Preparation of Silver Nanoparticles by Chemical Reduction Method

Article (PDF Available)inColloids and Surfaces A Physicochemical and Engineering Aspects 256(2-3):111-115 · April 2005with194 Reads
DOI: 10.1016/j.colsurfa.2004.12.058
Abstract
In the solution containing polyvinyl pyrrolidone (PVP), silver nitrate was reduced by the glucose, and silver particles were generated. The possible reaction process is discussed in this paper. Sodium hydroxide was used to enhance the reaction velocity. When the mole ratio of NaOH to AgNO3 was ranged from 1.4 to 1.6, the colloid kept stable and no Ag+ was traced. The particles and colloids were also analyzed by the X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV–visible (UV–vis) spectrophotometer. The TEM photo indicated that with the increase in PVP, the particles dispersed better; and if the weight ratio of PVP to AgNO3 is no less than 1.5, the particles dispersed individually in a colloid form. The agglomeration of particles also was influenced by the mixing speed of the reactants. The XRD spectrums showed that the particles were silver simple substance if the reductant was sufficient and the mixing speed of the reactants was slow enough.
Colloids and Surfaces A: Physicochem. Eng. Aspects 256 (2005) 111–115
Preparation of silver nanoparticles by chemical reduction method
Hongshui Wang
a
, Xueliang Qiao
a,
, Jianguo Chen
a
, Shiyuan Ding
b
a
State Key Laboratory of Plastic Forming Simulation and Die and Mould Technology,
Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR China
b
Hubei Xinyin Noble Metal Co. Ltd., Shiyan, Hubei, PR China
Received 16 March 2004; accepted 29 December 2004
Available online 30 January 2005
Abstract
In the solution containing polyvinyl pyrrolidone (PVP), silver nitrate was reduced by the glucose, and silver particles were generated. The
possible reaction process is discussed in this paper. Sodium hydroxide was used to enhance the reaction velocity. When the mole ratio of NaOH
to AgNO
3
was ranged from 1.4 to 1.6, the colloid kept stable and no Ag
+
was traced. The particles and colloids were also analyzed by the
X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV–visible (UV–vis) spectrophotometer. The TEM photo indicated
that with the increase in PVP, the particles dispersed better; and if the weight ratio of PVP to AgNO
3
is no less than 1.5, the particles dispersed
individually in a colloid form. The agglomeration of particles also was influenced by the mixing speed of the reactants. The XRD spectrums
showed that the particles were silver simple substance if the reductant was sufficient and the mixing speed of the reactants was slow enough.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Silver; Nanoparticles; Agglomeration
1. Introduction
The area of nanoparticles research has witnessed tremen-
dous growth due to the unusual chemical and physical prop-
erties demonstrated by this intermediate state of matter [1].
Due to their small size, these crystallites exhibit novel ma-
terial properties that largely differ from the bulk properties
[2]. Many reports on quantum size effect on photochemistry
[3–5], nonlinear optical properties of semiconductor [6,7] or
the emergence of metallic properties with the size of the par-
ticles [8–10] have appeared during the past.
Nanoparticles of noble metals are of great interest to-
day because of their possible applications in microelectron-
ics [11–14]. Silver particles play an important role in the
electronic industry. In recent years, with the higher inte-
grated density of the electronic components (small size and
precision of the electronic components), there are grow-
ing demands for a decrease in the thickness of conductive
Corresponding author. Tel.: +86 27 87541540; fax: +86 27 87541540.
E-mail address: xlqiao@public.wh.hb.cn (X. Qiao).
films and a further narrowing of the width of printed cir-
cuits and the space between these circuits. It is thus re-
quired that the powders (to form the conductive films and
printing the circuits on the basement) composing the paste
should have as small in diameter as possible, and the syn-
thesis of these particles is an important task. The chemical
way is often employed to synthesize silver colloidal metal
particles [15–17].
2. Experimental procedures
Allchemicalsusedintheexperimentwereanalyticreagent
(AR). The silver nitrate was provided by Hubei Xinying
Noble Metal Co. Ltd., glucose and polyvinyl pyrrolidone
(PVP) were obtained from China Medicine (Group) Shang-
hai Chemical Reagent Corp. The sodium hydroxide was pur-
chased from Tianjin Chemical Reagent Corp.
Silver nanoparticles were prepared by reducing the silver
nitratein PVP aqueoussolution. Glucose wasusedas reducer
and sodium hydroxide to accelerate the reaction.
