Silver Coating Using Latent Reductant from Silvercarbamate
Bull. Korean Chem. Soc. 2013, Vol. 34, No. 2 505
http://dx.doi.org/10.5012/bkcs.2013.34.2.505
Facile Preparation of Silver Nanoparticles and Application to Silver Coating Using
Latent Reductant from a Silver Carbamate Complex
Kyung-A Kim, Jae-Ryung Cha, and Myoung-Seon Gong*
Department of Nanobiomedical Science & WCU Research Center, Dankook University Graduate School,
Chungnam 330-714, Korea. *E-mail: msgong@dankook.ac.kr
Received October 23, 2012, Accepted November 19, 2012
A low temperature (65 oC) thermal deposition process was developed for depositing a silver coating on
thermally sensitive polymeric substrates. This low temperature deposition was achieved by chemical reduction
of a silver alkylcarbamate complex with latent reducing agent. The effects of acetol as a latent reducing agent
for the silver 2-ethylhexylcarbamate (Ag-EHCB) complex and their blend solutions were investigated in terms
of reducing mechanism, and the size and shape of silver nanoparticles (Ag-NPs) as a function of reduced
temperature and time, and PVP stabilizer concentration were determined. Low temperature deposition was
achieved by combining chemical reduction with thermal heating at 65 oC. A range of polymer film, sheet and
molding product was coated with silver at thicknesses of 100 nm. The effect of process parameters and heat
treatment on the properties of silver coatings was investigated.
Key Words : Silver 2-ethylhexylcarbamate, Chemical reduction, Latent reductant, Silver coating
Introduction
Silver powders with ultra fine and uniformly distributed
sizes are of considerable use in electronics, the chemical
industry, and medicine due to their unique properties such as
high electrical and thermal conductivity, high resistance to
oxidation, and bactericidal action (colloid silver).1-6 In this
regard, nano-sized powders and colloidal silver dispersions
have attracted a great deal of attention.7,8
Chemical approaches using a series of reductants and
stabilizers have been extensively developed to enable largescale production of metal NPs under simple and mild
conditions.9-12 The reducing and stabilizing abilities of the
compounds used in such reactions may play an important
role in metal NPs growth control. For example, a weak
reductant and an electron-rich stabilizer may be favorable
for control of the size and morphology of the metal NPs
obtained.13-16
Various chemical methods have been used to synthesize
metallic NPs dispersions, and the most common of these
approaches uses an excess amount of a reducing agent, such
as sodium borohydride or sodium citrate.17,18 Other simple
methods involve reducing metallic salts to prepare colloidal
dispersions.19 Latent reducing agents for silver precursors
are challenging due to the high reactivity of reducing agents.20
The precursor solution must be heated to achieve a sufficiently high temperature for efficient surface perform impregnation during the solution process, and the solution must
remain stable at those temperatures for an extended period.
In addition it is desirable for the solution blend to react
rapidly at reduced temperatures of about 60-65 oC to prepare
silver deposition coatings or aesthetic coatings within a few
minutes.
Acetol has motivated studies due to its mild reducing
power, which is related to in situ reduction of the silver
precursors to afford Ag-NPs in organic solvent such as N,Ndimethylformamide.21 The reducing power of acetol depends
on temperature. Additionally, the time needed to complete
the reduction ranges from several hours at room temperature
to a few minutes at high temperature for Ag+ concentrations
of 10−3-10−2 M in 2-propanol.22
We investigated the latent reduction characteristics of an
Ag-EHCB complex with acetol and the effects of reaction
temperature and time while preparing Ag-NPs in organic
solvent. Nano-silver layers were deposited by dipping the
film or spin coating on a polymer film substrate or polymer
molding product, followed by heating at 65 oC for 10 min
and 120 oC for 3 min.
Experimental
Chemicals and Instruments. The Ag-EHCB complex
(C8H17NHCOO)2Ag2·2C8H17NH2 solution used throughout
this study as a silver precursor was prepared as described
previously.23 2-Ethylhexylammonium 2-ethylhexylcarbamate
was prepared with carbon dioxide and 2-ethylhexyl amine.
