Morphology-control in microwave-assisted synthesis of silver particles in aqueous solutions

Article abstract of DOI:10.1246/bcsj.77.757

The morphologies of Ag particles can be controlled between distorted sphere and prism by employing microwave-promoted reductions of highly concentrated 0.1 M silver nitrate in aqueous solutions. Microwave (MW)-induced 1-min heating of aqueous solutions (≤ 0.1 M) of silver nitrate gave aggregates of sphere-nanoparticles in the presence of trisodium citrate (0.1-1.5 M) and excess of formaldehyde (1.5 M). Sample A1, prepared in the presence of an equal quantity (0.1 M) of trisodium citrate to silver nitrate, resulted in the formation of sphere-Ag particles with a smaller average particle size (24 nm) and a narrower particle size distribution (10-90 nm) than sample A2, prepared by the conventional heating (average size 71 nm, size distribution 20-360 nm). In addition to the MW effect, when the concentration of sodium citrate was increased, the inter-molecule hydrogen bonding of the citrate moieties associated with Ag particles should enhance the collision probability of the particles. Thus, large Ag particles with a wide particle distribution were produced in the presence of a larger amount of trisodium citrate under MW irradiation.

Full text of DOI:10.1246/bcsj.77.757

Ó 2004 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 77, 757–761 (2004)
757
Morphology-Control in Microwave-Assisted Synthesis
of Silver Particles in Aqueous Solutions
ꢀ
Tetsushi Yamamoto, Hengbo Yin,1 Yuji Wada, Takayuki Kitamura, Takao Sakata,2
Hirotaro Mori,2 and Shozo Yanagida
ꢀ
Material and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871
1Department of Chemical Engineering, Jiangsu University, Zhenjiang 212013, China
2Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, Ibaraki, Osaka 567-0047
Received October 7, 2003; E-mail: ywada@mls.eng.osaka-u.ac.jp
The morphologies of Ag particles can be controlled between distorted sphere and prism by employing microwave-
promoted reductions of highly concentrated 0.1 M silver nitrate in aqueous solutions. We can selectively synthesize either
sphere-Ag or prism-Ag particles by changing only an additive, i.e., either trisodium citrate/formaldehyde or poly(N-vi-
nyl-2-pyrrolidone) (PVP). The particle sizes and size distributions of sphere-Ag particles are strongly dependent on the
concentration of trisodium citrate.
Nanosized metallic materials, such as silver, gold, and other
noble metals, have displayed considerable phenomena due to
their strong size- and shape-dependent properties.1–5 Further-
more, new size-dependent properties of sub-micrometer parti-
cles, such as optical properties, may merge. The morphology
control of these particles has a strong dependence on the reac-
tion conditions. Sphere-Ag nanoparticles have been synthe-
sized by classical Turkevich methods,6,7 a reversed micelle
process,8 photoreductions,9,10 ultrasonic radiation,11 thermal
decomposition,12 and 60Co ꢀ-irradiation.13 Recently, morphol-
ogy controls of metal particles have been regarded as the next
target in the preparations of nanosized metals, for example,
wire-,4,9,14 prism-,10 and cube-Ag,15 polyhedron-, cube-, and
prism-Pt,5,16 rod-Co,17 prism-Pd or Ni15 by using capping mole-
