUSSION
The electrochemical characterization was made by cyclic voltammetry in order to define the
potential region of the deposition process both at changing ions concentration and supporting
electrolyte concentration. ¡Error! No se encuentra el origen de la referencia. shows typical
voltammetric curves at the glassy carbon electrode changing Ag(I) concentration in solution
from 1 to 30 mM.
Figure 2. Cyclic voltammograms from 0.7 to –0.7 V at glassy carbon electrode in electrolyte with
–2
changing Ag(I) concentration, Na2SO3 0.25 M, NaNO3 0.1 M at pH 8.60; Ag(I): (a) 3 x 10 M, (b) 2 x
–2
–2
–3
–3
10 M , (c) 1 x 10 M, (d) 5 x 10 M, (e) 1 x 10 M.
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3–
The peak potential for the cathodic reduction of the silver-sulfite complex [Ag(SO3)2] shifts
towards more positive value as the Ag(I) concentration increases, indicating the lowering of the
nucleation overpotential as the silver concentration is increased. The formal potential of the
complex may be estimated as about 0.02 V SCE by using the reported value of the stability
constant (log K ˜ 9.0 (i)).This is qualitatively confirmed by the voltammetric analysis, observing
that, as the Ag(I) concentration increases, the potential value midway between the anodic and
cathodic peaks tends to a similar value. The peak parameters, current density and potential,
change significantly as the Ag(I) concentration increases: the potential of both peaks shift toward
more positive values, as expected; besides, both peaks change in shape, showing broadening,
and the crossing of the forward and reverse scan becomes more and more evident, as a result of
increased nucleation intensity. In order to investigated effect of a supporting electrolyte on the
electrodeposition sodium nitrate NaNO3 was added in increasing concentration to the silversulfite electrolyte and a cyclic voltammetry characterization (Figure ) at the glassy carbon
electrode was performed. The current density and the potential of voltammetric peaks changed
significantly as the sodium nitrate concentration was varied in the deposition bath.
Figure 3. Cyclic voltammograms from 0.7 to –0.7 V at glassy carbon electrode in electrolyte with
–1
changing NaNO3 concentration, Na2SO3 0.25 M, Ag(I) 20 mM at pH 8.60; NaNO3 conc. : (a) 4 x 10 M,
–1
–1
–2
(b) 3 x 10 M , (c) 2 x 10 M, (d) 5 x 10–2 M. Scale bar 1 mA cm .
Gold deposition from the (NH4)3Au(SO3)2 10 mM, Na2SO3 0.1 M and C2H8N2 10 mM, solution at
pH 6.5, shows the characteristics of a highly irreversible process. In ¡Error! No se encuentra el
–1
origen de la referencia. cyclic voltammograms at the glassy carbon electrode at 50 mV s
scan rate are reported. From the first scan it may inferred that the discharge of the Au(I)
complex at the glassy carbon electrode is highly inhibited since no distinct feature, that could be
related to gold deposition, is clearly seen but a shallow current wave starting from about –300
V. This is soon superseded by a second current wave starting at about –500 V. The later can be
attributed to the reduction of sulphite to dithionite ion, in agreement with early reports on the
polarization behaviour of Au(I) sulfite solution [7]. Upon cycling, a steady voltammogram is
obtained where a distinct current peak for the discharge of the Au(I) sulfite complex appears at
about –300 V. This is followed by the reduction of sulfite to dithionite ion at the gold plated
surface at higher current density compared to the glassy carbon electrode.
Figure 4. SEM micrographs of gold particles on titanium deposited from (NH4)3Au(SO3)2 10 mM,
Na2SO3 0.1 M and C2H8N2 10 mM, at pH 6.5, at changing potential, as indicated (vs. SCE) and 50 ms
pulse duration. Scale bar is 1 µm.
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The voltammetric behaviour of the gold deposition solution strongly suggest that particles
formation may be controlled by a mechanism of progressive nucleation, as a result of kinetic
control on the Au(I) sulfite complex discharge. However, the evidence from SEM observation of
gold particles grown on titanium is not in line with this observation. The nucleation density
appears to be extremely sensitive to the deposition potential, possibly in connection with an
inhibition effect of the side reaction of sulfite reduction. In fact, at –200 mV potential pulse, a
uniform distribution of regularly sized gold particles is obtained, while at potential more negative
than –400 mV a strong reduction in particles density is observed. This behavior may be
explained taking into account the influence of surface inhibition related to the electroreduction of
sulfite, which could result in a twofold effect: inhibition of growth of already formed gold particles
and nucleation of new particle on the titanium surface as the deposition potential increases.
Besides, it is also worth noting that particles size obtained by potential sweep is sensibly larger
than obtained by potential pulse deposition.
Pulsed current electrodepositions were carried out on titanium plates; a single current pulse was
sent to each plate and thus a study on nucleation was performed.
The aim of these test was to optimize parameters like current density, i, and duration of the
pulses, t, to obtain nuclei the smallest and most numerous as possible.
SEM QBSD images are shown below:
Figure 5. Gold nuclei electrodeposited on a Titanium plate with a single
current pulse.
