Impact of microwave power on the preparation of silver nanowires via a microwave-assisted method

Article abstract of DOI:10.1039/c3ra00117b

The microwave-assisted method is rapid in synthesizing silver nanowires. However, a by-product of silver particles is often simultaneously produced. For better utilization of microwaves to produce silver nanowires, the effect of microwave power on the morphologies of silver nanostructures was studied. The synthesis was conducted at various single microwave powers, 80, 160, 320, 560 and 800 W and with a combination of 800 W and 320 W with different sequences and time assignments. The results showed that 320 W produced silver nanowires of 6-8 μm, while when a lower power was used, silver nanowires were shortened to 2-4 μm. At higher microwave power, vast silver particles dominated the product with short nanowires of about 3-4 μm. In addition, the combination of 800 W × 0.5 min + 320 W × 2.5 min ideally resulted in mainly uniform silver nanowires up to 10 μm in length, with a small quantity of silver particles. However, prolonged treatment at 800 W during the first heating phase and the reversed combination of 320 W + 800 W led to a high yield of silver particles. We believe that different microwave powers lead to products of various morphologies by impacting on the reduction rate and seed growth. Generally, this study highlights the microwave power dependent effects on silver nanostructures and a better utilization of microwave power combination to synthesize silver nanowires.

Full text of DOI:10.1039/c3ra00117b

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Impact of microwave power on the preparation of silver
nanowires via a microwave-assisted method3
Cite this: RSC Advances, 2013, 3,
8431
Yang Yang,a Yongyou Hu,*ab Xuhua Xiongc and Yanzhe Qina
The microwave-assisted method is rapid in synthesizing silver nanowires. However, a by-product of silver
particles is often simultaneously produced. For better utilization of microwaves to produce silver
nanowires, the effect of microwave power on the morphologies of silver nanostructures was studied. The
synthesis was conducted at various single microwave powers, 80, 160, 320, 560 and 800 W and with a
combination of 800 W and 320 W with different sequences and time assignments. The results showed that
320 W produced silver nanowires of 6–8 mm, while when a lower power was used, silver nanowires were
shortened to 2–4 mm. At higher microwave power, vast silver particles dominated the product with short
nanowires of about 3–4 mm. In addition, the combination of 800 W 6 0.5 min + 320 W 6 2.5 min ideally
resulted in mainly uniform silver nanowires up to 10 mm in length, with a small quantity of silver particles.
However, prolonged treatment at 800 W during the first heating phase and the reversed combination of
320 W + 800 W led to a high yield of silver particles. We believe that different microwave powers lead to
products of various morphologies by impacting on the reduction rate and seed growth. Generally, this
study highlights the microwave power dependent effects on silver nanostructures and a better utilization
of microwave power combination to synthesize silver nanowires.
Received 9th January 2013,
Accepted 19th March 2013
DOI: 10.1039/c3ra00117b
www.rsc.org/advances
such as the polyol method,8 wet chemical synthesis,9 and the
template technique,10 microwave irradiation possesses the
Introduction
As a special one-dimensional (1-D) nanostructure, silver
nanowires have been applied in many fields, such as
biomedical and chemical sensing, owing to their unique
surface-enhanced Raman scattering1 and surface-enhanced
fluorescence.2 They also play an important role in electronic
device fabrication. A small quantity of load can ensure the
composite polymer has high conductivity due to its high
aspect ratio.3 Recently, silver nanowires were reported as a
novel sterilizing material when electrified with a low direct
voltage, which stimulated an extremely high electric field on
the surface of the nanowires and promptly killed the passing
microbes.4 Silver nanowires are vital to these valuable
applications, so the shape-selective synthesis of silver nano-
wires is an important subject of research.
great advantage of remarkable reaction rate enhancement.
