Electrochemical method for the synthesis of silver nanoparticles

Abstract

The article deals with a novel electrochemical method of preparing long-lived silver nanoparticles suspended in aqueous solution
as well as silver powders. The method does not involve the use of any chemical stabilising agents. The morphology of the silver
nanoparticles obtained was studied using transmission electron microscopy, scanning electron microscopy, atomic force microscopy
and dynamic light scattering measurements. Silver nanoparticles suspended in water solution that were produced by the present
technique are nearly spherical and their size distribution lies in the range of 2 to 20nm, the average size being about 7nm.
Silver nanoparticles synthesised by the proposed method were sufficiently stable for more than 7years even under ambient
conditions. Silver crystal growth on the surface of the cathode in the electrochemical process used was shown to result in
micron-sized structures consisting of agglomerated silver nanoparticles with the sizes below 40nm.

Full text

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RESEARCH PAPER
Electrochemical method for the synthesis of silver
nanoparticles
Rashid A. Khaydarov Æ Æ Renat R. Khaydarov Æ Æ
Olga Gapurova Æ Æ Yuri Estrin Æ Æ Thomas Scheper
Received: 31 March 2008/Accepted: 3 September 2008/Published online: 21 September 2008
? Springer Science+Business Media B.V. 2008
Abstract
chemical method of preparing long-lived silver
nanoparticles suspended in aqueous solution as well
as silver powders. The method does not involve the
use of any chemical stabilising agents. The morpho-
logy of the silver nanoparticles obtained was studied
using transmission electron microscopy, scanning
electron microscopy, atomic force microscopy and
dynamic light scattering measurements. Silver nano-
particles suspended in water solution that were
produced by the present technique are nearly spher-
ical and their size distribution lies in the range of 2 to
20 nm, the average size being about 7 nm. Silver
nanoparticles synthesised by the proposed method
were sufficiently stable for more than 7 years even
The article deals with a novel electro-
under ambient conditions. Silver crystal growth on
the surface of the cathode in the electrochemical
process used was shown to result in micron-sized
structures consisting of agglomerated silver nanopar-
ticles with the sizes below 40 nm.
Keywords
Electrochemical synthesis ? Colloids
Silver ? Nanoparticle ? PVP ?
Introduction
The last decade has seen the development of a hoist
of different methods for the synthesis and charac-
terizationofmetalnanoparticles
thereof (Feldheim and Foss 2002). Owing to their
properties being distinctly different from those of
the bulk metal, nanoparticles are finding their way
into various areas of science and technology
(Mazzola 2003; Anselmann 2001; Bo ¨nnemann and
Richards 2001; Biswas and Wu 2005). In particular,
nanoparticles of silver have attracted much attention
because of a broad range of their possible applica-
tions including medicine (Salata 2004), catalysis
(Lewis 1993), textile engineering (Lee and Jeong
2005), biotechnology and bioengineering (Niemeyer
2001), water treatment (Solov’ev et al. 2007), elec-
tronics (Li et al. 2005) and optics (Murphy et al.
2005). Many approaches were developed to obtain
silver nanoparticles of various shapes and sizes,
andpowders
R. A. Khaydarov ? R. R. Khaydarov (&) ? O. Gapurova
Institute of Nuclear Physics, Uzbekistan Academy
of Sciences, Tashkent, Uzbekistan
e-mail: renat2@gmail.com
Y. Estrin
ARC Centre of Excellence for Design in Light Metals,
Department of Materials Engineering, Monash University
and CSIRO Division of Materials Science and
Engineering, Clayton, VIC, Australia
e-mail: yuri.estrin@eng.monash.edu.au
T. Scheper
Institute of Technical Chemistry, Leibniz University,
Hannover, Germany
e-mail: scheper@itc.uni-hannover.de
123
J Nanopart Res (2009) 11:1193–1200
DOI 10.1007/s11051-008-9513-x

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including laser ablation (Lee et al. 2001), gamma
irradiation (Long et al. 2007), electron irradiation
(Bogle et al. 2006), chemical reduction by inorganic
and organic reducing agents (Bo ¨nnemann and
Richards 2001), photochemical method (Mallick
et al. 2004), microwave processing (Yin et al.
