PVP protective mechanism of ultrafine silver powder synthesized by chemical reduction processes

Article abstract of DOI:10.1006/jssc.1996.0015

Polyvinyl pyrrolidone (PVP) as a protective agent plays a decisive part in controlling superfine silver particle size and size distribution by reducing silver nitrate with hydrazine hydrate. The particle size and particle aggregation decrease with the PVP/AgNO₃ mole ratio. Mechanisms of PVP protection were divided into three stages. First, PVP donates loan pair electrons of oxygen and nitrogen atoms to sp orbitals of silver ions, and thus the coordinative complex of Ag ions and PVP forms in aqueous solution. Second, PVP promotes the nucleation of the metallic silver because the Ag ions-PVP complex is more easily reduced by hydrazine than the pure Ag ions owing to Ag ions receiving more electronic clouds from PVP than from H₂O. Third, PVP prohibits silver particle aggregation and grain growth as a result of its steric effect. All of the hypotheses were supported by ultraviolet spectra, infrared spectra, and heterogeneous nucleation and grain growth by addition of silver nuclei.

Full text of DOI:10.1006/jssc.1996.0015

JOURNAL OF SOLID STATE CHEMISTRY 121, 105–110 (1996)
ARTICLE NO. 0015
PVP Protective Mechanism of Ultrafine Silver Powder Synthesized
by Chemical Reduction Processes
Zongtao Zhang, Bin Zhao, and Liming Hu
Technical Institute of Chemistry and Physics, East China University of Science and Technology, Shanghai 200237, P.R. China
Received January 3, 1995; in revised form August 14, 1995; accepted August 16, 1995
soluble silver salts was reacted with an appropriate reduc-
Polyvinyl pyrrolidone (PVP) as a protective agent plays a ing agent of both nonorganic and organic materials. How-
decisive part in controlling superfine silver particle size and
size distribution by reducing silver nitrate with hydrazine hy-
drate. The particle size and particle aggregation decrease with
the PVP/AgNO3 mole ratio. Mechanisms of PVP protection
were divided into three stages. First, PVP donates loan pair
electrons of oxygen and nitrogen atoms to sp orbitals of silver
ions, and thus the coordinative complex of Ag ions and PVP
forms in aqueous solution. Second, PVP promotes the nucle-
ation of the metallic silver because the Ag ions–PVP complex
is more easily reduced by hydrazine than the pure Ag ions
owing to Ag ions receiving more electronic clouds from PVP
ever, the conventional processes usually produce powders
with large size (sometimes more than 2 Ȑm), and irregular
shape and aggregation. This kind of powder was difficult
to use in thick-film and multilayer ceramic capacitors (1).
Fortunately, a newly developed chemical method,
known as the polymer protected reduction process, has
been used to prepare monodispersed silver powder with
submicrometer size and quasi-spherical shape (2–4). G.
Tosun and H. D. Glichsman (2) have reported a gelatin-
protected processing, in which silver nitrate was reduced by
than from H2O. Third, PVP prohibits silver particle aggregation alkylacid phosphate in aqueous solution and finely divided
and grain growth as a result of its steric effect. All of the
hypotheses were supported by ultraviolet spectra, infrared spec-
tra, and heterogeneous nucleation and grain growth by addition
silver particles with narrow particle size distribution and
with a mean particle size of about 0.1–1.0 Ȑm. They also
found that the gelatin played a key role in regulating the
silver particle size. F. Fievet et al. (3) and Ducamp-San-
guesa et al. (4) have employed polyvinyl pyrrolidone
(PVP) as a protective agent to synthesize ultrafine silver
powder with 300 nm diameter, by reducing silver nitrate
with hot polyol solution. We also prepared the superfine
of silver nuclei.
1996 Academic Press, Inc.
INTRODUCTION
Ultrafine silver powders have been widely used in the silver powders with spherical shape and about 100 nm
electronics industry for the manufacture of conductive diameter using a chemical reduction method protected by
thick film circuits and for the internal electrodes of PVP (5). However, the polymer protective mechanisms
multilayer ceramic capacitors. In recent years, with the have not been systematically studied.
small size and precision of electronic components, there
Polymer protective mechanisms originated from colloi-
are growing demands for a decrease in the thickness of dal chemistry. H. Hirai et al. (6) suggested that a complex of
conductive films to several micrometers and a further nar- the polymers and the metal ions was formed. According to
rowing of the width of printed circuits and the space be- chemical equilibrium, the effective metal ion concentration
tween these circuits (for instance, 100-Ȑm). It is thus re- decreased,sothepolymerhadimpairedthemetalionreduc-
quired that the conductive metal powder composing the tion, i.e., the nucleation of metal particles, the coordinative
paste should have small particle diameter and as close as bonds between the polymers and the ions being too strong.
possible to spherical shape with narrow particle size distri- On theother hand, the polymers usuallyhave the back bone
bution.
of polyvinyl as hydrophobic and hydrophilic groups. The
Many methods currently have been applied to prepare backbone of the vinyl polymer forms a hydrophobic do-
silver powder, such as physical processes of atomization main, which surround metal particles, whereas the hydro-
or milling, chemical methods of thermal decomposition, philic pendant groups of the polymer interact with water
and electrochemical processes. Silver powders used in elec- or polar solvent, i.e., the steric effect of the polymer in the
tronic applications are generally manufactured by chemical surface of metal particles prevents the particles from ag-
precipitation processes, in which an aqueous solution of glomeration. This hypothesis had, indeed, explained why
105
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All rights of reproduction in any form reserved.