This accelerates the nanowire synthesis by 1–2 orders of
magnitude11 and shortens the reaction time from hours to
minutes. However, silver nanowires are not favored products
on the nanoscale12 and easily mix with particles that have been
produced as a by-product, since quasi-spherical silver particles
have the lowest possible surface energy. To produce silver
nanowires as the main product, some impacting factors have
been investigated, such as the introduction of poly(vinylpyrro-
lidone) (PVP),13 Cl2,14 Cu+/2+ 15 Fe2+/3+ 16 shielding gas17 and
,
,
stainless steel,18 and microwave power has been demonstrated
to be related to the morphologies of the final nanostructures.
It was reported that 300 W and 400 W could be used to achieve
silver nanocubes, but 400 W was suitable for a high yield of
silver nanowires.19,20 Gou21 introduced in detail the high
efficiency of microwave power in synthesizing silver nanowires
and simply suggested that low (100 W), medium (300, 400 W)
and high (1000 W) microwave power leads to silver particles,
nanowires and short rods, respectively. In the hope of
identifying the impact of microwave power on the morphol-
ogies of silver nanostructures systematically, more in-depth
research on a wide range of microwave powers should be done,
and the reaction time and temperature, which would
intrusively influence the reaction along with the change of
microwave power, should be both taken into consideration
and well designed.
Microwave irradiation has been widely used in organic and
nanosized metallic synthesis, including silver nanowire
fabrication.5–7 When compared with traditional methods,
aMinistry of Education Key Laboratory of Pollution Control and Ecological
Remediation for Industrial Agglomeration Area, College of Environmental Science
and Engineering, South China University of Technology, Guangzhou, 510006, China.
E-mail: ppyyhu@scut.edu.cn; Fax: +86 020 39380508; Tel: +86 020 39380506
bState Key Lab of Pulp and Paper Engineering, College of Light Industry and Food
Science, South China University of Technology, Guangzhou, 510640, China
cSchool of Civil Engineering and Transportation, South China University of
Technology, Guangzhou, 510640, China
Electronic supplementary information (ESI) available. See DOI: 10.1039/
3
c3ra00117b
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The solution was then heated at microwave powers of 80, 160,
320, 560 and 800 W, representing a range from low to high
power. To perform a microwave power-combined synthesis,
microwave powers were quickly switched from one to the
other. Because the boiling point of EG is 197 uC, to avoid
overheating and bumping, the reaction time was exactly
controlled to maintain the final temperature of the reaction
at 178 ¡ 2 uC, which was measured by a thermometer. It is
believed that this limitation of temperature guaranteed a safe
reaction process and fairly represented the impact of micro-
wave power. After cooling in air down to room temperature,
the final products were then centrifuged and washed in
acetone, deionized water and ethanol to purify them for
further characterization.
A drop of the aqueous dispersion was air-dried on a piece of
aluminum conducting grid. The morphologies of the products
were observed by scanning electron microscopy (SEM, EVO
LS10) operating at an accelerating voltage of 10 kV. TEM,
HRTEM and selected-area electron diffraction (SAED) were
achieved on a JEOL JEM-2100HR. An Energy Dispersive
Spectrometer (EDS, INCA X-Max) was used to confirm the
existence of AgCl during the synthesis process. UV-visible
absorption spectra were taken on a HACH DR 5000 with the
dispersion solution in a quartz cuvette. The IR spectra were
taken on a Bruker Vector 33.
Herein, this study aims to reveal the relationship between
the microwave power and the preparation of silver nanowires
and silver particles as by-product, via a microwave-assisted
method. Note that the boiling point of the ethylene glycol used
here as the reductant, limits the synthesis temperature, which
is also determined by the microwave power and duration time.
The same final temperature was set for every synthesis at the
tested microwave powers, and different tested microwave
powers would thus last for different periods of time. The same
temperature rising range was the basis to fairly reveal the
direct impact of different heating rates at different microwave
powers on the preparation of silver nanostructures. Low,
medium and high microwave power was used in the synthesis.