2004), and thermal decomposition of silver oxalate
in water and in ethylene glycol (Navaladian et al.
2007). Reetz and Helbig (1994) were the first to
describe in detail an electrochemical technique for
the synthesis of nanoparticles, in which a metal
sheet was anodically dissolved and the intermediate
metal salt formed was reduced at the cathode, giving
rise to metallic particles stabilised by tetraalkylam-
monium salts. This work was successfully adopted
for the electrochemical synthesis of the silver
nanoparticles in acetonitrile containing tetrabutyl-
ammoniumsaltsbyRodrı ´guez-Sa ´nchez
(2000). Using a similar approach, silver nanoparti-
cles were obtained by potentiostatic or galvanostatic
polarisationofsilver in
Starowicz et al. 2006). In the work of Yin et al.
(2003),Poly(N-vinyl-2-pyrrolidone)
essential in the process of electrochemical synthesis
of silver particles. The main advantages of the
electrochemical methods lie in the high purity of
particles and the possibility of the nanoparticle size
control by adjusting the current density without a
need for expensive equipment or vacuum. The key
to the success of electrochemical methods is the
right choice of the chemical agents and the process
conditions. However, as pointed out earlier (Rodrı ´guez-
Sa ´nchez et al. 2000), the method has its limitations,
as the deposition of silver on the cathode during the
electrochemical process diminishes the effective
surface available for particle production. As the
entire cathode surface gets covered with the silver
electrodeposits, the particle production comes to a
halt altogether.
An important issue in the current search for
simpler and cost-effective methods for the synthesis
of nanoparticles is finding ways to avoid the use of
stabilising agents in the electrochemical processes
involved. In this article, we present a novel electro-
chemical method for preparing long-lived silver
nanoparticles suspended in aqueous solutions as
well as silver powders deposited on the electrodes,
which does not involve any chemical stabilising
agents.
et al.
ethanol solutionby
(PVP)was
Materials and methods
The proposed process for obtaining silver nanoparti-
cles is based on the use of an inexpensive two-
electrode setup in which the anode and the cathode
are made from the bulk Ag metal to be transformed
into Ag colloidal particles. We employed two
polished silver plates (85 mm 9 20 mm 9 4 mm)
as the anode and the cathode, being vertically placed
face-to-face 10 mm apart. The electrodes were
immersed in an electrochemical cell filled with
500 mL of distilled water obtained from an ordinary
commercially available
Russia). Electrolysis was performed in the tempera-
ture range 20–95 ?C at a constant voltage of 20 V.
Additional technological keys to the electrochemical
synthesis of silver nanoparticles lie in changing the
polarity of the direct current between the electrodes
every 30–300 s, and intensive stirring during the
process of electrolysis to inhibit the formation of
precipitates. The silver nanoparticle solutions pro-
duced in this way were stored under ambient
conditions in glass containers.
The concentration of the silver nanoparticles in
solutions was determined by neutron activation
analysis (Soete et al. 1972). The samples were
irradiated in the nuclear reactor of the Institute of
Nuclear Physics (Tashkent, Uzbekistan). The product
of the nuclear reaction109Ag(n,c)110mAg has the half-
life T1/2= 253 days. The silver concentration was
determined from the measurements of the intensity of
gamma radiation with the energy of 0.657 MeV and
0.884 MeV emitted by
with a resolution of about 1.9 keV at 1.33 MeV and a
6144-channel analyser were used for recording
gamma-ray quanta.
Dynamiclight scattering
ments were carried out at 25 ?C using a Zetasizer
(ZEN3600, Malvern Instruments, UK) to estimate the
size distribution of silver nanoparticles in solutions.