cules, such as poly(N-vinyl-2-pyrrolidone) (PVP),14,15 sodium
polyacrylate,5,16 and tetra-n-octylammonium carboxylate.17
Microwave (MW) dielectric loss heating is rapidly emerging
as a newly accepted heating technology for a variety of organic
and inorganic syntheses.18 In our MW-assisted syntheses of
nanosized nickel and CdS particles, we found that MW heating
can give a smaller size distribution of the nano-sized nickel and
CdS in a narrower range than the conventional outside heating.
We ascribe such interesting results to homogeneous and rapid
heating caused by a MW (2.45 GHz)-matched dielectric loss
of dipolar molecular circumstances.19,20 This interesting heat-
ing effect can be called ‘‘homogeneous MW-matched dielectric
heating’’. Keeping the concept of MW heating and these suc-
cesses in mind, we decided to try applying a MW heating tech-
nique for the morphology-control of the nano-size particle
shape in addition to the size and its distribution. We discuss
here how sphere- or prism-silver (Ag) particles are selectively
synthesized by the reduction of silver nitrate in aqueous solu-
tions up to 0.1 M concentration under microwave irradiation
in the presence such additives as trisodium citrate and poly-
(N-vinyl-2-pyrrolidone) (PVP), which may work as a heat gen-
erator in homogeneous MW-matched dielectric heating.
Experimental
A domestic MW oven (Sanyo EM-650T, having a maximum
power output of 650 W and a magnetron frequency of 2.45 GHz)
was modified by installing a condenser through a hole on the ceil-
ing and a magnetic stirrer at the bottom (Shikoku Keisoku Kogyo
Co., ZMW-002). A 100 mL Pyrex or quartz glass flask was placed
at the center of the oven as a reaction vessel. Additives, such as
trisodium citrate/formaldehyde or poly(N-vinyl-2-pyrrolidone)
(PVP; Wako; average Mw, 40000; concentrations of PVP ex-
pressed as polymer units of PVP in Table 1), were added into a sil-
ver nitrate aqueous solution. The mixture solution in the flask was
irradiated by MW at 650 W for an appropriate time. After MW
irradiation, as-prepared samples were washed with 50 mL of ion-
exchanged water three times to remove any unreacted additives,
and separated by centrifugation, giving product samples A1 and
A3–A8 (Table 1).
The product samples were kept in methanol for powder X-ray
diffraction (XRD) and scanning electron microscopy (SEM) analy-
ses. Transmission electron microscopy (TEM) was undertaken
with a Hitachi H-8000 electron microscope. An electron diffraction
analysis was carried out to certify the crystal phases of the Ag par-
ticles. A TEM sample was prepared by dropping a silver suspen-
sion onto an amorphous-carbon coated copper grid. The particle
sizes of the silver particles were determined by directly measuring
the sizes of randomly chosen particles on the TEM images. Ultra-
violet–visible (UV–vis) absorption spectral measurements were
carried out with a U-3300 UV–vis spectrophotometer.
Results and Discussion
Preparation of Nano-Sized Sphere-Silver Particles in
Aqueous Solution. It is well known as a silver mirror reaction,
where metal Ag is formed as silver metal film under a reflux of
aqueous solution of silver nitrate and formaldehyde; the form-
aldehyde therein works as a reducing agent of [Ag(NH3)2]þ
Published on the web April 2, 2004; DOI 10.1246/bcsj.77.757

