Silver nanostructure dependence on the stirring-time in

Materials Letters 138 (2015) 167–170
Contents lists available at ScienceDirect
Materials Letters
journal homepage: www.elsevier.com/locate/matlet
Silver nanostructure dependence on the stirring-time in a high-yield
polyol synthesis using a short-chain PVP
A. Gómez-Acosta a, A. Manzano-Ramírez b,n, E.J. López-Naranjo b, L.M. Apatiga c,
R. Herrera-Basurto d, E.M. Rivera-Muñoz c
a
Facultad de Ingeniería, Universidad Autónoma de Querétaro, Querétaro C.P. 76010, Mexico
CINVESTAV—I.P.N. Unidad Querétaro, Querétaro C.P. 76230, Mexico
c
Centro de Física Aplicada y Tecnología Avanzada, UNAM, A.P. 1-1010 Querétaro, Mexico
d
Programa de Mediciones para Nanotecnologías, CENAM, Km. 4.5 Carr. a Los Cués, El Marqués Querétaro, Querétaro C.P. 76246, Mexico
b
art ic l e i nf o
a b s t r a c t
Article history:
Received 3 July 2014
Accepted 22 September 2014
Available online 2 October 2014
This paper describes the size and shape evolution of silver nanostructures synthesized by a typical-polyol
method that yields high nanowire content using a short-chain (MW¼40,000) polyvinyl pyrrolidone (PVP).
Scanning Electron Microscopy, Transmission Electron Microscopy, UV–vis spectroscopy and X-ray diffraction, were used to characterize the nanostructures synthetized in this work. It was found that the
morphology of silver nanostructures changes as a function of the stirring time of the silver precursor salt
(silver nitrate, AgNO3) before the addition of the capping agent (PVP). Results showed that after 60 min of
stirring, the reaction yields silver nanowires ( 99%).
& 2014 Elsevier B.V. All rights reserved.
Keywords:
Silver-nanostructures
Morphology-evolution
Stirring-time
PVP (40,000)
1. Introduction
Polyol synthesis originally developed by Xia’s group to produce
silver nanostructures with controllable shapes, i.e. rods, wires, and
spheres is currently the most commonly-used preparation method
for silver nanowires (AgNWs) [1]. It is known that by controlling
reaction parameters such as molar ratio between capping agent
and metallic precursor, temperature, reaction time and the order
of addition of reactants, a reasonable control on the size and shape
can be achieved. The size and shape control of silver nanostructures is of particular significance as it determines their appropriate
application area. Studies have suggested that the selective adsorption of polyvinyl pyrrolidone (PVP) could lead to different growth
rates along different crystal planes; as a result the growth of silver
nanostructures yields different shapes [2,3]. It is also known that
during nanowires synthesis, Ag nanoparticles start to form via
homogeneous nucleation. As the process takes place, some of the
Ag nanoparticles start to dissolve and grow as nanowires via the
mechanism known as Oswald ripening. PVP is believed to passivate (1 0 0) faces of these Ag nanoparticles and leave (1 1 1) planes
active for anisotropic growth [4].
Although many research groups have explored different
approaches in order to improve the polyol process, only few
n
Corresponding author at: Libramiento Norponiente No. 2000, Fracc. Real de
Juriquilla, C.P. 76230 Querétaro, Mexico. Tel./fax: þ 52 442 211 99 18.
E-mail address: amanzano@qro.cinvestav.mx (A. Manzano-Ramírez).
http://dx.doi.org/10.1016/j.matlet.2014.09.109
0167-577X/& 2014 Elsevier B.V. All rights reserved.
parameters (i.e. temperature, injection rate, PVP:AgNO3 ratio and
PVP MW) have been investigated [4–8]. In this regard, the effect of
the addition of a short-chain PVP to AgNO3 solutions stirred at
different times has not been reported elsewhere, despite the role
of silver colloids of different sizes is still unclear. During the
formation of silver nanostructures, the force fields between silver
atoms influence these atoms to agglomerate and form silver
colloids. This agglomeration is attributed to high silver atom-toatom interaction energy. Throughout sintering, small silver particles are attracted towards each other and grow bigger in size.
Therefore, as time increases, silver agglomerates size increases as
well [2,5].
In the present work we report on the morphology evolution of
silver nanostructures synthetized by a polyol method using a shortchain PVP as a function of the stirring time of the precursor salt.
2. Experimental
Materials: Silver nitrate (AgNO3, 99.99%, Sigma-Aldrich), ethylene glycol (EG, anhydrous 99.8%, Sigma-Aldrich), polyvinyl pyrrolidone (PVP, MW ¼40,000, Sigma-Aldrich). All the chemicals were
used as received without any further purification.
Synthesis of Ag nanostructures: Silver nanostructures were
synthesized by a polyol method. For a typical synthesis, 5 ml of
pure EG and 0.160 g AgNO3 dissolved in 3 ml of EG were refluxed in a
three-necked flask at 160 1C under vigorous stirring (450 rpm). Then
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A. Gómez-Acosta et al. / Materials Letters 138 (2015) 167–170
a solution of 0.156 g PVP dissolved in 3 ml of EG was injected dropwise ( 5 min for injection). As the first drops of PVP solution were
added, the mixture turned yellow. With continuous addition, it
became gradually turbid for a final gray color. The PVP/AgNO3/EG
solution was refluxed at 160 1C during 60 min, after which the
reaction was stopped, allowing the product to cool to room temperature. In our experiments, three different samples were prepared
varying the AgNO3 solution stirring time; the preparation conditions
of samples are shown in Table 1. Finally, in order to separate polymer
from Ag nanostructures, the solution was diluted three times with
Table 1
Preparation conditions of silver nanostructures.
