Roles of chloride anions in the shape evolution of anisotropic silver nanostructures inpoly(vinylpyrrolidone) (PVP)- assisted polyol process

Article abstract of DOI:10.1246/bcsj.82.1304

The roles of Cl^- ions on the shape evolution of various silver (Ag) nanostructures including cubes, triangular bipyramids, and rods/wires in AgNO₃/poly(Vinylpyrrolidone) (PVP)/ethylene glycol (EG) solution have been investigated by c

Full text of DOI:10.1246/bcsj.82.1304

1304 Bull. Chem. Soc. Jpn. Vol. 82, No. 10, 1304–1312 (2009)
© 2009 The Chemical Society of Japan
Roles of Chloride Anions in the Shape Evolution of Anisotropic
Silver Nanostructures in Poly(vinylpyrrolidone) (PVP)-
Assisted Polyol Process
Xinling Tang,1 Masaharu Tsuji,*1,2 Michiko Nishio,1 and Peng Jiang3
1Department of Applied Science for Electronics and Materials, Interdisciplinary Graduate School
of Engineering Sciences, Kyushu University, Kasuga 816-8580
2Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga 816-8580
3National Center for Nanoscience and Technology, China, Beijing 100080, P. R. China
Received May 19, 2009; E-mail: tsuji@cm.kyushu-u.ac.jp
¹
The roles of Cl ions on the shape evolution of various silver (Ag) nanostructures including cubes, triangular
bipyramids, and rods/wires in AgNO3/poly(vinylpyrrolidone) (PVP)/ethylene glycol (EG) solution have been
investigated by changing the concentration of added NaCl, the reaction solution temperature, and the temperature
of injection of NaCl to the original AgNO3/PVP/EG solution under oil-bath heating. The addition of different
amounts of NaCl could tune morphologies and sizes of the final products from spheres via cubes to rods/wires.
Reaction temperature decided the reduction power and the yield of bipyramids increased with decreasing the reaction
temperature from 198 to 120 °C. Changing the solution temperature at which NaCl was introduced into the solution could
effectively influence the yields of different Ag structures by adjusting the ratios of Ag and AgCl seeds formed in the
solution. We found that AgCl could be an important precursor for the formation of the Ag cubes and bipyramids. The
¹
roles of Cl anions for the evolution of various Ag nanostructures have been discussed by taking account of the formation
¹
of AgCl as seeds, selective adsorption of Cl on {100} facets of Ag nanoparticles, and oxidative etching of the Ag
¹
particles by Cl /O2.
Recently, Ag nanostructures have attracted considerable
attention mainly due to their remarkable optical properties and
numerous applications in fields such as catalysis, surface
plasmonics, surface-enhanced Raman scattering, and chemical
or biological sensing.1-3 Since chemical and physical properties
of the Ag nanocrystals are highly related to orientations of
various crystalline surfaces, the controllable preparation of Ag
nanocrystals with different shapes and exposed surfaces is very
important and challengeable.
Polyol reduction is a typical technique for the preparation of
Ag nanocrystals by reducing Ag+ ions in the presence of a
polymer surfactant such as poly(vinylpyrrolidone) (PVP) in
ethylene glycol (EG).4 We have carried out a systematic study
of the synthesis of Ag nanostructures by the polyol method
under microwave (MW) and oil-bath heating.5 There are two
purposes in our studies. One is to develop a rapid shape and
size control synthesis method for Ag nanostructures. The other
is to understand the mechanism of the formation of various Ag
nanostructures in the polyol reduction. In a previous paper, we
examined the effects of bubbling gas and found that bubbling
O2 gas into the reacting solution facilitated the formation of
monodispersed Ag nanowires with a high yield (>90% without
isolation).5f
these truncated Ag nanocrystals. We have also found that the
presence of Cl anions is necessary for the preparation of such
¹
anisotropic Ag nanostructures as cubes, bipyramids, and nano-
wires with sharp corners/edges in the polyol process.5 However,
optimum conditions for preparing each kind of different aniso-
tropic Ag nanostructure have not been obtained under oil-bath
heating. In this work, to determine the optimum conditions for
the preparation of each anisotropic Ag nanostructure under
¹
oil-bath heating and to clarify roles of Cl such as the formation
¹
of AgCl seeds, adsorption of Cl to specific facets of Ag
nanoparticles, and shape-selective etching of Ag nanoparticles
¹
by Cl /O2, we examined the dependence of shapes and sizes of
the anisotropic Ag nanostructures on the concentration of added
NaCl, the solution temperature, and the injection temperature of
NaCl to the original AgNO3/PVP/EG solution. We discuss
¹
the roles of Cl anions for the preparation of these anisotropic
Ag nanostructures in the polyol experiments.