106
ZHANG, ZHAO, AND HU
In this paper, we focus on the effect of the PVP/AgNO3
mole ratio on the silver particle size and the size distribu-
tion. Based on ultraviolet spectra, infrared spectra, and
other experimental data, a PVP protective mechanism for
synthesizing superfine silver particles by chemical reduc-
tion of silver nitrate with hydrazine hydrate in aqueous
solution will be proposed.
EXPERIMENTAL PROCEDURES
All chemicals were reagent grade. Both silver nitrate and
hydrazine hydrate were purchased from Xin Da Chemical
Company, Shanghai, China. PVP (K30, polymerization de-
gree 360) was imported from the United States. The above
materials were dissolved in deionized water to form an
aqueous solution. The reactant solutions were mixed drop
by drop with the aid of a peristaltic pump. The hydrazine
was in stoichiometric excess of five times the silver nitrate
FIG. 1. Silver particle size and the size geometric standard deviation
(GSD) as a function of PVP/AgNO3 mole ratio.
the colloidal metal particles in polymer solution were well to achieve complete reduction of the silver salt. The reac-
dispersed. The coordinate bonds between the polymers and tion solution was kept under agitation at a rate sufficient
metal ions, however, have not been supported by experi- to maintain the uniformity of the system and to keep the
ments. Recently, Fievet et al. (7) reported that a protective precipitated particles dispersed until the reduction was
polymer agent, D-sorbitol, enhanced the nucleation of the completed. Also, the dipping rate was sufficiently low to
copper phase during the reduction of copper ions by ethyl- avoid foaming of the reaction dispersion. At the end of all
ene glycol, which was not inconsistent with the model’s in- reactions, the solids were separated from the liquid solu-
ductionofpolymershinderingthemetalparticlenucleation. tion by centrifugation, and subsequently were washed with
Furthermore, this hypothesis cannot explain the fact that acetone or alcohol several times until a clear solution was
polymers absorbed on the surface of metal particles cannot obtained. The particles were then dried at 40ЊC in air.
be washed out by several applications of water and acetone
(5, 7), if only physical absorption between the metal surface by different techniques. The phase composition was deter-
and hydrophobic groups of the polymers occurred. mined with X-ray diffractometer using CuKͰ radiation.
Characterization of the metallic particles was achieved
FIG. 2. TEM micrographs of silver powders synthesized at the mole ratios of (A) PVP/AgNO3 ϭ 0 and (B) PVP/AgNO3 ϭ 1.5.