Considering it requires different reaction rates for silver seeds
to form and grow into nanowires, the effect of combined
microwave powers, representing a change in reaction rate, on
the silver nanostructures was also studied. SEM and UV-vis
absorption spectra were mainly used to study the evolution of
the products’ morphologies. The impacting mechanism of
microwave power on the silver nanowires’ growth and the
particles’ output as a by-product was discussed. Generally, this
study enhances the existing knowledge of silver nanowire
synthesis by a microwave-assisted method, and proposes the
optimal utilization of microwave power to fabricate silver
nanowires of high quality and purity.
Results and discussion
Experimental section
Impact of single microwave powers
Materials
The results of single microwave powers on product morphol-
ogy are shown in Table 1. It was observed that silver nanowires
of various shapes could be fabricated at all tested microwave
powers, no matter whether it was low (80 W 6 18–22 min, 160
W 6 10–12 min), medium (320 W 6 3–4 min), or high (560 W
6 2–2.5 min, 800 W 6 1.25–1.5 min). The higher the power
was, the less time it took to achieve silver nanowires. It should
be noted that each synthesis was carried out using a gradual-
heating route, not all silver turned into a 1-D morphology and
silver particles coexisted with silver nanowires. At 80 W 6 18–
22 min or 160 W 6 10–12 min, short nanowires (2–4 mm) and
nanorods with a diameter up to 200 nm were mainly obtained
(Fig. 1(A) and (B)). When the microwave power was increased
to 320 W 6 3–4 min, longer nanowires that were uniform with
an average diameter of 70 nm and a length of 6–8 mm were
dominant (Fig. 1(C)). However, as shown in Fig. 1(D) and (E),
the elevation of microwave power to 560 W 6 2–2.5 min and
AgNO3 was purchased from Sinopharm Chemical Reagent Co
Ltd; PVP (K30) was purchased from Shanghai Bio Science &
Technology Co Ltd. Ethylene glycol (EG) and NaCl was
purchased from Guangzhou Chemical Reagent Factory. A
commercially available Galanz microwave oven working at 80–
800 W was utilized as the heating source to fabricate silver
nanostructures. The oven works using a cyclic method. By
changing the ratio of working time per cycle (10–100%),
different microwave powers could be acquired. The working
conditions are detailed in Table S1, ESI .
3
Methods
As described by Gou,21 the synthesis involved the following
steps: 90 mg of AgNO3, 110 mg of PVP, 5 mg of NaCl, and 20
mL of EG were first mixed inside a 50 mL Teflon vessel. The
mixture became opaque due to the formation of AgCl colloids.
Table 1 Single microwave heating conditions for synthesis of silver nanostructures
Single powera
Heating duration
Product morphology
Average length range of nanowires
80 W
18–22 min
10–12 min
3–4 min
2–2.5 min
1.25–1.5 min
Mainly nanowires/rods + quasi-spherical/cubic particles
Mainly nanowires/rods + quasi-spherical/cubic particles
Mainly nanowires + quasi-spherical/cubic particles
Mainly quasi-spherical/cubic particles + nanowires
Mainly quasi-spherical/cubic particles + nanowires
2–4 mm
2–4 mm
6–8 mm
3–4 mm
3–4 mm
160 W
320 W
560 W
800 W
a
Each reaction was finished when the final temperature reached 178 ¡ 2 uC.
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Fig. 2 UV-vis spectra of products obtained at different microwave powers: (a)
80 W 6 18–22 min, (b) 160 W 6 10–12 min, (c) 320 W 6 3–4 min, (d) 560 W
6 2–2.5 min, (e) 800 W 6 1.25–1.5 min.
320 W 6 3–4 min must remarkably accelerate the reduction
rate of silver ions, as well as the deposition of atoms on the
end of nanowires, thus, longer nanowires could be achieved.