In general, Ag nanostructures can be prepared under both
oil-bath and rapid MW heating. An advantage of the oil-bath
heating is that the temperature of reaction solution increases
more slowly than that under MW heating. Furthermore, the
reaction flask is open to outside. Therefore, taking the reacting
solution sample in each reaction stage, that is necessary for
evaluating the growth mechanism of the Ag nanostructures, is
easy. This is a major reason why the oil-bath heating method
has been used to preparing the Ag nanostructures in the present
study.
Wiley et al.6 prepared single-crystal cubes and tetrahedrons
of silver with truncated corners/edges in high yields by reduc-
ing AgNO3 with EG heated to 148 °C in the presence of PVP
and a trace amount of NaCl. It took more than 45 h to prepare
Published on the web October 8, 2009; doi:10.1246/bcsj.82.1304

X. Tang et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 10 (2009) 1305
room temperature to 198 °C in an oil bath. After cooling the
solution down to room temperature, 0.6 mL of NaCl (10 mM in
EG) was added. The concentration of NaCl was 0.3 mM. Then the
solution was heated to 198 °C.
Experimental
Materials. AgNO3 (99.8%), NaCl (99.5%), and EG (99.5%)
used for this study were purchased from Kishida Chemical
Industries Ltd. The PVP powder (average molecular weight in
monomer units, MW: 40000) was purchased from Wako Pure
Chemical Industries Ltd. All these reagents were used without
further purification.
Effect of Concentration of NaCl. First, 17.4 mL EG solution
containing 0.585 g PVP (MW: 40000) and 0.6 mL NaCl solution
(0-15 mM in EG) were placed in a 100 mL three-necked flask
to form a homogeneous solution. Then, 2 mL AgNO3 solution
(0.465 M in EG) was added to the above solution. The concen-
trations of AgNO3, NaCl, and PVP in EG were 46.5, 0-0.45, and
264 mM, respectively. The mixture was heated from room temper-
ature (20 °C) to the boiling point of EG (198 °C) in an oil bath.
Magnetic stirring was applied throughout the entire synthesis in
this work. After the reaction finished, the system was naturally
cooled to room temperature.
Effect of AgCl. First, 19.1 mL EG solution containing 0.585 g
PVP (MW: 40000) and 0.6 mL NaCl (0.01 M in EG) was placed
in a 100 mL three-necked flask. Then, 0.3 mL AgNO3 solution
(0.02 M in EG) was added to the resulting solution. Finally,
the mixture including equal molar NaCl (0.3 mM) and AgNO3
(0.3 mM) was heated from room temperature to 198 °C in an oil
bath and kept it at 198 °C for 10 min.
Effect of Temperature at a Constant NaCl Concentration.
The experimental procedure used for studying the effect of
temperature at a constant NaCl concentration was similar to that
described in “Effect of concentration of NaCl” except for the final
temperature. After reaction solutions were heated to 120 and
148 °C, solutions were kept at the same temperatures for 1-2.5 h.
Effect of Addition of NaCl on Spherical Ag Nanoparticles.
Spherical Ag nanoparticles were prepared without the addition
of NaCl. 20 mL EG solution containing 46.5 mM AgNO3 and
264 mM PVP in a 100 mL three-necked flask was heated from
Effect of Addition Temperature of NaCl on Reagent Solu-
tion. First, 17.4 mL EG solution containing 0.586 g PVP (MW:
40000) and 2 mL AgNO3 solution (0.465 M in EG) were placed in
a 100 mL three-necked flask. Then the mixture was heated from
room temperature in an oil bath. During the heating process,
0.6 mL NaCl solution (0.01 M in EG) was added to the mixture at
different temperatures. The concentrations of AgNO3, NaCl, and
PVP in EG were 46.5, 0.3, and 264 mM, respectively. The reaction
system was then cooled naturally to room temperature after
reaching the boiling point of EG (198 °C). The reacting solution
experienced a series of color changes before the color became
stable.
Characterization. Morphologies of the Ag nanoparticles were
observed using transmission electron microscopy (TEM; JEOL
JEM-2010 at 200 kV). Product solutions were centrifuged at
12000 rpm three times for 30 min to ensure complete collection
of the products each time. The precipitates were collected then
re-dispersed in ethanol. Samples for TEM measurements were
prepared by placing a droplet of the colloidal solutions on the
carbon-coated grids. Ultraviolet/visible (UV-vis) extinction spec-
tra were obtained (UV-3600; Shimadzu Corp.) using a quartz cell.
The sample solution was diluted with EG. Aliquots of the solution
were taken separately at various temperatures in the course of one
reaction using glass pipettes when the reaction was monitored by
UV-vis spectra. The original solutions used for UV-vis extinction
spectra detection were diluted by a factor of 100 with an EG
solvent.
Results and Discussion
Effect of NaCl Concentration. Ag nanostructures were
prepared at various NaCl concentrations from 0 to 0.45 mM.