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.colsurfa.2004.12.058
112 H. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 256 (2005) 111–115
Silvernitrate solution (A) wasprepared by adding 3.4g of
AgNO
3
into 20 ml distilled water. The PVP solution (B) was
prepared by dissolving PVP, glucose and sodium hydroxide
in 60ml distilled water together. Solution B was heated to
60
C and stirred hard, and solution A was added into B drop
by drop. After all the silver nitrate solution was added, the
mixed solution was stirred for 10min more. The particles
wereseparated bycentrifugation, and thesolid products were
washedwithdistilledwaterseveraltimesuntilnoNO
3
could
be traced.
Characterizations of the particles were achievedby differ-
ent techniques.
X-raydiffraction(XRD)dataweretakenwith Cu K radi-
ation (λ = 1.5418
˚
A), on the powder diffractometer operated
in the θ/2θ mode primarily in the 35–85
(2θ)range and step-
scan of 2θ =0.5
. Samples were prepared as uniform thin
films supported on the slides.
The transmission electron microscopy (TEM) was per-
formed with a Jeol electron microscope (model JEM 100CX
II).Sampleswereprepared by dispersing adropofthecolloid
on a copper grid, which was covered by carbon film, and the
solvent was evaporated.
Extinction spectra were measured on a UV-2010 UV–vis
spectrophotometer. All spectra were obtained from the parti-
cles immersed in water.
3. Results and discussion
3.1. Possible reaction process
As shown in Fig. 1, the solution without PVP can be kept
for 50 h at 60
C, and the UV–vis spectra were superposed
completely with the silver nitrate solution, but 10min after
the addition of PVP, an obvious change can be seen on the
curve, the peak at 300 nm was decreased dramatically, and a
new peak appeared at about 420nm. These changes can be
explained as follows: the silver ions were reduced and the
silver particles were generated.
The possible reaction between glucose and silver ion in
PVP solution can be written as follows:
Ag
+
+ PVP Ag(PVP)
+
(1)
CH
2
OH (CHOH)
4
CHO + 2[Ag(PVP)]
+
+ 2OH
CH
2
OH (CHOH)
4
COOH + 2Ag(PVP) ↓+H
2
O
(2)
2Ag
+
+ 2OH
Ag
2
O + H
2
O (3)
Ag
2
O + CH
2
OH (CHOH)
4
CHO + 2PVP
CH
2
OH (CHOH)
4
COOH + 2Ag(PVP) (4)
Fig. 1. The absorption spectra of the silver ions solution. (A) The UV–vis
spectrum of silver nitrate with a concentration of 1mol/l. (B) The UV–vis
spectrum of silver nitrate with glucose solution was kept for 2, 10 and 50h
at 60
C. (C) The UV–vis spectrum after 10min of the PVP was added
in B.
In the solution containing PVP, silver ions were re-
duced in two possible paths. The first is Eqs. (1) and (2).
Ag
+
was compounded with PVP firstly and complex ions
were generated. The hydroxyl ion may undergo a nucle-
ophilic addition reaction to glucose producing gluconate
ions [15], and then it reduces silver ion to silver atom.
The other two equations were the second path of the re-
action. Ag
+
reacted with hydroxyl ion, and then the prod-
uct, Ag
2
O, was reduced by glucose and silver particles were
generated [18]. In the process of reaction, the disperser
formed a protection layer on the surface of Ag
2
OorAg
particles.
The pH of the solution also plays an important role during
the reaction process. Under lowerpH, the reaction proceeded
in the first pattern, and with the increasing of the pH, the
second one became the dominant pattern gradually.
The relative ratio of glucose to silver was actually kept
at 2 throughout this study. This ratio should be sufficient to
reduce all silver ions in the solution.
H. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 256 (2005) 111–115 113
Table 1
Effects of sodium hydroxide
Sample number NaOH:AgNO
3
(mole ratio)
Ag
+
was traced Deposition
11YesNo
21.2 Yes No
31.4 Yes No
41.5NoNo
51.6NoYes
3.2. Effects of sodium hydroxide
According Eq. (2), the addition of alkali is favored for
higher reducing ability; however, it had an adverse effect on
particle agglomeration. Silver colloids were destabilized by
sodium hydroxide and deposited.
Samples with different sodium hydroxide were prepared.
Each sample was divided into two parts. The first part was
separated by centrifugation, and the Cl
was introduced to
check whether Ag
+
exsited in the supernate. The other part
was standing to observe whether there was deposition pre-
cipitated 24h later.
As shown in Table 1,noAg
+
could be monitored until the
mole ratio (n
NaOH
: n
AgNO
3
) is over 1.4, and when it came to
1.6, particles were deposited out after 24h.