According to the theory of crystal growth, silver atoms were
first achieved by the reduction of EG. The silver atoms would
then self-assemble to form silver seeds, including single-
crystal seeds and multiply twinned particle seeds (MTPs).26
The former would grow into spherical/cubic particles, while
the latter would grow into silver nanorods and nanowires with
the assistance of PVP.13 Meanwhile, single-crystal seeds are
more kinetically stable than MTPs so more time is needed for
the original silver atoms to form MTPs.27 Consequently, if the
microwave power was too high, such as 560 W 6 2–2.5 min or
800 W 6 1.25–1.5 min, the rate of reduction would exceed that
of seed formation and single-crystal seeds would be preferen-
tially formed. Single-crystal seeds would finally grow into
larger particles known as the Ostwald ripening process.28 On
the contrary, when the microwave power was turned down to
320 W 6 3–4 min, the reduction rate would be slowed down
and would closely match that of seed formation, providing a
suitable environment for MTPs and nanowires to grow.
The crystal structure of the silver nanowires was also
studied. The TEM image in Fig. 3(A) shows the straight
appearance of a silver nanowire. The inset displays the
electron diffraction pattern by focusing the electron beam
onto the wire and confirms a twinned structure. The TEM
image in Fig. 3(B) clearly shows different nanostructures and
proves that single-crystal and twinned particle seeds co-exist.
The twin plane can be also observed as previously reported
and again proves the twinned structure of silver nanowires.29
The HRTEM image in Fig. 3(C) shows a well-resolved lattice
spacing of 0.23 nm which refers to the [111] plane of the silver
seeds. Silver seeds selectively grow in the [110] direction along
their longitudinal axis, owing to the assistance of PVP
preferentially binding on the surface of silver nanowires and
blocking the deposition of silver atoms. And all organic
reagents including PVP can be easily removed from the surface
Fig. 1 SEM images of samples obtained at different microwave powers: (A) 80
W 6 18–22 min, (B) 160 W 6 10–12 min, (C) 320 W 6 3–4 min, (D) 560 W 6
2–2.5 min, (E) 800 W 6 1.25–1.5 min.
800 W 6 1.25–1.5 min led to much shorter nanowires only 3–4
mm in length and a large amount of quasi-spherical/cubic
silver particles became the main products.
The UV-vis absorption spectra (Fig. 2) can clearly illustrate
the evolution of product morphology under different micro-
wave power because silver nanostructures with different
morphologies exhibit surface plasmon resonance (SPR) at
different frequencies.22,23 The change in optical features
observed in these spectra corresponds to SEM observed above.
The curves in Fig. 2 show two peaks at 350 nm and 390 nm,
which could be attributed to the bulk silver film and transverse
plasmon resonance of the silver nanowires, respectively.24 This
phenomenon indicates silver nanowires were achieved at the
five tested single microwave powers. The co-existing silver
particles exhibited absorption at y455 nm. With the micro-
wave power increased, successional enhancement in intensity
at this wavelength can be observed (Fig. 2(a)–(e)) and leads to a
higher peak than that at 390 nm, which means silver particles
were becoming dominant in the solution.
It is well known that microwave heating is rapid due to its
dielectric superheating mechanism.25 The increase of micro-
wave power from 80 W 6 18–22 min, to 160 W 6 10–12 min to
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Fig. 3 (A) TEM image of the end of a silver nanowire; the inset presents the selected-area electron diffraction pattern of the nanowire. (B) TEM image of the product
in the early stages of the wire growth process, the twin planes are indicated by the arrows. (C) HRTEM image of the silver nanowires, showing the growth direction
and lattice spacing of the [111] planes.
by wash-centrifugation, which was characterized by IR spectro-
particles prospered (Fig. 5(F)). The appearance of some co-
existing short wires ,1 mm indicated silver seeds were formed
and started to assemble. Hence, it is believed that 800 W 6 0.5
min can be used to activate the reduction of silver ions and the
switch to 320 W 6 2.5 min will provide a suitable rate for
MTPs to form and silver nanowires to grow. But prolonged
heating at 800 W to 1 min over-accelerates the formation of
silver seeds from silver atoms and single-crystal seeds are
achieved preferentially at a high reaction rate. And the switch
to 320 W 6 1.25 min did not help to produce long silver
nanowires due to the high consumption of silver sources in
the first heating phase. It is thus believed that a high
microwave power can be used in the first heating phase, but
the reaction time should be carefully controlled to prevent the
premature assembling of silver seeds before the switch to a
medium microwave power.