(b) 0.12 mM
(a) 0.075 mM
{100}
cube
{100}
200 nm
200 nm
triangular
bipyramid
(c) 0.30 mM
(d) 0.45 mM
{111}
rods, wires
{100}
2 µm
500 nm
Figure 1. Typical TEM images obtained at 198 °C at various concentrations of NaCl: (a) 0.075, (b) 0.12, (c) 0.30, and (d) 0.45 mM.

¹
1306 Bull. Chem. Soc. Jpn. Vol. 82, No. 10 (2009)
Roles of Cl in Synthesis of Ag Nanostructures
When the Ag nanostructures were prepared without addition of
NaCl, spherical particles with diameters of 20-100 nm were
obtained, as will be shown in “Effect of addition of NaCl to
spherical Ag nanoparticles.” Figures 1a-1d show TEM images
of the products obtained at NaCl concentrations of 0.075,
0.12, 0.30, and 0.45 mM, respectively, where cubes, triangular
bipyramids, one-dimensional (1-D) rods/wires, and quasi-
spherical particles are produced. Based on TEM observations
from various view angles and their selected area electron
diffraction patterns, as reported previously,5c-5e cubes, bipyra-
mids, and nanowires have the crystal structures presented in
Figure 1. Yields and average sizes of each Ag nanostructure at
various NaCl concentrations are presented in Tables 1 and 2.
At a low NaCl concentration of 0.075 mM, although products
were mainly composed of spherical particles (65%) with
diameters of 85 « 26 nm, small amounts of cubes (18%),
bipyramids (11%), and short rods (6%) were obtained. Some of
the cubes, bipyramids, and short rods do not have sharp edges.
It could be attributed to an extreme low NaCl concentration so
that few AgCl nanoseeds were supplied for the further growth
of well crystalline structures and slow oxidative etching by O2/
the Ag cubes, 0.30 mM for the preparation of the monodis-
persed 1-D products, and 0.45 mM for the formation of the 1-D
products with larger diameters.
Figure 2 shows the UV-vis spectra of product solutions at
198 °C at various NaCl concentrations. As well-known, Ag
nanostructures with different shapes and sizes exhibit different
surface plasmon resonance (SPR) bands.1,5-10 The UV-vis
spectrum of the product solution prepared without addition
of NaCl shows a typical symmetric SPR band with a peak
at 436 nm. This peak can be attributed to the extinction of
spherical particles. When 0.075 mM of NaCl was added, the
SPR band becomes broad with a flat peak in the region of 400-
500 nm. This peak arises from overlap of SPR bands resulting
from spherical particles, cubes, bipyramids, and nanorods of
various sizes. When the concentration of NaCl increased to
0.12 mM, a sharp and strong SPR band with a peak at 466 nm
appears. This peak can be assigned to SPR band of the Ag
cubes with an average diameter of ca. 50 nm. At a relatively
high NaCl concentration of 0.30 mM, the strongest SPR peak
at 395 nm and a weak shoulder peak at 355 nm occur. These
peaks are assigned to two different transversal modes of the
1-D products with a pentagonal cross section, corresponding
respectively to the out-of-plane dipole resonance and out-of-
plane quadrupole resonance modes.10c,10d Another weaker
shoulder peak with a long tail band above 450 nm can be
ascribed to the overlapping of the in-plane quadrupole and
dipole resonance modes of the Ag nanowires with the peaks at
445 and 514 nm, respectively.10d When the NaCl concentration
¹
Cl . At the NaCl concentration of 0.12 mM, significant change
in the product shapes occurred. Major products were cubes
(61%) with average diameters of 46 « 8 nm and quasi-spherical
particles (30%) with average sizes of 68 « 19 nm. At the
NaCl concentration of 0.30 mM, the yield of cubes greatly
decreased to 8% and 1-D products with diameters of 37 « 8 nm
and lengths of 3-24 ¯m became major products (75%). At
the highest NaCl concentration of 0.45 mM, the yield of 1-D
products decreased to 47%. But the diameters of the 1-D
products increased to 100 « 60 nm with shortening of their
lengths (0.3-3 ¯m). On the basis of these observations,
obviously, the addition of different amounts of NaCl could
tune morphologies and sizes of the final products from spheres
via cubes to 1-D rods/wires. We have found that the NaCl
concentration of 0.12 mM could be the best for the synthesis of
2.5
0 mM
0.075 mM
2.0
0.12 mM
0.30 mM
1.5
1.0
0.5
0.0
0.45 mM
Table 1. The Yield of Each Product (%) Obtained by
Heating AgNO3 (46.5 mM)/NaCl/PVP (264 mM)/EG
Solutions to 198 °C at Different NaCl Concentrations
Concentration
of NaCl/mM
Triangular Rods and
Cubes
Quasi-spheres
bipyramids
wires
300
400
500
600
700
800
0.075
0.12
0.30
0.45
18
61
8
11
6
10
18
6
3
75
47
65
30
7
Wavelengh / nm
Figure 2. UV-vis spectra of the reaction solution at NaCl
concentration of 0, 0.075, 0.12, 0.30, and 0.45 mM.
