In this work, a range of factors was investigated for the synthesis of silver nanowires. Nickel chloride showed to be
a promising controlling agent for the scale-up synthesis of silver nanowires. Other factors such as reaction temperature,
concentrations of surfactant, precursor, controlling agent, and the effect of degassing were studied and optimized. The
synthesized silver nanowire sample was applied for flexible electrode fabrication. The fabricated electrode showed to
have high transparency (90 %), low sheet resistance (26 Ω/sq), low surface roughness (Rq = 6 nm), and high bending
stability (withstand 100 bending cycles at bending radius of 4 mm), which satisfied the requirement for applications in
flexible optoelectronic device.
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Cite this paper: Vietnam J. Chem., 2021, 59(1), 98-105 Article
DOI: 10.1002/vjch.202000131
98 Wiley Online Library © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH
Optimization of silver nanowire synthesis for flexible transparent
conductive electrodes
Nguyen Thanh Nhan1, Pham Duy Linh1, Doan Tien Dat1, Min Ju Cho2, Dong Hoon Choi2*,
Hoang Mai Ha1*
1Institute of Chemistry, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay,
Hanoi 10000, Viet Nam
2Department of Chemistry, Research Institute for Natural Science, Korea University
Submitted July 31, 2020; Accepted October 13, 2020
Abstract
In this work, a range of factors was investigated for the synthesis of silver nanowires. Nickel chloride showed to be
a promising controlling agent for the scale-up synthesis of silver nanowires. Other factors such as reaction temperature,
concentrations of surfactant, precursor, controlling agent, and the effect of degassing were studied and optimized. The
synthesized silver nanowire sample was applied for flexible electrode fabrication. The fabricated electrode showed to
have high transparency (90 %), low sheet resistance (26 Ω/sq), low surface roughness (Rq = 6 nm), and high bending
stability (withstand 100 bending cycles at bending radius of 4 mm), which satisfied the requirement for applications in
flexible optoelectronic device.
Keywords. Silver nanowires, flexible, transparent, conductive electrodes.
1. INTRODUCTION
Flexible transparent conductive electrode (FTCE) is
an indispensable part of many electronic and
optoelectronic devices. Among various materials for
the fabrication of FTCE, silver nanowires (AgNWs)
emerges as the most potential candidate due to its
high conductivity, high flexibility and
transparency.[1] Researches have focused on the
application of AgNW-based electrode for organic
light-emitting diodes,[2] thin-film solar cells,[3,4]
touch panels,[5] sensors,[6] transparent heaters,[7] etc.
Until now, AgNW synthesis procedures are
mostly based on the polyol method. The core
difference between procedures is the use of halide
salt as controlling agent. Recently, several groups
reported the use of Br- or Cl- as co-additives to
produce AgNW with different diameters.[8–14] Those
procedures also concentrate on the effect of some
other factors such as reaction time,[9,10] controlling
agent,[11,15] surfactant chain length,[11,12] precursor
and surfactant concentration on the morphology of
AgNWs.[13,14] However, this work still attracts much
effort to find the best condition to synthesize
AgNWs for specific applications.
Herein, we try to optimize the procedure for the
synthesis of AgNWs by investigating various factors
like controlling agent, precursor, surfactant, solvent,
temperature, and degassing process. The optimized
procedure utilized chloride salt as controlling agent
to achieve AgNWs with low content of by-product,
and hence simplifying the purification step and
achieving AgNWs in good yield. Moreover, the
synthesis required no degassing step and the
purification used only ethanol, which made our
method greener and more applicable in industry. The
optimized AgNW sample was applied for the
fabrication of FTCEs through spin-coating
technique, showing good electrode morphology, low
sheet resistance and high transparency.