The morphology of silver powder deposited on the
surface of the cathode during the electrochemical
process was observed by field emission scanning
electron microscopy (FE-SEM; JSM-6700F, JEOL,
Japan). The size and shape of the nanoparticles in
solution were determined by transmission electron
microscopy (TEM) (LEO-912-OMEGA, Carl Zeiss,
Germany). Energy dispersive X-ray spectrometer
(EDS) attached to the TEM was used to determine
waterdistiller(DE-25,
110mAg. A Ge(Li) detector
(DLS)measure-
1194 J Nanopart Res (2009) 11:1193–1200
123

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the chemical composition of the samples. The size
values were averaged over more than 150 nanopar-
ticles from different TEM micrographs of the same
sample.
The silver nanoparticles suspended in aqueous
solutions were also imaged by atomic force micros-
copy (AFM) (Solver P47Bio, NT-MDT Co., Russia)
in contact mode using freshly cleaved mica surfaces
as substrates.
Results and discussion
As distinct from most non-electrochemical methods
(Lee et al. 2001; Long et al. 2007; Bogle et al. 2006;
Mallick et al. 2004; Yin et al. 2004; Navaladian et al.
2007), the proposed approach of electrochemical
synthesis does not require the use of vacuum
chambers, sophisticated preparation or expensive
equipment. The process of synthesis proposed here
involves three major stages, which are described
below.
Stage I: formation of a colloidal solution of silver
nanoparticles
This stage comprises the following steps (Fig. 1):
1.Oxidative dissolution of the sacrificial Ag anode:
Ag0? e?! Agþ1
2. Release of the oxygen gas:
2H2O ? 4e?! O2" þ4Hþ
due to the electrolysis of water, with simultaneous
Ag2O film deposition on the surface of the anode;
3.
4.
Ag?ions migration to the cathode;
Reductive formation of zero-valent Ag atoms on
the cathode:
Agþ1þ e?! Ag0
and release of the hydrogen gas:
2H2O þ 2e?! H2" þ2OH
5.Formation of silver nanoparticles via nucleation
and growth due to attractive van der Waals forces
between Ag atoms;
Separation of synthesised silver nanoparticles
from the cathode and their further migration
caused by vigorous stirring of the solution.
6.
Upon application of a voltage, the reactions
described above set in and the electrical current
continuously increased until it reached a saturation
level. The warmer was the water, the faster was the
reaction and the higher was the value of the saturation
current (cf Fig. 2).
In the described first stage of the electrochemical
synthesis, the polarity of the direct current between
the electrodes was periodically changed, which
significantly reduced the electrodeposition of silver
on the cathode. As soon as the polarity changes,
Ag2O on the surface of the former cathode starts
reducing in a reaction with hydrogen
Fig. 1 Electrochemical formation of silver nanoparticles in
distilled water
0
10
20
30
40
50
60
70
80
02040 60 80100 120
t, min
I, mA
353 K
338 K
328 K319 K
313 K
Fig. 2 Current versus process time for different temperatures
J Nanopart Res (2009) 11:1193–1200 1195
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Ag2O þ H2! 2Ag þ H2O
at temperatures above 50 ?C. We varied the period of
the polarity changes in the range of 30 to 300 s. It
was found empirically that 4 min was an optimum
period. At smaller values, agglomeration of particles
in the solution was enhanced at the cost of silver film
deposition at the cathode. For values of the period
exceeding 4 min, a gradual decrease of the effective
surface for the particle production occurred, owing to
a significant growth of silver electrodeposition on the
cathode. By periodically changing the polarity every
4 min, the film deposition at the electrodes was
reduced as compared to known electrochemical
methods (Reetz and Helbig 1994; Rodrı ´guez-Sa ´nchez
et al. 2000; Starowicz et al. 2006), and the produc-
tion of nanoparticles was enhanced resulting in larger
concentrations of silver colloids.
The solutions were tested for sizes of suspended
silver nanoparticles as soon as current saturation was
reached. TEM and DLS studies demonstrated that the
average size of silver nanoparticles in the colloidal
solution decreases with increase in the current density,
aswasshownin(ReetzandHelbig1994)theoretically.