Acetol (Aldrich Chemical Co., St. Louis, MO, USA) was
used as the reducing agent without further purification. 2Propanol (Aldrich Chemical) was used as received. X-ray
diffraction (XRD) patterns of the Ag-NPs produced were
measured using a Shimadzu XD-D1 X-ray diffractometer
and Cu Kα radiation (λ = 1.54056 Å) at a scanning rate of
2°/second in a 2θ range of 30°-90°. Transmission electron
microscopy (TEM) images were obtained using a JEOL
electron microscope (JEM-2000EXII). TEM samples were
prepared by dropping a small amount of silver colloid
solution onto a carbon-coated copper grid. UV-Vis spectra
(200-800 nm) were obtained using a Shimadzu UV-1601PC
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Bull. Korean Chem. Soc. 2013, Vol. 34, No. 2
spectrometer. Silver nanocolloid solutions were diluted with
2-propanol and placed in a quartz cuvette cell.
Preparation of Ag Nanocolloid Solution by Reduction
with Acetol. Acetol (0.25 g, 3.8 × 10−3 mol) in 2-propanol (3
mL) at 20 oC was added to the Ag-EHCB complex (1.62 g,
3.0 × 10−3 mol/L) and PVP (1.0 g, 6.0 × 10−3 mol/L) in 2propanol (30 mL). The reaction mixture gradually formed a
greenish black dispersion to produce the silver nanocolloid
solution. Reduction at 65 oC was performed by placing the
flask in an oil bath heated to 65 oC for 20 min. Heating the
solution at 65 oC for 10 min caused a gradual reduction to
produce a brownish black dispersion. The nano-colloidal
solution was cooled and stored under room temperature. The
nanocolloid solution with different PVP concentrations at
various temperatures and times for the acetol reduction was
prepared by the procedures described above.
Deposition of Silver Coating on a Polymer Film and
the Molding Product. The silver coating solution with 9.4
wt % solvent content was prepared by mixing the Ag-EHCB
complex IPA solution (Ag content 10%, 100 g) with acetol
(6.8 g). Thin silver films were coated at 20 oC on plasmatreated PET substrate surfaces by spin coating using the
above mentioned coating solution. Sintering was performed
at 65 °C for 10 min in air, followed by heating to 120 °C for
3 min. The silver coating was deposited on the polymer
molded products by simple immersion under the same
sintering conditions.
Results and Discussion
The Ag-EHCB complex was prepared by reacting silver
oxide with 2-ethylhexylammonium 2-ethylhexylcarbamate.
The ionic bond between the silver ions and carbamate anions
was weakened by coordination between the alkyl amine and
the silver ions. Thus, the Ag-EHCB complex can be reduced
by most chemical reducing agents.24-33
In this study, the type and content of stabilizer was expected to be an important influence on particle size, distribution, and stability of the Ag-NPs. Polyvinylpyrrolidone
(PVP) was selected as the stabilizer for dispersing the AgNPs in 2-propanol.34 A large number of experiments were
conducted while changing reaction time, temperature, and
stabilizer concentrations in the reaction mixture to determine
the optimal conditions for preparing stable Ag-NPs.
Acetol was used as the latent reducing agent for the AgEHCB complex. When acetol was poured into the AgEHCB complex solution, the reaction did not proceed. As
the temperature was increased to 65 oC, the precursor solution quickly turned dark green in color upon adding the
acetol and then changed gradually to greenish black color as
the reaction was completed. The dark green color suggested
that the silver ions were reduced into extremely fine Ag-NPs
nanoparticles, which then gradually agglomerated into stable
Ag-NPs with a particular size distribution in the presence of
the PVP molecules. Evidently, when the reduction rate was
slow, the PVP molecules had sufficient time to coat these
newly formed silver colloids. Therefore PVP molecules
Kyung-A Kim et al.