2. MATERIALS AND METHODS
2.1. Materials
Silver nitrate (AgNO3, 99.9 %), ethylene glycol
(EG) and diethylene glycol (DEG) (analytical grade)
were provided by Fisher Scientific. Sodium bromide
(NaBr, 99 %), sodium chloride (NaCl, 99 %),
anhydrous ferric chloride (FeCl3, 99.99 %), nickel
chloride (NiCl2, 98 %), and poly(vinylpyrrolidone)
with MW ~ 55000 and 360000 g/mol (PVP 360 and
PVP 55, respectively) were obtained from Sigma-
Aldrich. Ethanol, acetone, and isopropanol were of
high purity and used without further purification.
PET substrate 300 mm×300 mm, thickness of 0.25
Vietnam Journal of Chemistry Hoang Mai Ha et al.
© 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 99
mm was purchased from Sigma-Aldrich.
2.2. Characterization
The morphology and distribution of the AgNWs on
PET substrate were investigated using scanning
electron microscopy (SEM, S-4800, Hitachi, Japan).
The UV-Vis absorption/transmittance spectra of
electrodes were obtained using UV-Vis
spectrophotometer (SP-3000 nano). The surface
roughness was measured by atomic force
microscopy (AFM, XE-100, PSIA). The sheet
resistance of FTCEs was measured using a four-
point probe (Keithley, 2634B).
2.3. Synthesis of silver nanowire
2.3.1. Using sodium bromide as controlling agent
AgNWs were synthesized with some modification
from the procedure reported by Xia et al.[16] Firstly,
the solutions of 100 mM AgNO3 and 50 mM NaBr
were prepared in EG. Then 250 mg PVP 360 was
dissolved in 40 mL of EG and heated to 160 °C for 1
h. Next, 500 µl NaBr was pipetted to the PVP
solution. After 5 min, 5 mL AgNO3 solution was
added dropwise into the PVP solution at the speed of
0.15 mL/min. The magnetic stirrer was turned off
after 10 min of stirring, and the reaction was allowed
to react for a further 30 min at 160 °C.
2.3.2. Using sodium chloride as controlling agent
The method for synthesizing AgNWs using NaCl as
controlling agent was based on the procedure
reported by Yan et al. with some modifications.[17]
First, 1800 µM sodium chloride solution was
prepared (solution A). Then, 0.2 g PVP 360 was
dissolved in 25 mL EG (solution B). After the
complete dissolution of PVP in EG, 0.25 g AgNO3
was added to solution B, followed by the addition of
3 mL of solution A. This solution was stirred for 15
min before heating to 140 °C. Once it reached 140
°C, the magnetic stirrer was turned off and the
reaction was allowed to react for 5 h.
2.3.3. Using ferric chloride as controlling agent
The procedure for synthesizing AgNWs using FeCl3
was similar to the above procedure but the
controlling agent was FeCl3 instead of NaCl.
2.3.4. Using nickel chloride as controlling agent
The procedure reported by Zhang et al. was
modified for the synthesis of AgNWs using NiCl2 as
controlling agent.[13] Specifically, 416.5 mg PVP
was dissolved in 30 mL EG. Then, 254.5 mg AgNO3
was dissolved in 20 mL EG. After all compounds
were completely dissolved, 0.65 mL of 7.5 mM
NiCl2 solution was pipetted to the PVP solution,
followed by the addition of AgNO3 solution. This
final solution was stirred well for 10 min before
heating to 140 °C. The reaction system was held at
this temperature for 6 h at low stirring speed.
All the raw solutions were purified. Specifically,
10 mL of each raw solution was transferred into a
centrifugation tube and mixed with 20 mL ethanol.
The solution was shaken gently before being
centrifuged at speed of 7000 rpm for 5 min. The
supernatant was discarded. The precipitation was
washed twice with ethanol by centrifugation at 4000
rpm for 5 min before being dispersed in 5 mL
isopropanol for further usage.
2.4. Fabrication of transparent electrodes
PET substrates were cut into small pieces with the
size of 2.5×2.5 cm. They were thoroughly cleaned
with DI water for 10 min twice and isopropanol for
10 min before being dried with a nitrogen gun at
room temperature. Then, the purified silver
nanowire solution was dropped on the PET substrate
and spin-coated at 2000 rpm. The as-prepared
FTCEs were annealed at proper temperature for 10
min before characterization.