10
25
Table 2. Average Sizes of Each Product Obtained by Heating AgNO3 (46.5 mM)/NaCl/PVP (264 mM)/EG
Solutions to 198 °C at Different NaCl Concentrations
Edge length/nm
Rods and wires
Quasi-spheres
Diameter/nm
Concentration
of NaCl/mM
Triangular
Cubes
Diameter/nm
Length/¯m
bipyramids
0.075
0.12
0.30
0.45
49 « 11
46 « 8
50 « 14
74 « 20
74 « 22
52 « 6
105 « 19
66 « 10
27 « 8
30 « 3
37 « 8
0.08-0.45
0.2-1.5
3-24
85 « 26
68 « 19
77 « 12
51 « 9
100 « 60
0.3-3

X. Tang et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 10 (2009) 1307
was increased to 0.45 mM, the UV-vis extinction has peaks at
355 and 403 nm and a broad tail band in the longer wavelength
region above 400 nm. This indicates that the products included
a large number of nanorods with larger diameters.
Figure 3a depicts a typical TEM image of the Ag nano-
particles obtained under oil-bath heating at 120 °C for 2.5 h.
Major products were nanorods (57%) with an average diameter
of 39 « 21 nm and length range of 0.4-3.5 ¯m. Among
them, bipyramids with an average size of 116 « 35 nm take a
relatively high yield (31%) and the yield of cubes is only 2%.
Figure 3b presents a typical TEM image obtained at 148 °C
The yield of 1-D nanostructures increases to 68%, whereas
that of the bipyramids decreases to 18% and the yield of the
cubes remains low (3%). We have synthesized 1-D Ag products
in a high yield (75%) at 198 °C. These findings suggest that
keeping the reaction solution at low temperatures such as 120
Effect of Temperature at a Constant NaCl Concentra-
tion. Based on the findings described above, we believe that
the shape evolution of Ag nanostructures depends strongly on
¹
the concentration of Cl . Further experiments were carried out
to examine the effect of temperature on the final products.
Keeping the concentration of NaCl at a constant value of
0.30 mM, the Ag nanostructures were synthesized at lower
temperatures of 120 and 148 °C. Original reaction solutions
were heated from room temperature to 120 °C and kept at this
temperature for 2.5 h or heated to 148 °C and kept at this
temperature for 1 h. Since the reduction rate of Ag+ at 120 °C is
much slower than that at 148 °C, a longer heating time is
necessary to complete the reduction of Ag+ ions. Yield and
average size of each product are given in Tables 3 and 4. For
comparison, the corresponding Ag products synthesized at
198 °C are also present. In this case, the reaction was stopped
just after the solution was heated from room temperature to
198 °C for about 20 min.
Table 3. The Yields of Each Product (%) Obtained
by Heating AgNO3 (46.5 mM)/NaCl (0.3 mM)/PVP
(264 mM)/EG Solutions to Different Temperatures
Temperature
Triangular Rods and
Cubes
Quasi-spheres
/°C
bipyramids
wires
120
148
198
2
3
8
31
18
10
57
68
75
10
11
7
Table 4. Average Sizes of Each Product Obtained by Heating AgNO3 (46.5 mM)/NaCl (0.3 mM)/PVP (264 mM)/
EG Solutions to Different Temperatures
Edge length/nm
Rods and wires
Quasi-spheres
Diameter/nm
Temperature
Triangular
/°C
Cubes
Diameter/nm
Length/¯m
bipyramids
120
148
198
66 « 7
79 « 12
50 « 14
116 « 35
135 « 45
105 « 19
39 « 21
42 « 23
37 « 8
0.4-3.5
0.3-3.7
3-24
37 « 17
72 « 24
77 « 12
(a-1) 120oC
(b-1) 148oC
1 µm
500 nm
(b-2)
(a-2)
200 nm
200 nm
Figure 3. Typical TEM images of Ag nanoparticles obtained at (a) 120 and (b) 148 °C.

¹
1308 Bull. Chem. Soc. Jpn. Vol. 82, No. 10 (2009)
Roles of Cl in Synthesis of Ag Nanostructures
(a)
(b)
100 nm
100 nm
Figure 4. Typical TEM image of Ag nanostructures obtained from (a) AgNO3 (0.3 mM)/NaCl (0.3 mM)/PVP (264 mM)/EG
system and (b) addition of 1 mL of AgNO3 (0.93 M) at 185 °C to the same solution as that used in experiment (a) and heated for 1 h.
and 148 °C will be favorable to the production of single-twin
triangular bipyramids, while yields of single crystal cubes and
1-D products become lower. No significant differences were
observed in average sizes between the cubes and bipyramids
prepared at 120 and 148 °C and those obtained at 198 °C.
Although average diameters of the 1-D products obtained at
120 and 148 °C are similar to that obtained at 198 °C (ca.
40 nm), lengths of the 1-D products obtained in the lower
temperatures (0.3-3.7 ¯m) are shorter than that at 198 °C
(3-24 ¯m) by about one order of magnitude.