might provide some physical barrier to the Ag-NPs surface
and slow down the agglomeration process.35
Figure 1 shows the UV-Vis spectra of the silver colloidal
solutions obtained from acetol reduction at different temperatures for 10 min. Absorption peaks at 356 nm appeared
during the early stage, which corresponded to the formation
of a fixed number of Ag-NPs silver nuclei seeds. The spectrum changed (red shift) to a double peak with a larger
shoulder at 413 nm when temperature was increased, which
corresponded to particle size growth. Mie’s theory predicts
only a single surface plasmon resonance (SPR) band in the
absorption spectra of spherical Ag-NPs, whereas the anisotropic particles can produce two or more SPR bands.36
According to the discrete dipole approximation method, the
355 nm and 430 nm peaks are the out-of-plane dipole and
quadrupole plasmon resonance, respectively.37 This result
clearly indicates that the Ag-NPs in our study retained their
shape as spherical particles in 2-propanol with the aid of the
PVP stabilizer an increased temperature.
Figure 2 shows the UV-Vis absorption spectra of the AgNPs colloidal solution stabilized with PVP taken at different
reaction times between 3 min and 10 min at 65 oC. The UVVis absorption peaks also appeared at 360 nm with a
shoulder at 410 nm at the initial time. The main peak was
red-shifted to 410 nm with increasing reaction time and
reached a stable level. The optimum reaction time was 10
min because of the characteristic features of the reducing
steps; acetol was oxidized to piruvaldehyde, which was
further oxidized to piruvic acid as shown in Scheme 1.38
Stabilizers have two purposes such as to generate a complex compound with the precursor and to protect particles
from growth and agglomeration. Figure 3 shows the effects
of PVP concentrations at 65 oC for 10 min. The main absorption spectra of the Ag-NP solution were similar to those
described in Figures 1 and 2, but the distribution was more
uni-modal. Protection from agglomeration was unfavorable
at a low PVP concentration, because PVP probably could
not arrange itself to provide better coverage on the silver
colloid surface. The PVP concentration of 6.0 × 10−3 mol/L
Figure 1. UV-Vis spectra of the silver nanoparticles (Ag-NPs)
colloid solution obtained from reducing Ag-EHCH with acetol at
various temperatures for 10 min.
Silver Coating Using Latent Reductant from Silvercarbamate
Figure 2. UV-Vis spectra of Ag-NPs colloidal solution obtained
from reducing Ag-EHCH with acetol for various times at 65 oC.
was sufficient to stabilize the Ag-NPs with respect to further
growth. However the final particle size increased slightly
with decreasing PVP concentrations, so further amounts of
PVP were not used.
Figure 4(a) shows typical TEM images of Ag-NPs after a
reaction time of 3 min. The derived size distribution histograms are displayed in Figure 4(b). The size distribution was
uniform except for some small grain seeds during the initial
period. Ag-NPs morphology was spherical when reaction
time was short, and particle size increased gradually from 6
Figure 3. UV-Vis absorption spectra of silver colloidal solutions
obtained by chemical reduction with different PVP concentrations
at 65 oC in 2-propanol.
Scheme 1. Reduction of Ag-EHCB with acetol.
Bull. Korean Chem. Soc. 2013, Vol. 34, No. 2
507
nm to 15 nm with increasing reaction time. However, the
change in morphology was less significant than the growth
in size from 3 to 10 min, as shown in Figure 4(c). TEM
revealed a particle diameter of 5-20 nm, and various particle
shapes were found. Several elongated and partially fused
particles were observed beside spherical particles. Particle
diameters increased slightly and aggregation of particles
began to occur with an increase in reaction temperature to 65
o
C (Figure 4(c)). According to the histogram depicted in
Figure 4d, these spherical Ag-NPs displayed a narrow size
distribution of 5-20 nm. The aggregation of silver nuclei was
smoothly suppressed and the growth of core silver was
limited to an average diameter of 15 nm, which coincided
with the UV-visible absorption spectral analysis. We divided
a typical synthesis into nucleation and growth stages. The
PVP stabilizer played a role capturing the zero-valent Ag
atoms. Once the concentration of atoms reaches a point of
supersaturation, the atoms start to aggregate into small
clusters.39 Under our conditions, the supersaturation point
Figure 4. TEM images of silver colloidal solutions obtained from
the Ag-EHCB complex and acetol solution after chemical reduction
at 65 oC for (a) 3 min, (b) histogram, (c) 10 min and (d) histogram.