PROTECTION OF ULTRAFINE SILVER POWDER
107
The shape of the final products was observed by transmis- 2A). In contrast, the well-dispersed quasi-spherical-shaped
sion electron microscope (TEM, Model H-800, Japan). The silver particles and a narrow particle size distribution (Fig.
particle size and size distribution were measured with a 2B) were obtained in the cease of PVP/AgNO3 ϭ 1.5.
BI-90 laser particle sizer.
The experimental results demonstrated that the PVP as a
protective agent plays a decisive role in controlling the
metallic silver size, size distribution, and particle agglomer-
ation. The mechanisms will be discussed in the next section.
RESULTS AND DISCUSSION
1. Influences of PVP/AgNO3 Mole Ratio on the Silver
Particle Size and the Size Distribution
2. Mechanisms of PVP Protection
Varying amounts of the protective agent PVP were used
for investigating the effect of the surfactant on silver parti-
cle size and particle size distribution (GSD, geometric stan-
dard deviation) (8). This is shown in Fig. 1, where the silver
ion concentration was 0.5886 mol/liter and the reaction
was performed at room temperature for 20 min. We show
in Fig. 1 that the average particle size of the silver powder
was more than 1000 nm and the GSD was 2.7 at PVP/
AgNO3 ϭ 0. With an increase in the PVP/AgNO3 ratio
(one mole of PVP means one unit of PVP, shown in Eq.
[1]), both particle size and GSD decreased. When PVP/
PVP protection in the silver nitrate–hydrazine system
was generally proposed on the basis of the following reac-
tions. PVP has a structure of a polyvinyl skeleton with
polar groups, shown in the formula
[CH2 CH]n
O
[1]
N
AgNO3 Ն 1.5, the particle size and GSD fell weakly. This where n is the polymerization number. The donated lone
means that PVP/AgNO3 ϭ 1.5 is a critical value. If the pairs of both nitrogen and oxygen atoms in the polar groups
ratio was less than 1.5, the PVP protection effect was not of one PVP unit may occupy two sp orbitals of the silver
complete, whereas if the ratio was more than 1.5, further ion to form a complex compound. Also, because sp orbitals
polymer protection became unnecessary.
form a linear coordinative bond, 1 mole PVP and 1 mole
Figure 2 shows TEM micrographs of the silver powders. silver ion may form one mole complex. We suppose that
It can be seen that the particle shape and degree of aggrega- these two kinds of possibilities were equal. The reaction
tion distinctly depend on different PVP/AgNO3 mole ra- of PVP and silver ions may be as in Eq. (2). Subsequently,
tios. When PVP/AgNO3 ϭ 0, it reveals irregular shape and hydrazine hydrate may react with the complex of silver
strong sinter-like particles several micrometers in size (Fig. ion and PVP as in Eq. (3):
[CH2 CH]
[CH2 CH]
[CH2 CH]
N
[CH2 CH]
+
:
:
Ag
+
O + 2 Ag+
O
+
[2]
:
O Ag
:
O
N
N
N
3
[CH2 CH]
[CH2 CH]
N
[CH2 CH]
[CH2 CH]
[CH2 CH]
: :
O Ag O
N
[CH2 CH]
+
:
:
:
:
Ag
Ag
+
:
:
O
+
O Ag
O
+
2e
O
+
N
N
N
N
[3]
The PVP protective mechanism of the silver reduced by ultraviolet absorption at lower wave length than solutions
hydrazine hydrate can be explained as three steps. not containing PVP. Furthermore, the higher the concen-
The first step was the formation of coordinative bonding tration of the PVP and AgNO3 solution, the greater was
between PVP and silver ions, which was identified by com- the fraction at the lower wave length absorption. These
paring ultraviolet spectra (Fig. 3) of the AgNO3 solutions ultraviolet absorption valley shifts may be attributed to the
containing and not containing PVP. It is noted that pure PVP–Agϩ complex substituted for the H2O–Agϩ complex,
PVP has no absorption peak in ultraviolet spectra. All the since the nitrogen and oxygen of the PVP have a stronger
samples contained silver ions; however, the exhibit peaks coordinative field than H2O.
at 320 nm resulted from the coordinative bonds of
The second step involves PVP-promoted silver nucle-
H2O : Ag : OH2 . At the same silver ion concentration, the ation, which is supported by the photoreduction experi-
addition of PVP to the AgNO3 solution produces more ments. Two silver nitrate solutions containing and not con-