758 Bull. Chem. Soc. Jpn., 77, No. 4 (2004)
Morphology-Control in MW-Assisted Synthesis of Ag
Table 1. Preparation of Silver Particles under MW (2.45 GHz) Irradiation at 650 W
Sample
Silver nitrate
/M
Trisodium
citrate/M
Formaldehyde
/M
PVP
/M
MW
irradiation
time/min
Conventional
heating timeaÞ
/min
A1
A2
A3
A4
A5
A6
A7
A8
0.1
0.1
0.1
0.1
0.01
0.01
0.1
0.1
0.1
0.1
1.0
1.5
—
—
—
—
1.5
1.5
1.5
1.0
—
—
—
—
—
—
—
—
3
3
3
3
1
—
1
—
8
—
—
—
—
—
—
1
30
90
20
75
a) 50 mL aqueous solution containing reactants was heated from room temperature to the boiling temperature
ꢁ
(taking about 4 min), and kept boiling for a given time under vigorous stirring in an oil bath (150 C). The
conventional heating time was the total reaction time.
(Tollens’ reagent) to metal silver. We found that MW-induced
1-min heating of aqueous solutions (up to 0.1 M) of silver ni-
trate gave aggregates of sphere-nanoparticles in the presence
of trisodium citrate (0.1–1.5 M) and excess of formaldehyde
(1.5 M) (Table 1). Conventional heating using an oil bath also
gave comparable spherical particles (sample A2 in Table 1).
SEM and XRD analyses revealed that all of the spherical par-
ticles were identified as face-centered-cubic (fcc) metal Ag
(powder diffraction file, PDF #4-783). As shown in Fig. 1, sam-
ple A1, prepared in the presence of an equal quantity (0.1 M) of
trisodium citrate to silver nitrate, resulted in the formation of
sphere-Ag particles with a smaller average particle size (24
nm) and a narrower particle size distribution (10–90 nm) than
sample A2 prepared by the conventional heating (average size
71 nm, size distribution 20–360 nm). Therefore, we speculated
that MW irradiation should induce homogeneous and fast nu-
cleation of silver metal from the reduction of silver cation,
and their successive flash growth to nano-sized spherical parti-
cles, as observed in the MW-assisted preparation of nano-sized
nickel metal.19 On the other hand, when the concentration of
trisodium citrate was increased excessively from 0.1 M to
1.0–1.5 M, the average particle sizes were increased (65 nm
at A3 and 132 nm at A4) and the size distributions became wid-
er with an increase of trisodium citrate under MW flash heating.
When trisodium citrate (Na3C6H5O7, 0.1 or 1.0 M) was
added into silver nitrate aqueous solutions, silver citrate
(Ag3C6H5O7, PDF #1-30) was formed as a white powder,
which was confirmed by powder XRD analysis. Therefore,
the formation of sphere-Ag particles should be formed through
silver citrate formation. At a low concentration of trisodium cit-
rate (0.1 M), citrate works as an efficient stabilizer.21 Small
sphere-silver particles were formed under microwave irradia-
tion (Scheme 1). However, at a high concentration of trisodium
citrate (1.5 M), a large excess of trisodium citrate may promote
the ‘‘coalescence’’13 of silver particles, and may affect the di-
electric loss of the reaction medium, leading to rapid heating
under MW irradiation. Therefore, a large excess of trisodium
citrate may spontaneously promote the ‘‘coalescence’’13 of sil-
Fig. 1. SEM images and size distributions of sphere-Ag par-
ver particles and, subsequently, successive particle growth
should occur by a localized-‘‘super heating’’22 or ‘‘hot spot’’23
induced under MW irradiation, as observed in the MW-assisted
preparation of nano-sized CdS.20 In addition to the MW effect,
when the concentration of sodium citrate was increased, the in-
ticles synthesized in silver nitrate (0.1 M) aqueous solution
under MW irradiation by changing trisodium citrate at 0.1
M (A1), 1.0 M (A3), and 1.5 M (A4) or by a conventional
heating in trisodium citrate at 0.1 M (A2). In size distribu-
tions, D is average particle size.