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Kyung-A Kim et al.
Figure 5. XRD pattern of silver powder collected from the nanocolloidal solution.
was achieved on a small timescale and nucleation occurred
immediately. Thus, the nucleation and growth processes
could not be separated from each other. Coordination occurs
between the pyrrolidone ring of PVP and silver surface
atoms of the Ag-NPs.34,35
The silver colloidal solutions were centrifuged to precipitate Ag-NPs, which were isolated as a greenish black
colored powder. These powders are stable for several months
at room temperature and well dispersed in both polar and
nonpolar solvents such as 2-propanol and toluene.
The precipitated Ag-NPs were subjected to powder X-Ray
diffractometry. The XRD pattern of the Ag-NPs showed 2θ
peaks of 38.00 (111), 44.4 (200), 64.5 (220), and 77.5º (310)
representing the face-centered cubic (fcc) phase of silver and
confirming the presence of Ag(0) in the sample as shown in
Figure 5.
Finally, the sinterability of this Ag-EHCB complex blended with the acetol reducing agent was tested briefly. The
solution was coated on a PET film (50 × 50 mm2) and then
heated to 65 oC for 10 min and 120 oC for 1 minute. Figure 6
shows a typical scanning electron micrograph of the silver
film deposited on the PET substrate held at 65 and 120 oC.
Comparing Figure 6(a) and 6(b), it can be seen that despite
the lower temperature, the film was close to a continuous
structure, whereas the film consisted of continuous islands at
higher temperatures (120 oC). Furthermore, agglomeration
increased considerably as deposition rate increased for a
given temperature, and large clusters with small separations
were formed. The surface resistivity of the silver-coated film
was investigated at room temperature. It was evident that the
surface resistivity decreased as reduction time increased up
to 10 min. Although the temperature was 120 oC, the thin
silver layer with thickness of 100 nm showed a resistivity of
1.7 × 108 Ω·m, which was comparable to bulk silver (1.6 ×
10−8 Ω·m).
When the silver coating was performed on the PET film at
120 oC, the visible reflectivity of the freshly coated silver
mirror surface was 88% at 1.0 μm thickness, as illustrated in
Figure 7(a). Silver coatings are often used in electronics to
provide a corrosion-resistant electrically conductive layer on
a solderable surface, typically in electrical connectors and
Figure 6. SEM images at two different magnifications showing
silver-coated films generated after 10 min heating (a) at 65 oC and
(b) at 120 oC after silver coating by dipping.
Figure 7. Optical images of (a) silver-coated film after spin coating of Ag-EHCB complex and acetol solution and (b) polymer
molding product after silver coating by chemical reduction at 65 oC.
printed circuit boards. In addition, a low temperature (65 oC)
thermal deposition process excels for plating applications on
small to large-size products that have the highest aesthetic,
selective coating or functional performance requirements.
Figure 7(b) shows an optical image of a glossy and glittered
silver-coated plastic molded product by thermal heating at
65 oC after simple dip-coating.
Conclusion
In summary, we developed a novel acetol latent reducing
Silver Coating Using Latent Reductant from Silvercarbamate
agent for the Ag-EHCB complex as a silver precursor. The
Ag-EHCB and acetol solution stably existed in alcohol
solution at room temperature with no changes occurring
over time. The Ag-EHCB complex gradually lost organic
moieties afforded to an Ag-NPs colloid solution when heated to 65 oC and was protected by the PVP matrix at temperatures < 65 oC. Silver coatings were also deposited using a
combination of chemical reduction and thermal treatment.
This method successfully demonstrates the deposition of a
silver coating on thermally sensitive polymer substrates.
Acknowledgments. This work was supported by Priority
Research Centers Program (grant#: 2009-0093829), WCU
(World Class University) program (grant#: R31-10069) and
Strategic Technology Project (grant# 10031791) through the
National Research Foundation (NRF) funded by the
Ministry of Education, Science and Technology and the
Ministry of Knowledge Economy.
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