Sample AgNO3
(g)
PVP
(g)
A
B
C
0.156 160
0.156 160
0.156 160
0.160
0.160
0.160
Temperature
(1C)
Stirring
time (min)
Reaction
time (min)
Stirring
rate (rpm)
0
30
60
60
60
60
450
450
450
acetone and three times with deionized water (both in a ratio of
1:2.5) and centrifuged after each dilution at 4000 rpm during 7 min.
Characterization: The morphologies and crystal structures of
the obtained products were characterized using scanning electron
microscopy (SEM) employing a Philips XL 30 ESEM device and
transmission electron microscopy (TEM) using a JEOL JEM-1010
electron microscope. In this case, a silver nanostructures drop was
deposited on a FF 300 square mesh copper grid for observation.
X ray diffraction (XRD) performed using a Rigku Ultima IV
difractometer operated at 40 kA and 30 mA with CuKα radiation
wavelength of λ ¼1.5406 Å. Optical absorption spectra for the
diluted samples were recorded on an Agilent 8453 spectrophotometer. Samples were scanned from 190 to 1100 nm at a resolution of 2 nm.
3. Results and discussion
Morphology: Typical SEM and TEM images of silver nanostructures are shown on Fig. 1. Results showed that the final morphologies of Ag nanostructures at the end of the polyol process are
Fig. 1. (a), (c), (e) SEM micrographs of silver nanostructures synthesized after 0, 30 and 60 min of AgNO3 stirring time respectively, and (b), (d), (f) TEM micrographs
corresponding to samples A, B and C.
A. Gómez-Acosta et al. / Materials Letters 138 (2015) 167–170
strongly dependent on the time that the silver solution was stirred
before the addition of the capping agent. From Fig. 1a, it can be
seen that the reaction products of sample A (0 min) consisted of a
large number of nanoparticles with an average diameter of
150 nm (Fig. 1b). When the stirring time increased from 0 to
30 min (sample B), Ag atoms agglomerated and formed a thin
layer on the walls of the flask. The morphology of the synthesized
products changed, and a mixture consisting of rods, triangularshapes and particles was obtained (Fig. 1c and d). Finally, after
60 min of stirring (sample C) a thicker layer of silver agglomerates
was observed, and Ag nanowires were formed as it can be seen on
Fig. 1e and f. It has been reported that using a short-chain PVP
(MW¼40,000) and PVP:AgNO3 2.5, the product mainly consists
of nanorods and nanoparticles when a typical-polyol method is
used [8,9], and of 28% of nanorods and nanowires using a
microwave-polyol method [10]. However, no report on the influence of stirring time before the addition of the capping agent is
found in the literature. Additionally, in the best case, with a shortchain PVP (MW ¼38,000) and a PVP/AgNO3 ratio (R¼1) only a
maximum of 50% of nanowires has been obtained [6]. However,
Fig. 1e shows that is possible to obtain 99% nanowires using a
PVP with MW ¼40,000 and after 60 min of AgNO3 stirring.
Optical properties: UV–vis spectrum measurements of synthesized nanostructures were used to track the morphological evolution since different shapes and sizes exhibit characteristic surface
plasmon resonance bands at different frequencies. Fig. 2 shows the
optical absorption spectra of samples A, B and C.
169
After PVP addition, a plasmon peak at 410 nm appears
immediately in samples where silver nanoparticles are the main
product. It is observed that the peak at 410 nm is located near to
the maximum absorbance peak in the case of samples A and B,
indicating a high concentration of nanoparticles. It is also clear
that for sample C, peaks can be observed at 350 nm and,
380 nm. In this regard, it is well known that AgNWs show two
characteristic absorption peaks around 350 nm and 380 nm
[9]. The peak located at 350 nm corresponds to the quadruple
resonance excitation of AgNWs while the peak at 380 nm is
attributed to the transverse plasmon resonance of the AgNWs.
Thus, the presence of nanowires in sample C is clearly indicated.
Finally, a long tail over the wavelength around 400 nm to 800 nm
range (sample B) indicates that silver nanorods with a wide range
of size are obtained [2,9,11]. These morphologies are observed
mostly after 30 min of stirring.
Crystal structure: Fig. 3 shows the XRD results for samples A,
B and C which show the two major peaks corresponding to the
diffraction of (1 1 1) and (2 0 0) planes of face centered cubic (fcc)
silver. A relative high intensity ratio of the (1 1 1) to (2 0 0) peak
indicates that samples have a preferential growth direction [1].
This is observed specially in sample C. In this case, the observed
(1 1 1)/(2 0 0) intensity ratio (15.11) is much higher than that of
the standard card for silver (PDF# 87-0720 file). This indicates that
Ag nanowires are growing preferentially in (1 1 1) direction. In the
case of sample B, (2 0 0) peak shows a higher intensity than in
sample C, while (1 1 1) peak shows a lower relative intensity. In
this case, the ratio between (1 1 1) and (2 0 0) peaks exhibits a
lower value (4.23) than in the case of sample C, indicating the
presence of nanorods. Finally, in sample A the crystalline structure
of silver is barely noticed and no preferential growing direction is
observed.
4. Conclusions
It is showed that by using the Polyol method employing a
short-chain PVP, without additional reactants than those used in a
typical synthesis, silver nanowires can be obtained. By controlling
stirring time of the precursor salt before addition of capping agent,
adequate change in morphology can be achieved either nanoparticles or nanowires. It was found that at longer stirring times
acicular morphology is enhanced.
Fig. 2. UV–vis absorption spectra of samples A, B and C.
Acknowledgements
The authors gratefully thank José Eleazar Urbina-Sánchez, Adair
Jiménez-Nieto at CINVESTAV and Ma. de Lourdes Palma-Tirado,
Beatriz Millán-Malo at UNAM for the technical support during
this work.
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