The converse combination (320 W 6 1.25 min + 800 W 6 1
min and 320 W 6 2.5 min + 800 W 6 0.5 min) did not have
any positive effect on reducing the production of particles or
promoting the growth of nanowires. Both these combinations
gave a high yield of silver particles and relatively short silver
nanowires (Fig. 5(C) and (D)). These results correspond to the
strong absorbance at y500 nm for the silver particles and the
comparatively weaker absorbance at 390 nm for the silver
nanowires (Fig. 4(B)). Moreover, the red-shift of the peak from
y455 nm to y500 nm demonstrates the increase in size of the
silver particles in those samples. As discussed before, a high
temperature and a high microwave power leads to an over-
accelerated reduction rate which is suitable for the formation
of kinetically-stable single-crystal seeds instead of MTPs. So
the utilization of a high microwave power of 800 W in the
second heating phase would just immensely restrain the
generation of MTPs and cause a boom of single-crystal seeds,
scopy (shown in Fig. S1, ESI ).
3
Impact of combined microwave powers
As mentioned above, silver nanowires could be achieved under
high, medium or low microwave power. But higher microwave
power shortens reaction time and medium microwave power
guarantees long silver nanowires as the main product. So the
combination of 800 W and 320 W was tested to reveal whether
this synthesis operation could make a high quality product in
the shortest reaction time. The results are shown in Table 2.
It was proved by UV-vis spectroscopy that 800 W 6 0.5 min +
320 W 6 2.5 min greatly reduced the amount of silver particles
formed in comparison with single 800 W 6 1.25–1.5 min,
according to the weakened intensity of the peak at y455 nm
(Fig. 4(A)). As supported by the SEM images, this microwave
power combination gave a product of uniform nanowires up to
10 mm of high purity (Fig. 5(A)), while the product taken from
the 800 W 6 1 min + 320 W 6 1.25 min still contained a
remarkable amount of silver particles (Fig. 5(B)), and the
lengths of the silver nanowires were shortened to 4–5 mm. To
study the impact of these two sets of combined microwave
powers on the silver nanostructures, the samples synthesized
at 800 W 6 0.5 min and 1 min were analyzed. At 800 W 6 0.5
min, AgCl colloids could be observed (Fig. 5(E)) and were
confirmed by EDS (Fig. S2, ESI ). These are different from the
3
conjunctive shapes found in the freshly prepared reaction
solution (Fig. S3, ESI ). The AgCl colloids formed after 800 W
3
6 0.5 min heating were individually independent and were
covered with sphere-like particles. This indicated that the
reaction was at the beginning of AgCl dissolution and
reduction of silver ions. But when the first heating phase of
800 W was prolonged to 1 min, a large amount of silver
Table 2 Combined microwave heating conditions for synthesis of silver nanostructures
Combined powersa
Product morphology
Average length range of nanowires
800 W 6 0.5 min + 320 W 6 2.5 min
800 W 6 1 min + 320 W 6 1.25 min
320 W 6 1.25 min + 800 W 6 1 min
320 W 6 2.5 min + 800 W 6 0.5 min
Mainly nanowires + quasi-spherical/cubic particles
Mainly quasi-spherical/cubic particles + nanowires
Mainly quasi-spherical/cubic particles + nanowires/rods
Mainly quasi-spherical/cubic particles + nanowires
8–10 mm
4–5 mm
1–2 mm
3–5 mm
a
Each reaction was finished when the final temperature reached 180 uC.
8434 | RSC Adv., 2013, 3, 8431–8436
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