3. RESULTS AND DISCUSSION
3.1. Synthesis of silver nanowires
3.1.1. Effect of controlling agent on the formation of
silver nanowires
The effect of controlling agents on the morphology
of AgNWs was shown in figure 1. Comparing
between chloride and bromide salt, bromide ions
promote the formation of thinner silver nanowires
(figure 1a, 1b). However, in the case of bromide
ions, a significant amount of nanoparticles was
formed as a by-product. The formation of these
silver nanoparticles and silver bromide nanoparticles
was explained by the difference in the interaction
affinity between silver nanocrystal and halide
ions.[18] Generally, halide ions can bind with silver
ion to generate silver halide nanoparticles which
reduce the free silver ion concentration and slow
down the reduction rate.[8] Because silver bromide
has a lower solubility than silver chloride, the
synthesis of AgNWs using sodium bromide as
Vietnam Journal of Chemistry Optimization of silver nanowire synthesis for
© 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 100
controlling agent led to the production of a larger
amount of silver halide nanoparticles, which lowered
the yield of AgNW synthesis.
The effect of metal ions in halide salts on the
morphology of AgNWs was evaluated. There was a
slight difference in the effect of sodium, iron, and
nickel ion. The average diameters of nanowires
using sodium chloride, ferric chloride, and nickel
chloride were 52, 40, and 35 nm, respectively. When
using NaCl, the surface of AgNW was very rough
with many tiny particles grew on it (figure 1b-1 and
1b-2), suggesting the growth of silver nanoparticles
on the surface of AgNW crystal. Noticeably, in the
case of FeCl3, the morphology of AgNWs was
unusual. As obviously seen from figures 1c-1 and
1c-2, the silver nanowires were slightly wavy with
many bending points; and the diameter was uneven
throughout a nanowire. The size distribution of
silver nanowires in this sample was unequal as well.
The reason for this unusual morphology is suggested
to be the uneven oxidative etching of Fe3+ on
AgNWs.[17] In both the case of NaCl and FeCl3, the
morphology of AgNWs led to a higher proportion of
electron scattering, thus increasing the resistivity of
silver nanowire electrode. However, when using
NiCl2 (figures 1d-1 and 1d-2), the surface of
AgNWs was very smooth, and AgNWs had a narrow
size distribution with the mean value of 35 nm in
diameter and 15 µm in length. Moreover, the by-
product was removed effectively after a simple
purification process. This observation proved that
nickel chloride is the suitable controlling agent for
the synthesis of AgNWs.
10 µm
200 nm
c-1)
c-2)
5 µm
200 nm
d-1)
5 µm
b-1)
5 µm
a-1)
a-2)
5 µm
200 nm
b-2)
200 nm
d-2)
Figure 1: The morphology of silver nanowires synthesized using NaBr (a-1, a-2), NaCl (b-1, b-2),
FeCl3 (c-1, c-2), NiCl2 (d-1, d-2) as controlling agents
3.1.2. Effect of temperature
The effect of temperature on the synthesis of AgNW
was studied. At 120 °C, there were no AgNWs
formed, implying that this temperature was not
sufficient for the formation of AgNWs (figure 2a).
When the temperature was above 130 °C, AgNWs
with different diameters were obtained, and the
diameter rose in association with the increase in
temperature. Noticeably, at 130 °C, AgNWs with
unequal diameters were obtained while at 140 °C,
the diameters of nanowires were more equal (figures
2b and 2c). It is explained that at the first stage of
the synthesis, the nucleation rate was significantly
higher at 140 °C than that at 130 °C. The fall in the
concentration of precursor at 140 °C hampered the
formation of nuclei at the later state, thereby forming
nanowires with more uniform morphology.