The growth process of the Ag nanostructures was monitored
by UV-vis spectra (Figures S1a and S1b in Supporting
Information). UV-vis spectra of the Ag nanoparticles prepared
at 120 °C (Figure S1a) present a symmetric SPR band with a
peak maximum at ca. 410 nm due to the appearance of smaller
Ag seeds. The peak intensity increases after heating for 25 min.
After heating for 80 min, two weaker peaks of Ag nanorods
appear at 350 and 390 nm and a stronger peak of cubes and
bipyramids is observed at 440 nm. The SPR peaks of the 1-D
products become distinguished, whereas that of the cubes
and bipyramids becomes weak, when heating time reaches
100 min. After that, the intensities of SPR peaks of 1-D
products enhance continuously but peak positions are nearly
unchanged at 2.5 h. Because the UV-vis spectrum for the 1-D
Ag nanostructures is significantly stronger than those for other
shaped particles such as cubes and bipyramids, the extinction
peaks for the Ag cubes and bipyramids are submerged when
their yields are less.
particles. Initially, large spherical particles with the diameter
range in 20-100 nm were prepared under heating at 198 °C
without addition of NaCl in the original solution. Then, NaCl
solution was added to the original solution. The final concen-
tration of NaCl in the mixture was 0.3 mM, which was the same
concentration as that used for the preparation of the 1-D Ag
nanowires in high yield. The mixture was heated again to
198 °C for about 20 min. To examine the shape transition of the
spherical Ag particles, UV-vis spectra of the reaction solution
were measured at various temperatures (Figure S2a in Sup-
porting Information). The UV-vis data indicate little changes in
peak intensity and band width of the typical symmetric SPR
band for the spherical Ag nanoparticles. This implies that
the shapes and sizes of the Ag spherical particles are not
significantly influenced by the addition of NaCl at 198 °C.
Figure S2b (Supporting Information) shows the TEM image
of the final product obtained at 198 °C. All of the particles are
still spherical and little anisotropic product is obtained. The
TEM observation suggests that it is difficult to etch larger
¹
spherical Ag particles by Cl /O2. Apparently, the shape tran-
sition of the Ag particles from spherical to anisotropic crystals
does not occur once the larger spherical particles are formed.
This is also consistent with little change in UV-vis spectra of
the above reaction solution containing the spherical Ag
particles by the addition of NaCl at 198 °C. The contrast study
¹
reveals that the presence of Cl ions at the early stage before
the formation of the large Ag particles could be important for
the formation of the anisotropic Ag nanostructures.
Evolution of the SPR bands with reaction time at 148 °C is
very similar to that at 120 °C though the reduction rate is much
faster at the higher temperature (Figure S1b). A very weak SPR
band appears at 100 °C, and an obvious symmetric peak for the
Ag seeds is observed at 148 °C. SPR bands observed after heat-
ing for 8, 10, 17, and 60 min at 148 °C are similar to those after
heating for 80, 100, 120, and 180 min at 120 °C. These findings
are consistent with TEM image observations of the Ag products
where no significant differences are found for the distributions
and sizes of final products between 120 and 148 °C.
Effect of AgCl for the Formation of Anisotropic Ag
Nanostructures. An important role of the addition of NaCl at
low temperature may be the initial formation of AgCl, which
acts as seeds for the formation of anisotropic Ag nano-
structures. Since the solubility constant of AgCl in EG is
expected to be very low at room temperature, almost all Cl in
NaCl can be converted to AgCl when 0.3 mM AgNO3 and
0.3 mM NaCl were premixed at room temperature. When
the solution was heated to 198 °C, cubes and bipyramids with
less sharp edges were obtained (Figure 4a). However, when
new AgNO3 solution was added to the solution at 185 °C
dropwise, and subsequently the whole reaction system was
heated to 198 °C and kept at this temperature for 1 h, mixtures
of Ag cubes and bipyramids with sharp edges were obtained
(Figure 4b). These results indicate that the nanoparticles
Effect of Addition of NaCl to Spherical Ag Nanoparti-
cles. An important effect of the addition of NaCl is shape
¹ 1,5f,6
selective oxidative etching by O2/Cl .
Here, O2 gas can
dissolve in EG. We examined whether large spherical Ag
¹
particles can be etched by O2/Cl and changed into anisotropic

X. Tang et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 10 (2009) 1309
produced at the first step become seeds for overgrowth of the
cubes and bipyramids. There are few 1-D products grown from
these seeds. Since Cl /O2 (dissolved in EG) can etch cubes and
bipyramids,5f quasi-spherical particles were observed under our
experiment conditions. An important finding in this section is
that AgCl seeds prepared at room temperature can be converted
to cubes and bipyramids during heating from room temperature
to 198 °C. However, the transition from AgCl seeds to 1-D
products via decahedrons does not happen.