108
ZHANG, ZHAO, AND HU
FIG. 3. Ultraviolet ray absorption spectra.
taining PVP with the same silver nitrate concentration 100 by Hirai et al. (6). Fortunately, these phenomena can be
g/liter were subjected to photo radiation by sunlight of the understood in terms of chemical bonding. Since the ligand
same intensity for 20 min. The ultraviolet (UV) spectra of of C–N and CuO in PVP contributes more electronic
the samples are illustrated in Fig. 4, where the first peaks density to the sp orbital of silver ions than does H2O, the
at 320 nm are the silver ion UV absorption and the second silver ions in the Agϩ–PVP complex may obtain electrons
peaks at about 520 nm are the silver particle UV absorp- from oxidizing agents more easily than those in Agϩ–H2O.
tion. It is obvious from the results in Fig. 4 that the area
The subsequent step is PVP-accelerated formation of a
of the silver absorption peak for PVP/AgNO3 ϭ 1.5 is large amount of silver nuclei. It was found that the color
much larger than for PVP/AgNO3 ϭ 0, whereas the area of the solutions changes gradually from pink at the initial
of the silver ion absorption peak shows the reverse trend reaction to violet, black, and gray at the end of the reaction
in comparison with the silver absorption ones. PVP, there- for system containing PVP, whereas a gray color always
fore, increases the silver ion reduction rate. The same result appears during the silver nitrate and hydrazine hydrate
was reported by Fievet et al. (7) in copper oxide reduced reaction without PVP. TEM observation showed that the
by polyol using D-sorbitol as protective agent.
scattering of sunlight by nuclei with diameters of 10, 20,
Cleary, the above experimental results cannot be ex- 50, and 100 nm is responsible for the pink, violet, black,
plained according to the traditional mechanisms suggested and gray solutions. It was concluded that a large amount
of small silver nuclei at the beginning of the reductive
reaction was produced when PVP was used; conversely, a
small amount of larger silver nuclei was generated if PVP
was not employed.
The effect of the nuclei on the silver particle size was
investigated by designing two experiments, as shown in
Table 1. The synthesized nuclei, prepared by refluxing alco-
holic silver nitrate solution using PVP as a protective agent
as reported as literature (9), were subsequently added to
the AgNO3–N2H4 и H2O–PVP reaction system. After the
TABLE 1
Influence of Silver Nuclei on Silver Powder Size
Nuclei size (nm)
Nuclei content (wt%)
Silver particle size (nm)
30
90
5
5
50
110
FIG. 4. Ultraviolet spectra of AgNO3 solution after radiation of solar
ray for 20 min.