However, at 150 °C, the quality of AgNWs was
lower with higher average diameter; and some tiny
c)
a)
d)
b)
200 nm 200 nm
200 nm200 nm
Figure 2: SEM images of AgNWs synthesized using
NiCl2 as controlling agent at different temperatures:
120 °C (a), 130 °C (b), 140 °C (c), 150 °C (d)
Vietnam Journal of Chemistry Hoang Mai Ha et al.
© 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 101
nanoparticles formed on the surface of nanowires
(figure 2d). It was reasoned that at such high
temperature, silver atom deposited on silver
nanowire less selectively, thereby increasing the
AgNW diameter. In the meantime, this fast
deposition process also favored the epitaxial growth
of nanoparticles on silver nanowires.
3.1.3. Effect of precursor concentration
The morphology of AgNWs synthesized at different
AgNO3 concentrations is shown in figure 3. At low
concentration of Ag+, the formation of nanoparticles
was more favorable while the nanowires formed
with a short length and large diameter (figure 3a). It
is reasoned that after forming seeds, the
concentration of Ag+ was too low for the growth of
silver nanowires. At high concentration (0.04 M),
the high deposition rate of Ag atom favored the
formation of not only AgNWs but also nanoparticles
(figure 3d). At moderate concentrations (0.02 M and
0.03 M), the AgNWs formed with less byproduct
(figures 3b and 3c). Comparing these two
concentrations, the AgNWs synthesized at the
concentration of 0.03 M formed with higher
homogeneity. So, this concentration was used for
further optimization.
d)
a) b)
c)
500 nm 500 nm
500 nm500 nm
Figure 3: SEM images of AgNWs synthesized with
different concentrations of silver nitrate: 0.01 M (a),
0.02 M (b), 0.03 M (c), 0.04 M (d)
3.1.4. Effect of Ag+/Cl-
As mentioned earlier, controlling agents play a
crucial role in the formation of nanoparticles and
nanowires. To investigate the effect of Cl- ion on the
morphology of AgNWs in more detail, silver
nanowires with four different Ag+/Cl- molar ratios
were synthesized (figure 4a-d). At low Ag+/Cl- molar
ratio, a large amount of seed was formed, and Ag+
precursor depleted quickly, leading to short-length
silver nanowire. Moreover, Cl- at high concentration
also passivated all surfaces of crystal, thereby
producing more silver nanoparticle (Figure 4a).
Decreasing Cl- concentration enabled to reduce the
amount of by-product (figure 4b and 4c).
Nevertheless, at very high Ag+/Cl- molar ratio, a
smaller number of seed produced, leading to big-size
silver nanowires (figure 4d). The optimized ratio of
Ag+/Cl- was 150/1.
c)
a) b)
d)
500 nm
500 nm
500 nm
500 nm
Figure 4: SEM images of AgNWs synthesized at
different Ag+/Cl- molar ratios: 50/1 (a), 100/1 (b),
150/1 (c), 200/1 (d)
3.1.5. Effect of PVP/AgNO3 molar ratio
PVP surfactant plays a crucial role in the anisotropic
growth of AgNWs by strong binding to the lateral
surfaces of crystals (these surfaces have higher
energy than the end surfaces), thereby inhibiting the
lateral growth.[19] The effect of PVP/AgNO3 molar
ratio was investigated and illustrated in figure 5
(these AgNW samples was not purified to observe
the byproducts). Note that the calculation of [PVP]
is monomer based. When this ratio was small, a
significant amount of nanoparticles was produced
(figure 5a) since there was not enough PVP to
passivate the lateral crystal surfaces. Increasing the
PVP/AgNO3 molar ratio led to a decrease in by-
product (figures 5b and 5c). However, when the
ratio of PVP/AgNO3 was higher, a large amount of
silver nanoparticle and short-length silver nanowires
were obtained because PVP layer covering the silver
nanocrystal was too thick and it passivated not only
the lateral but also the end surfaces (figure 5d).
Hence, the optimum molar ratio of PVP/AgNO3 was
2.5/1.