When n
NaOH
: n
AgNO
3
was ranged from 1.4 to 1.6, Ag
+
had not reacted completely, but it could not be monitored by
Cl
, and they were absorbed on the surface of particles, and
formed electric double layer; it protected the colloid from
agglomeration. With increased n
NaOH
: n
AgNO
3
,OH
dif-
fused into the layer, and the adsorption layer was thinned.
When n
NaOH
: n
AgNO
3
came to 1.6, the layer was destroyed
by OH
, particles with a PVP layer have more chance to col-
lide with each other, and the particles would agglomerate by
the intertwist of PVP or the conjunction of silver atoms.
3.3. The influence of dispersant
The use of dispersant has two purposes, one is to generate
complex compound with the precursor, and control the pro-
cess of the reaction as discussed in part 3.1, the other is to
protect particles from growth and agglomeration (Fig. 2).
Fig. 3 is the TEM photograph with different ratio of
disperser to AgNO
3
, with the increase in the disperser, the
particle size has no obvious changes (the diameters of parti-
cles ranges from 20 to 80nm), but the dispersibility becomes
far better. In the reactions, the pH value of solution B was
very high, reaction will follow the second pattern, when the
AgNO
3
solution was added. The particle size was controlled
by the size of Ag
2
O (20–70nm), because the generation of
Ag
2
O is more quickly than the formation of protecting layer.
When the amount of the disperser is not enough, it cannot
form a complete protection layer, and the particles will ag-
glomerate easily. With more disperser added, it can form a
moreperfectlayer quickly, andthelayerprotectsthe particles
from agglomeration and growth. As shown in Fig. 3,ifthe
Fig. 2. The possible process of the agglomarate.
Fig. 3. The TEM photograph with different rate of disperser to AgNO
3
.
114 H. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 256 (2005) 111–115
Fig. 4. The influence of the mixing speed of reactants.
weight ratio (w
PVP
: w
AgNO
3
) is about 1.5, the particles can
disperse individually.
3.4. The influence of the mixing speed of reactants
When the reactants were mixed, the generated particles
needed time to disperse in the solution, and disperser also
needed time to form a protection layer. If the mixing speed
is too high, new particles will generate by side of the former
ones and form bigger particles together.
As shown in Fig. 4, the agglomerations in Fig. 4c are the
biggest,andtheparticlesinFig. 4baredispersedindividually.
When the speed is about 1 drop per second, the particles can
disperse sufficiently before the new ones are generated, but
if the speed was too slow, the reaction time become longer,
andthe particleswill impact witheach otherand agglomerate
because of the Brownian movement [19].
3.5. XRD study
All samples were analyzed by XRD. The spectrum of the
sample used for Fig. 2 was shown in Fig. 5. There are five
peaks corresponding to the interplanar distance (d): 2.3571,
2.0420,1.4438, 1.2316and 1.1795;theyare all agreeing with
Fig. 5. The XRD spectrum of silver particles.
Fig. 6. The XRD spectrum of the samples containing Ag
2
O.
the spectrum of the silver. This indicated that the particles
were silver simple substance. The curves of other samples
were also agreed with Fig. 5.
If the solutions were mixed in a high velocity, the XRD
spectrums would be like Fig. 6. There are strong peaks at
the point where d is 2.7233 and 1.6665; they are matching
with the diffraction curves of the Ag
2
O. The reason that the
oxidewasremained canbeexplainedasfollows:asthe reduc-
ing process was slower than the generation of the Ag
2
O, the
Ag
2
O particles are generated too fast to be reduced entirely
beforeagglomerating, andthen the agglomerationswere pro-
tected by PVP immediately. So Ag
2
O formed in the middle
of the agglomeration with part of Ag and part of PVP on the
surface, which made the reducing reaction not keep on. As a
result, the Ag
2
O was remained in the agglomerations.
4. Conclusions
Well-dispersed silver particles with 20–80 nm size and
spherical shape were prepared by reducing silver nitrate with
glucose in the presence of protective agent PVP. The addi-
tion of the sodium hydroxide enhanced the reaction velocity.
When the mole ratio of NaOH:AgNO
3
is in range from 1.4
to 1.6, the colloid keeps stable and no Ag
+
could be traced.
The PVP protected the silver particles from growth and ag-
glomeration, when the weight ratio of PVP:AgNO
3
is no less
than 1.5, the particles could be dispersed individually. When
the mixing speed of the reactant was about 1 drop per sec-
ond, the colloid got excellent dispersing ability. If the mixing
speed of the reactants was too high, the Ag
2
O remained in
the particles.
H. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 256 (2005) 111–115 115
Acknowledgement
Thanks are due to Shuizhou Cai for the test of UV–vis.
The authors are also grateful to Xiaoxia Yang for the help in
the experiment.
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