TEM images (not shown here) of the final products obtained
by injecting NaCl at 80 °C show little difference in comparison
to those obtained by the injection of NaCl at room temperature
(20 °C), shown in Figure 1c. This implies that the injection of
NaCl before nucleation starts has little effect on the shapes and
sizes of final products. Figures 6a-6d show TEM images of the
final products obtained by the addition of NaCl when reaction
solution temperature reaches 120, 154, 160, and 170 °C,
respectively. Distributions of each product and their average
sizes are listed in Tables 5 and 6. Results obtained by injecting
NaCl at 198 °C (not shown here) are essentially identical with
those produced at 170 °C. With the increase in the injection
temperature of NaCl from 20 to 170 °C, the yield of 1-D
products decreases from 75 to 1%, whereas that of spherical Ag
particles increases from 7 to 99%. Yields of the Ag cubes and
bipyramids increase from about 10 to 20% with the temperature
change from 20 to 120 °C, and then they decrease to zero when
increasing the addition temperature from 120 to 170 °C.
Average sizes of the Ag cubes and bipyramids decrease from
50 « 14 and 105 « 19 nm to 37 « 5 and 49 « 11 nm, respec-
tively, with increasing the addition temperature from 20 to
160 °C. The length of 1-D products becomes much shorter from
3-24 to 0.08-0.1 ¯m with increasing the addition temperature
from 20 to 170 °C.
Figures S3a-S3d (Supporting Information) show the UV-vis
spectra measured from the solutions sampled at various
temperatures. In all cases, before injection of NaCl, a
symmetric SPR band with a peak at ca. 410 nm appears at ca.
100 °C and becomes stronger and broader with the increase of
the reaction solution temperature. The shapes and wavelengths
of SPR bands depend on the injection temperature of NaCl. The
extinction spectra obtained by the injection of NaCl at 120 °C
give peaks A and B related to 1-D products and weak peak C
related to cubes and bipyramids. With the increase in the
injection temperature of NaCl, peaks A and B become less
prominent and a symmetric SPR peak D related to quasi-
spherical particles becomes prominent. When NaCl was added
at 160 and 170 °C, peaks A and B become weak or disappear
and broad SPR peaks D related to spherical particles are
strongly observed. These observations are consistent with the
observations from TEM images of final products.
¹
Effect of Addition Temperature of NaCl to Reaction
Solution. We have shown that the concentration of NaCl in
the initial solution is an important factor for the preparation of
anisotropic Ag nanostructures. In the previous section, we
have also demonstrated that AgCl nanoparticles formed at low
temperature can act as precursors of the produced Ag nano-
particles with well-defined shapes. To further understand the
effect of NaCl, we have performed a series of experiments by
adding NaCl to the original solution at different reaction
temperatures (80, 120, 154, 160, 170, and 198 °C) and heating
the solution to 198 °C or keeping it at 198 °C when NaCl was
added at 198 °C. The change of the reaction solution with
temperature has been continuously traced by observing UV-vis
spectra. We have found that no SPR peak appears below 90 °C
(not shown in Figure 5), indicating that the reduction of Ag+
ions and the nucleation of Ag nanoparticles do not happen at
this low temperature because EG molecules decompose at the
temperature above ca. 90 °C on the basis of TG measurement of
pure EG. In Figure 5, a weak symmetric SPR peak at ca.
410 nm appears at 120 °C due to the formation of small Ag
seeds. With the increase of the solution temperature from 154
to 198 °C, the SPR band gradually becomes stronger and wider
due to the continual growth of the spherical Ag particles.
Assuming that the intensity of the SPR peak is proportional to
amount of formed metallic Ag, we estimate that about 20, 50,
90, and 100% of the total amount of AgNO3 could be reduced
and converted into Ag nanoparticles at 120, 154, 160, 170, and
198 °C, respectively.
1.2
120 oC
154 oC
The most important finding in this session is as follows. To
obtain anisotropic Ag nanoparticles in high yields, injection of
1.0
¹
160 oC
170 oC
NaCl at low temperature is required. This indicates that Cl
0.8
plays an important role just at nucleation and initial crystal
growth stages. It is difficult to transfer spherical particles into
198 oC
¹
0.6
anisotropic particles by Cl /O2 oxidative etching at high
temperatures with the injection of NaCl. The concentration of
AgCl decreases when increasing the injection temperature
because the solubility constant of AgCl in EG increases with
the increase in the solution temperature. We have found that
AgCl also is an important precursor for the formation of cubes
and bipyramids. The decrease in yields of cubes and bipyra-
mids with the increase of injection temperature of NaCl might
be due to the decrease in the concentration of AgCl at higher
injection temperatures.
0.4
NaCl
injection
0.2
0.0
300
400
500
600
700
800
Wavelength / nm
Formation Mechanism of Ag Nanostructures. From the
present experiments, we have distinctly revealed that morphol-
ogies of the final products are related to various experimental
Figure 5. UV-vis spectra of Ag nanoparticles when
AgNO3/PVP/EG solution was heated from room temper-
ature (20 °C) to 198 °C.