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i = 50 mA/cm , t = 200 ms
Figure 6. Silver nuclei electrodeposited on a Titanium plate with a single
current pulse.
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i = 50 mA/cm , t = 200 ms
From SEM images and following image analysis it results that a single current pulse enable the
electrodeposition of the metal; both gold and silver nuclei have average size smaller than 150
nm; gold nuclei are smaller than 100 nm for the 50.7%, while silver nuclei for the 67.4%.
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It seems that silver makes it possible to produce slightly smaller nanoparticles than gold, and in
general we can state that short current pulses (200 ms) allow producing nanoparticles.
SEM images worked out with “Image Pro Plus” Software are shown in figure 7. The software
allows to extract informations from SEM images about the distribution of nuclei’s size; it’s better
work with very short current pulses in order to minimize enhancement and coalescence of the
nuclei on the titanium plate.
Gold nuclei
Silver nuclei
40%
40%
35%
35%
30,7%
30%
23,9%
25%
21,8%
frequency
frequency
30%
19,5%
20%
14,9%
15%
25%
20%
18,7%
18,0%
17,0%
15%
10%
7,3%
2,8%
0,0%
1,4%
1,1%
0,7%
4,7%
5%
1,0%
0,0%
0%
< 25
8,7%
10%
6,7%
5%
25-50
50-75
75-100 100-125 125-150 150-175 175-200 200-225 225-250
> 250
diameter (nm)
0,7%
0,0%
0,5%
0%
< 25
25-50
50-75
75-100 100-125 125-150 150-175 175-200 200-225 225-250
> 250
diameter (nm)
(a)
(b)
Figure 7. Bar graphs about size distribution of (a)gold and (b)silver nuclei
on a titanium plate after a single current pulse
After this study gold and silver nanoparticles were produced with the pulsed current
sonoelectrochemical technique.
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Nanopowders were synthesized with current density of 50 mA/cm and the following pulses
management: tON = 0.2s, tUS = 0.3s, tp = 0.2s. The duration of the whole process was 30’.
SEM and TEM images about morphological and dimensional features of gold nanoparticles are
shown:
Figure 8. SEM images of silver (left) and gold (right) nanoparticles
Figure 9. TEM images of silver (left) and gold (right) nanoparticles
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Morphological characterization performed by TEM and SEM showed that nanoparticles were
strongly aggregated. TEM images in Fig. 9 showed the smallest particles synthesized with the
method, which had a minimum size of about 10 nm. However particles presented a wide size
distribution and it was quite difficult to determine their average dimension.
Besides they were agglomerate in clusters and formed three-dimensional structures with mean
size of ca. 400 nm with a rather round shape and which in turn aggregate and build complex
structures, as shown in SEM images (Fig. 8).
It seems to be impossible to separate each particle from the others; this phenomenon is
probably due to the physical nature of nanopowders, which have high surface energy and tend
to aggregate to minimize system energy.
A solution containing nanopowders was analysed with light scattering to measure the average
diameter of nanoparticles.
Figure 10. Light scattering pattern of gold nanoparticles
This technique made it possible to analyze particles in solution without filtering it; the result
support the first interpretation given to the presence of relatively big clusters in SEM images;
there is a peak of intensity positioned at 400 nm and it is due to the formation of aggregates in
the solution, after the formation of single nanoparticles.
CONCLUSIONS
Silver and gold particles with size spanning from the nanoscale to the mesoscale were
deposited from sulfite based solutions. The proposed solution chemistry is reasonably stable
and is shown to be a viable process for the electrochemical deposition of both Ag and Au
particles. The voltammetric behavior of the electrolytes is studied at glassy carbon and titanium
surface, showing that kinetic control plays a role in the discharge of the sulfite complex of either
Ag(I) or Au(I).
Pulsed current electrodepositions were carried out on titanium plates to perform a study on
nucleation. Gold and silver nuclei electrodeposited showed size distribution and density
dependent on parameters like current density and duration of the pulses.
The sonoelectrochemical method used to produce nanoparticles gave interesting results: gold
and silver nanopowders appeared strongly aggregated in three-dimensional clusters and the
smallest ones showed a minimum size of about 10 nm. In particular gold nanoparticles were
quite spherical but seem to be bigger than silver ones.
References
[1] K. Iwasaki, T. Itoh, T. Yamamura, Materials Transactions 46(6) (2005) 1368-1377.
[2] S. Cattarin, M. Musiani, Electrochimica Acta 52(3) (2006) 1339-1348
[3] L.P. Balogh, S.S. Nigavekar, A.C. Cook, L. Minc, M.K. Khan, PharmaChem 2(4) (2003) 94-99
[4] J.L. Delplancke, J. Dille, J. Reisse, G.J. Long, A. Mohan, F. Grandjean, Chem. Mater 12 (2000) 946955
[5] V. Mancier, et al., J. Magn. Magn. Mater 281 (2004) 27-35
[6] J. Reisse, et al., Ultrason. Sonochem 3 (1996) S147-S151
[7] J.-P. Derivaz, A. Resin , S. Losi, Surf. Technol. 5 (1977) 369
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