The rate of the reaction was shown to increase with
decrease in the distance between the electrodes and
increaseinthevoltage.Alongerreactiontime foreach
individual trial resulted in a larger average size and a
higher concentration of silver nanoparticles. In order
to obtain stable silver nanoparticles with a concentra-
tion in the range of 20 to 40 mg/L, it was necessary to
ensureareactiontimeof50–70 mininthetemperature
range of 50 to 80 ?C.
Stage II: filtration
As was shown by DLS and TEM measurements, a
typical sample of silver nanoparticles solution con-
tains a small amount of large Ag colloids after the
first stage as well (see Fig. 3). In order to remove
these larger colloids and to provide reduction of the
silver ions present in the solution, we propose to
implement a filtration stage. Passing the colloidal
solution through a paper filter, narrows the range of
size distributions of the synthesised silver nanopar-
ticles (see Fig. 4) and also provides additional
reduction of Ag ions:
Agþ1þ e?! Ag0
As a result, the ratio c(Ag?)/c(Ag) decreases, where
c(Ag?) is the concentration of silver ions and c(Ag) is
the concentration of silver nanoparticles suspended in
the solution, respectively.
Fig. 3 Typical DLS-images of size distribution by intensity (left) and number (right) of silver nanoparticles obtained after Stage I
Fig. 4 Typical TEM image
and size distribution of
silver nanoparticles
obtained after Stage II
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Stage III: additional treatment
A third stage of Ag nanoparticle synthesis involves
the additional treatment of the smallest-size fraction
of silver nanoparticles remaining in solution after the
filtering stage. It consists in adding hydrogen peroxide
to a level of up to 0.005% concentration of H2O2to
the solution. Due to the reaction
Ag2O þ H2O2! 2Ag þ H2O þ O2;
silver oxide is reduced to Ag which is released in the
solution. Due to this process, the size of the silver
nanoparticles is reduced while new Ag nanoparticles
may be forming as well. The details of this process,
whose outcome consists in a significant refinement of
the nanoparticle size, are not fully understood as yet.
Examination of TEM images taken 2 weeks after the
addition of H2O2revealed that silver nanoparticles
suspended in water solution were nearly spherical and
that their size distribution fell in the range of 2 to
20 nm, the average size being about 7 nm (cf. Fig. 5).
EDS analysis of silver nanoparticles (cf. Fig. 6)
has shown a presence of Ag2O in the solution after
Stage II and an efficiency of reduction of silver
oxides in the solution during Stage III.
In our studies, as a variant of the processing, PVP
was also used as a protective medium for the silver
colloids. PVP is known to affect the molecular
motion of reduced silver, inhibiting the aggregation
of nanoparticles (Yin et al. 2004). Sustainability of
the solution of nanoparticles after Stage II (‘Sample
A’) and Stage III (‘Sample B’) of the proposed
synthesis was monitored over 300 days and com-
pared with that of the samples containing PVP
(‘Sample A ? PVP’ and ‘Sample B ? PVP’). PVP
was added to the samples in such a way as to produce
a mass relation of Ag colloids and PVP of 1:20. As
one can see from Table 1, the nanoparticles prepared
can be stored for months under ambient conditions
with only little coarsening; small additions of PVP to
electrochemically synthesised silver nanoparticle
solutions act to enhance their stability.
DLS and TEM measurements showed that an
increase in the PVP concentration enhances the
stability of colloids. On the other hand, it leads to
more viscous colloidal solution potentially compli-
cating the process of impregnating different materials
with silver nanoparticles. This effect may be relevant
for paints modified with silver nanoparticles, textile
fabrics (Jeong et al. 2005; Lee and Jeong 2004) etc.,
Fig. 5 Typical TEM image
and size distribution of
silver nanoparticles
obtained after Stage III
Fig. 6 EDS analysis after
stages II (left) and III
(right). Carbon and copper
peaks are from the carbon-
coated copper sample grid
J Nanopart Res (2009) 11:1193–12001197
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