¹
1310 Bull. Chem. Soc. Jpn. Vol. 82, No. 10 (2009)
Roles of Cl in Synthesis of Ag Nanostructures
¹
parameters such as the concentration of NaCl, temperature of
reaction solution, and the injection temperature of NaCl. There
are three possible roles played by NaCl. The first is the
formation of AgCl, which may become seeds of anisotropic Ag
Cl anions can more easily adsorb on {100}-type facets of the
Ag particles so that the {100}-facet-dominated Ag nanocrystals
¹
are preferentially produced. The third is that Cl ions can also
act as an etchant in the presence of O2 in EG. The first effect is
especially important at low temperature, where the solubility
constant of AgCl is very low. Once increasing solution
temperature AgCl is slowly dissolved and decomposed into
¹
nanostructures. The second is that Cl ions act as adsorbers.
Table 5. The Yield of Each Product (%) Obtained
by Heating AgNO3 (46.5 mM)/NaCl (0.3 mM)/PVP
(264 mM)/EG Solutions to 198 °C at Different Injection
Temperatures of NaCl
Ag+ and Cl in EG. Ag+ ions are further reduced into
¹
elemental Ag0. In general, shapes of the final products depend
not only on the stability of crystal structures against etching by
¹
Cl /O2 but also the density of Ag0 atoms around the growing
Injection
temperature/°C
Triangular Rods and
nuclei. A high related ion concentration around the nuclei can
create an environment with high twinning probability for the
formation of such twinned particles as twin-plates and five-
twinned decahedra from which pentagonal 1-D rods and wires
are grown. Thus, five-twinned 1-D products are favorable at
higher concentration of Ag+ ions, whereas single crystal cubes
are preferentially formed at lower concentration of Ag+ ions.
Cubes
Quasi-spheres
bipyramids
wires
20
120
154
160
170
8
21
8
5
0
10
20
12
11
0
75
43
31
19
1
7
16
49
65
99
Table 6. Average Sizes of Each Product Obtained by Heating AgNO3 (46.5 mM)/NaCl (0.3 mM)/PVP (264 mM)/
EG Solutions to 198 °C at Different Injection Temperatures of NaCl
Edge length/nm
Rods and wires
Quasi-spheres
Diameter/nm
Injection
temperature/°C
Triangular
Cubes
Diameter/nm
Length/¯m
bipyramids
20
120
154
160
170
50 « 14
46 « 11
43 « 4
37 « 5
105 « 19
73 « 9
72 « 18
49 « 11
37 « 8
38 « 8
22 « 7
23 « 10
51 « 6
3-24
0.3-0.2
0.1-2
0.1-0.4
0.08-0.1
77 « 12
50 « 27
54 « 22
97 « 49
47 « 15
(b) 154oC
(a) 120oC
500 nm
500 nm
(c) 160oC
(d) 170oC
200 nm
200 nm
Figure 6. Typical TEM images of as-synthesized Ag nanostructures by injection of NaCl into reaction solution at (a) 120, (b) 154,
(c) 160, and (d) 170 °C.

X. Tang et al.
(1) [NaCl] / [AgNO3] = 0
Bull. Chem. Soc. Jpn. Vol. 82, No. 10 (2009) 1311
O2
B
C
A
-
AgNO3
Ag+ + NO3
Ag0 nuclei
EG
Ag0 seeds
Ag0 particles
(2) [NaCl] / [AgNO3] = 1
O2
AgNO3 + NaCl
AgCl + NaNO3
EG
B
C
A
-
Ag+ + Cl
Ag0 nuclei
Ag0 seeds
Ag0 crystals
(3) [NaCl] / [AgNO3] = 0.0065
O2
A
B
C
-
6AgNO3 + NaCl
AgCl + NaNO3 + 5NO3 + 5Ag+
Ag0 nuclei
EG
-
Ag+ + Cl
Ag0 seeds
Ag0 crystals
Scheme 1. Possible nucleation and growth mechanisms of Ag nanostructures at different molar ratios of [NaCl]/[AgNO3].
On the basis of our present study, mechanisms for the
formation of Ag nanostructures can be classified into three
cases as shown in Scheme 1, depending on the molar ratios of
[NaCl]/[AgNO3]. Case (1) is [NaCl]/[AgNO3] = 0. Without
the addition of NaCl, Ag+ ions are reduced to Ag0, nucleation
occurs, and then the Ag seeds are formed and their growth leads
to spherical particles formed randomly. Case (2) is [NaCl]/
[AgNO3] = 1. When equal molar concentrations of AgNO3 and
NaCl were mixed at room temperature, AgCl is formed because
of the small solubility constant of AgCl. In this case, AgCl is
of 80, 120, 154, 160, 170, or 198 °C where the reaction stage
is different. At 80 °C, the reduction does not start (A in
Scheme 1). At 120 °C, just small seeds start to produce (B in
Scheme 1), while at 154 and 160 °C, Ag seeds become larger
(C in Scheme 1). The reduction reaction nearly finishes at
170 °C and completely finished at 198 °C. We have found that
the yield of spherical particles increases from 7 to 16% when
the injection temperature of NaCl increases from 20 to 120 °C.