3.1.6. Effect of PVP chain length
To investigate the effect of PVP chain length on the
morphology of AgNWs, two types of PVP with
Vietnam Journal of Chemistry Optimization of silver nanowire synthesis for
© 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 102
different average molecular weights were used and
the SEM images of produced AgNWs were shown
in figure 6. In the case of PVP 55, the obtained
product contained a large amount of byproducts; and
the average diameter of the nanowire was large (50
nm) (figure 6a). However, when using PVP 360,
byproducts were produced but much less; and the
average diameter was also smaller (figure 6b). It is
explained by the steric effect.[12] Short-chain PVP
imposed incomplete covering of nanowire surface,
passivate the lateral surfaces ineffectively, thereby
allowing silver atom to deposit onto the lateral
surfaces and producing a great deal of nanoparticle
and large-diameter silver nanowire.
d)
a) b)
300 nm
300 nm
300 nm
c)
300 nm
Figure 5: SEM images of AgNWs synthesized at
different PVP/AgNO3 molar ratio: 0.8/1 (a), 1.5/1
(b), 2.5/1 (c), 3.5/1 (d)
a) b)
5 µm
5 µm
c) d)
5 µm
5 µm
Figure 6: SEM images of AgNWs synthesized using
different chemicals: different PVP: PVP 55 (a) and
PVP 360 (b), AgNWs synthesized in DEG (c),
AgNWs synthesized in innert environment by
nitrogen bubbling into the system (d). The scale bar
in the insets: 500 nm
3.1.7. Effect of solvent
EG plays a vital role in the success of silver
nanowire synthesis because it is not only a solvent
for the reaction to take place but also a reducing
agent. For a deeper understanding of the role of EG,
AgNW was synthesized in DEG instead of EG and
the result was shown in figure 6c. It was interesting
that there were no AgNW produced when using
DEG as a solvent. It is suggested that the reduction
potential of DEG is slightly different from that of
EG, which led to the tendency of forming
nanoparticles rather than nanowires.
3.1.8. Effect of removing oxygen
Generally, oxygen has a profound impact on the
synthesis of silver nanowire. It oxidizes ethylene
glycol, thereby lowering the reducing power of
EG.[13] Thus, the impact of oxygen-removing was
studied to guarantee the best condition for the
synthesis of AgNWs.
So far, most of the procedures for the synthesis of
AgNWs reported the degassing process by bubbling
nitrogen into the solution. Nevertheless, after
investigation, it was found that bubbling nitrogen
directly into solution led to the generation of a great
amount of big-size silver nanoparticles which are
very difficult to be removed by usual purification
methods like centrifugation or sedimentation (Figure
6d). This result indicates that bubbling nitrogen into
the solution created a negative impact on the
AgNWs, and the step of removing oxygen was not
necessary for this procedure.
3.2. Electrode fabrication
After optimization process, we synthesized AgNWs
with the following condition: NiCl2 as controlling
agent, PVP 360 as surfactant, AgNO3 concentration
of 0.04 M, Ag+/Cl- ratio of 150/1, PVP/AgNO3
molar ratio of 2.5/1, and the reaction was conducted
at 140 °C in ethylene glycol. The AgNW sample
was used for the fabrication of electrodes on flexible
substrates. To find the best condition for electrode
fabrication, AgNW solutions with different
concentrations were prepared. The electrodes
fabricated with these solutions were characterized.
The transmittance spectra and FoM values (FoM =
T10/R, where T is transmittance measured at 550 nm
and R is sheet resistance [20]) of those electrodes
were shown in figures 7a and 7b for better
comparison. The optimized concentration for the
fabrication of the electrode was 3 mg/mL. The
electrode obtained from this solution was further
checked. SEM images of the AgNW electrode
(figure 7c) showed even distribution of silver
Vietnam Journal of Chemistry Hoang Mai Ha et al.
© 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 103
nanowire on the PET substrate. Through AFM
image (figure 7d), the surface roughness of the
electrode was 6.12 nm, qualified for the application
of the elec