PROTECTION OF ULTRAFINE SILVER POWDER
109
1
pure PVP, but the C–N absorption peak at 1019 cmϪ is
1
separated into two peaks at 1037 and 1021 cmϪ , and the
CuO absorption peak at 1663 cmϪ1 was slightly widened.
This implies that there is weak coordinative chemical bond-
ing of C–N to Ag at the interface between PVP and the
powder. Furthermore, for Ag particles covered by PVP
during the reduction, the degree of separation of the C–N
vibrations becomes more obvious, the one peak at 1019
1
1
cmϪ becoming two peaks at 1078 and 1021 cmϪ . Also,
1
the absorption peak of the CuO bond at 1663 cmϪ for
pure PVP is shifted to 1643 cmϪ1 for Ag particles covered
by PVP. This decrease in wavenumber for CuO absorp-
tion may result from bond weakening via partial donation
of oxygen loan pair electrons of PVP to the vacant orbitals
of the silver surface. All the IR spectra prove that the
chemical bonds of N : Ag : O remain in the PVP–Ag pow-
der, i.e., reaction [3] is correct.
PVP strongly absorbed on the surface of silver particles
is an obstacle to silver diffusion and may decrease the
silver grain growth in the aqueous suspension. In addition,
because of the excellent dispersion of the silver particles
owing to the PVP in water, the diffusion distance of the
silver particles is larger that of the silver particles without
PVP covering, due to poor dispersion of the silver. The
diffusion barrier and the large diffusion distance may be
another mechanism for PVP protection against silver
grain growth.
FIG. 5. FTIR spectra of PVP and silver powder.
PVP preventing the silver particles from aggregation is
themain roleofthesurfactant, whichoccursboth duringthe
growth step and the washing processes. For the former case,
thesteric effectarisingfrom the longpolyvinylchain of PVP
on the surface of silver particles may contribute to the anti-
agglomeration. Because the steric effect is largely deter-
mined by the covered fraction of PVP on the surface of the
silver particles, it was, therefore, necessary that there be
enoughPVPtoabsorbonsilverparticles. Forthelattercase,
chemical bonding between the PVP and silver powders may
play an important role in prohibiting aggregation of silver
particles, because washing with water and acetone six times
did not completely remove the PVP from the surface of the
silver particles. These experiments imply that the steric ef-
fect also exists during the washing processes.
reaction ended, silver particles 50 and 110 nm in diameter
were observed with TEM, using 30 and 90 nm nuclei, re-
spectively. Thus the small generation of nuclei at the initial
stage of the reaction by addition of PVP protectant should
be an important factor of the average small particle size
of the silver powder.
The third step is PVP prohibiting grain growth and parti-
cle aggregation. There is no doubt that the PVP was physi-
cally bonded with silver, according to previous works (5).
The chemical bonding between the PVP and silver powder
(reaction [3]) is demonstrated in Fig. 5. The wavenumbers
1
2920 and 3428 cmϪ correspond to C–H and O–H bond
1
vibrations, respectively. The values 1463 and 1427 cmϪ
were obtained for
On the basis of the steric effect, we can explain the
relationship between PVP/AgNO3 mole ratio and the ag-
glomeration behavior (shown in Fig. 2). The more strongly
aggregated or sinter-like silver particles (Fig. 2A) are due
N
of PVP bond absorption. The wavenumbers 1363 and 1288 to lack of steric protection by PVP. Although the steric
1
1
cmϪ were due to bond vibrations of the NO3Ϫ group and effect has been seen in solutions of PVP/AgNO3 ϭ 0.4–0.8,
the N Ǟ H–O complex, respectively. Now we concentrate partial agglomeration also takes place as a result of incom-
1
on the wavenumbers of about 1019 and 1663 cmϪ , which plete covering of the silver particles by PVP. In the case
were, respectively, generated by the C–N and CuO bond of PVP/AgNO3 Ն 1.5, the entire surface of the silver parti-
vibrational absorption. It is found that most of the IR cles was coated by PVP. Consequently, the steric influence
spectrum (Fig. 5) of PVP mechanically mixed with pure against the agglomeration was fulfilled and the silver parti-
silver powder (PVP ϩ Ag) is nearly identical to that of cles were separated from each other by PVP.

110
ZHANG, ZHAO, AND HU
CONCLUSIONS
REFERENCES
1. Well-dispersed silver powder with 50–100 nm size and
quasi-spherical shape has been prepared by reducing silver
nitrate with hydrazine in the presence of PVP as a protec-
tive agent.
1. Tian, Yuzi, ‘‘Metallurgy of Noble Metals.’’ Atomic Energy Press,
Beijing, P150 (1992).
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(1989).
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ultraviolet spectra, infrared spectra, and experiments with
added silver nuclei, were divided into three stages. First,
the complex of PVP–silver ions via coordinative bonding
was constructed. Second, the complex promotes silver nu-
cleation, which tends to produce small silver particles.
Third, the steric effect of PVP covering the silver surface
via physical and chemical bonding inhibits particle–particle
contact and thus the agglomeration of the powder. The
steric effect worked both during the grain growth step and
during the washing process.
6. Hirai, H., Nakao, Y., and Toshima, N., J. Macromol. Sci. Chem. A13,
727 (1979).
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`
J. Mater. Chem. 3, 627 (1993).
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haven Instrumentals Corporation, Holtsville, NY, 1986.
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