This implies that complete re-dissolution of spherical seeds to
Ag+ and re-reduction leading to anisotropic Ag nanostructures
does not occur even when NaCl was added to the solution just
after the production of small Ag seeds. Thus, we conclude that
major roles of NaCl have two points in our study. One is to
form AgCl seeds at lower temperatures and another is to act as
adsorbers to selectively stablize {100} facets during crystal
growth.
¹
dissolved as Ag+ and Cl with increasing temperature. After
then Ag+ ions are reduced to Ag0 and nucleation, seed
formation, and crystal growth take place. Since the concen-
tration of Ag+ ions is low, twinning probability is low.
Therefore, single crystal cubes and single-twin bipyramids are
produced preferentially. Case (3) is an intermediate between
the above two cases: [NaCl]/[AgNO3] = 0.0065. In this case
although AgCl is formed, a large excess amount of AgNO3 is
present in the solution. The standard electrode potential of
We have also found that the concentration of NaCl strongly
affects the final shapes of products. Cubes are produced
preferentially at the NaCl concentration of 0.12 mM, whereas
1-D products are major products at the NaCl concentration
of 0.30 mM. In our previous experiments under bubbling
O2,5f we found that cubes and bipyramids are etched by
¹
¹
AgCl + e ¼ Ag + Cl (+0.2233 V vs. SHE) is lower than
that of Ag+ + e ¼ Ag (+0.799 V vs. SHE). Under such
¹
conditions, most of the Ag0 atoms do not arise from AgCl but
from free Ag+. In this case, major products are five-twinned
1-D particles because the concentration of Ag+ ions is
sufficient to provide a high probability for twinning.
¹
¹
Cl /O2, whereas decahedrons can survive Cl /O2 etching
and grow into 1-D rods or wires under certain conditions.
Thus, one reason for the favorable formation of the cubes
at lower NaCl concentration may be explained in terms of
a slower etching of the cube seeds due to the low NaCl
concentration.
¹
Although the real role of Cl anions leading to different
anisotropic Ag nanostructures in each condition is still unclear,
the injection of NaCl at different temperature provides some
¹
information about whether O2/Cl etching is significant or not
When AgNO3 was reduced at 120 and 154 °C at the NaCl
concentration of 0.30 mM, major products were the 1-D
products and bipyramids and little cubes could be prepared.
for the formation of anisotropic Ag nanostructures under the
present conditions. We injected NaCl at solution temperatures

¹
1312 Bull. Chem. Soc. Jpn. Vol. 82, No. 10 (2009)
Roles of Cl in Synthesis of Ag Nanostructures
Under these conditions, seeds of cubes are either difficult to be
formed or their etch rates by O2/Cl are faster than those of the
bipyramids and 1-D products.
Supporting Information
¹
TEM images and UV-vis spectra of Ag nanostructures prepared
from AgNO3/PVP/EG and AgNO3/NaCl/PVP/EG mixtures at
various conditions. This material is available free of charge on the
web at http://www.csj.jp/journals/bcsj/.
We have demonstrated that the final Ag products depend on
the injection temperature of NaCl. If NaCl was injected into the
¹
solution at a low temperature, most of the Cl ions exist as
AgCl nuclei because the solubility constant of AgCl in EG is
very small at low temperature. The AgCl particles first nucleate
and then serve as the seeds for the growth of Ag nanocrystals.
On the contrary, because the solubility constant of AgCl
enhanced rapidly at high temperature, some AgCl nuclei
dissolve in EG solution. In addition, the standard electrode
potential of Ag+ is higher than AgCl. It means that Ag+ ions
can be reduced into Ag atoms much easier than AgCl. Usually
without the introduction of NaCl, the reduction process was
finished at a relatively low temperature. Therefore, when we
injected NaCl into the solution including AgNO3 beyond
170 °C, most of the Ag+ ions have been reduced and large
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of various Ag nanostructures. Various experimental parameters
examined here give some new information on optimum
experimental conditions for the preparation of various aniso-
tropic Ag nanostructures by the polyol reduction method.
This work was supported by the Joint Project of Chemical
Synthesis Core Research Institutions, a Grant-in-Aid for
Scientific Research on Priority Areas “Unequilibrium electro-
magnetic heating” and Grant-in-Aid for Scientific Research
(B) from the Ministry of Education, Culture, Sports, Science
and Technology of Japan (Nos. 19033003 and 19310064),
and Kyushu Univ. GCOE program “Novel Carbon Resource
Sciences.” We thank IMCE of Kyushu University for financial
support to Prof. P. Jiang as a visiting professor during 2007.4-
2007.6.