T. Yamamoto et al.
Bull. Chem. Soc. Jpn., 77, No. 4 (2004)
759
Scheme 1.
ter-molecule hydrogen bonding of the citrate moieties associat-
ed with silver particles should enhance the collision probability
of the particles, resulting in the coalescence of silver nanopar-
ticles. As a result, large Ag particles with a wide particle distri-
bution were produced in the presence of a larger amount of tri-
sodium citrate under MW irradiation. In other words, the nucle-
ation and growth of Ag particles is dependent on the concentra-
tion of citrate ion as a MW-matched heating source; thus, the
heating speed should be properly controlled for the MW-assist-
ed synthesis of sphere-Ag particles having a narrow size distri-
bution.
Preparation of Prism-Silver Particles in Aqueous
Solution. Prism-Ag particles were synthesized under MW ir-
radiation in silver nitrate aqueous solutions when PVP was add-
ed instead of trisodium citrate/formaldehyde. When a sample
containing a low concentration of silver nitrate (0.01 M) was
irradiated under MW irradiation for 30 min, the formation of
sphere-Ag particles having an average particle size of 25 nm
with a size distribution of 10–35 nm was observed (Fig. 2,
A5). The selected-area-electron diffraction showed that the
sphere-particle can be assigned to fcc metal silver. By prolong-
ing the MW irradiation time for up to 90 min, the average par-
ticle size of the sphere-Ag particles was increased to 50 nm
with a size distribution of 40–60 nm, accompanying the forma-
tions of several two-dimensional triangles with edge sizes of
70–250 nm (Fig. 2, A6). When the concentration of silver ni-
trate was increased to 0.1 M, both sphere-Ag particles with
an average particle size of 70 nm and triangular Ag particles
with edge lengths of 200–380 nm were formed under MW irra-
diation for only 20 min (Fig. 2, A7). After MW irradiation for
75 min, nearly all of the initial spherical particles were convert-
ed to prismatic structures with edge lengths of 230–420 nm
(Fig. 2, A8). At low concentrations of silver nitrate (0.01 M)
and PVP (0.3 M) after 120 min under MW irradiation, we could
not obtain prism-Ag particles. The prismatic particles were
composed of both perfectly triangular and truncated triangular
prisms. Triangular prisms were ascribed to fcc structures by
electron-diffraction analysis (Fig. 2, inset A8). From {220}
Fig. 2. TEM images of Ag particles synthesized in the pres-
ence of PVP (3 M) under MW irradiation for 30 min (A5)
and 90 min (A6) in silver nitrate (0.01 M) aqueous solution
or for 20 min (A7) and 75 min (A8) in silver nitrate (0.1 M)
aqueous solution, respectively. Inset of A8 shows an elec-
tron diffraction analysis of a prism-Ag particle.
sorption maximum peak shifted slightly towards longer wave-
lengths as the particles became larger. However, in the case
of prismatic particles composed of both perfectly triangular
and truncated triangular prisms, we observed another absorp-
tion shoulder-peak at ca. 470 nm in addition to a ca. 420-nm
peak for the sample synthesized at a high concentration of sil-
ver nitrate (0.1 M) and PVP (Fig. 3b). The shoulder-peak at ca.
470 nm may be due to an in-plane quadrupole resonance caused
by a prism shape, as reported previously.10 However, we could
not detect a strong peak at 670–770 nm, which was observed
in previous work as being due to an in-plane dipole plasmon
resonance.10 This difference can be explained as being due to
the presence of large-size prism-Ag particles with edge sizes
of 230–420 nm in the present case.
ꢀ
Bragg reflections at a lattice spacing of 1.44 A, the top crystal
face of a prism-Ag particle must be (111). An additional weak
1
PVP is a homopolymer having an individual unit containing
an aprotic polar imide group. The oxygen atoms of the imide
groups should have a strong affinity to silver cations,24 supply-
ing electrons to silver cations, causing the thermal reduction
and stabilizing the resulting silver particles through surface ad-
sorption of the PVP chain. When the concentration of silver ni-
trate is increased to 0.1 M, sphere-Ag particles are converted to
prisms with increasing the MW irradiation time, but no small
prismatic embryos can be found, even at the initial stage of
crystallization (Scheme 2). The prism-Ag particles should be
formed by a shape-transformation from a sphere to a prism dur-
ꢁ
diffraction spot corresponding to f422g Bragg reflections at a
3
ꢀ
lattice spacing of 2.50 A was observed due to the local hexag-
onal-like structure in an automatically-flat silver.10 We con-
cluded that high-concentrated silver nitrate (0.1 M) was favor-
able for the formation of prism-Ag particles.
From a UV–vis spectroscopy measurement we observed an
absorption peak at ca. 410 nm due to a characteristic surface
plasmon resonance of a sphere-silver particle synthesized at
the low concentrations of silver nitrate (0.01 M) and PVP
(Fig. 3a). With a longer MW irradiation time, the plasmon ab-

760 Bull. Chem. Soc. Jpn., 77, No. 4 (2004)
Morphology-Control in MW-Assisted Synthesis of Ag
Conclusions
The size and size distribution of sphere-Ag particles can be
controlled by changing the concentration of trisodium citrate.
Furthermore, we have succeeded to switch the shape of Ag par-
ticles between sphere and prism by changing the concentration
of a polar polymer, PVP. The synthesis shown in the present
work was carried out in water under a high concentration of
Ag by 100 times compared to the previously reported methods.
These findings should open a new area of size- and morpholo-
gy-controlled synthesis of nano metals under homogeneous
MW-matched dielectric heating.
The present study was mainly supported by ‘‘21st Century
Center of Excellence Program’’. A part of this work was sup-
ported by ‘‘Nanotechnology Support Project’’ of the Ministry
of Education, Culture, Sports, Science and Technology
(MEXT).
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