A new method for thin silver coating reaction using Ag2O and polyethylene glycol 400

Article abstract of DOI:10.1246/cl.2007.782

A new method for thin silver coating reactions using Ag₂O and polyethylene glycol 400, an environmentally benign solvent, was devised. The reactions were investigated using ¹H NMR, IR, UV-vis, SEM, and XPS measurements. Copyright

Full text of DOI:10.1246/cl.2007.782

782
Chemistry Letters Vol.36, No.6 (2007)
A New Method for Thin Silver Coating Reaction Using Ag2O and Polyethylene Glycol 400
Kiyoshi Tanemura,ꢀ1 Takanobu Koike,2 Shigeki Komatsu,1 Shin-ichi Goto,1
Yoko Nishida,1 Tsuneo Suzuki,1 and Takaaki Horaguchi2
1School of Life Dentistry at Niigata, The Nippon Dental University, Hamaura-cho, Niigata 951-8580
2Department of Chemistry, Faculty of Science, Niigata University, Ikarashi, Niigata 950-2181
(Received March 7, 2007; CL-070246; E-mail: tanemura@ngt.ndu.ac.jp)
A new method for thin silver coating reactions using Ag2O
and polyethylene glycol 400, an environmentally benign solvent,
was devised. The reactions were investigated using 1H NMR, IR,
UV–vis, SEM, and XPS measurements.
Thin silver coating reactions play a prominent role on the
manufacture of many electric devices1 as well as the fabrication
of silver mirrors.2 Normally, silver mirrors are produced by the
reaction of silver nitrate in aqueous ammonia with reducing
agents such as formalin or glucose, which is known as an elec-
troless plating method.3 However, the disposal of large amounts
of oxidized materials derived from formalin or glucose has
caused many problems. Furthermore, explosive compounds
such as silver fulminate (AgONC) are generated in the reaction
mixture on standing.3
Polyethylene glycol (PEG) is widely used as a safe and
environmentally friendly material.4 During the course of our
investigation concerning PEG,5 we were interested in the fact
that Ag2O exhibited appreciable solubility in polyethylene
glycol 400 (PEG400) having an average molecular weight of
4006 to give a colorless solution, which gave the silver mirror
on the inside surface of the flask by heating at 90 ꢁC. In this
paper, we wish to report the new method for thin silver coating
reactions using Ag2O and PEG400.
Figure 1. (a) The silver mirror fabricated on the inside surface
of the flask. (b) The SEM image of silver particles fabricated
on the cover glass after 3 h and (c) after 8 h. (d) The SEM image
of the cross sectional view of the silver film on the cover glass
after 8 h.
First, we examined the reactions of various silver com-
pounds in PEG400. When the solution of Ag2O in PEG400
was heated at 90 ꢁC for 8 h, the silver mirror was coated on
the inside surface of the flask (Figure 1a). Heating the solution
of AgO in PEG400 also gave the silver mirror; however, we
chose Ag2O as a silver source because AgO is more expensive
than Ag2O and is difficult to obtain in pure form. The treatment
of Ag2CO3 and AgOAc with PEG400 afforded silver precipi-
tates on the bottom of the flask. The reaction of AgNO3 with
PEG400 at 90 ꢁC for 8 h did not occur to give colorless solution.
It has been recently reported that silver nanoparticles7 were
generated via the redox reactions between AgNO3 and
PEG2000 at 80 ꢁC8 or AgNO3 and ethylene glycol at 148 ꢁC.9
We compared the reactions of Ag2O in the different molecu-
lar weight of PEG. The reaction of Ag2O in PEG200 at 90 ꢁC led
to the formation of silver deposits on the bottom of the flask.
Although the silver mirror was also generated in PEG600, we
chose PEG400 as a reaction medium because PEG600 is a solid
under 17–22 ꢁC.
particles existed as larger aggregates with the size of 150–
300 nm (Figure 1c). As seen in Figure 1d, the cross sectional
view indicated that thickness of the silver film is 150–300 nm
after 8 h.
The X-ray photoelectron spectrum (XPS) of the fabricated
silver film was measured. The spectra showed the peaks of
silver at 368.3 and 347.2 eV. In addition, the shoulders at
367.6 and 373.8 eV was also observed owing to the incorpora-
tion of Ag2O in the silver film.
The optical properties of silver particles were characterized
by UV–vis absorption spectroscopy. We performed measure-
ments on dilute suspensions (1:0 ꢂ 10ꢃ3 mol dmꢃ3) of Ag parti-
cles in PEG400 (Figure 2). The appearance of a plasmon peak
at 428 nm belonging to the dipole resonance of silver particles
suggested the formation of the particles with the size of
ca.100 nm.2,8
We first supposed that the reactions proceeded via the redox
reactions between Ag2O and PEG400. However, the oxidation
of the hydroxy groups in PEG to the corresponding aldehydes
Next, we employed scanning electron microscopy (SEM) in
order to access the size and morphology of silver particles.
Figure 1b shows the SEM image of silver particles fabricated
on the cover glass after 3 h. The image demonstrated that silver
particles existed as the single particles with the size of ca.
100 nm or aggregates of a few particles. After 8 h, most of silver
1
or carboxylic acids was not observed in the H NMR spectrum
of the recovered PEG.8,10 The redox reactions between Ag2O
and PEG400 were too slow to be detected because of low
reactivities of PEG400, although Ag2O was reduced to Ag in
MeOH by the redox reaction even at room temperature. The
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.6 (2007)
783
metal coating reactions did not proceed at 90 and 180 ꢁC in all
cases. Since metal oxides are insoluble in water and most of
organic solvents, and generally only soluble in aqueous acids
or bases, it is noteworthy that some metal oxides are soluble
in PEG400. We suppose that metal oxides are dissolved in
PEG400 to make the complex with the PEG molecule.6 The
structure of the complex is now under investigation.
In conclusion, we constituted a new methodology for
thin silver coating reactions using Ag2O and PEG400. This
method is environmentally friendly because no additives
such as reducing agents are needed in the procedure. PEG is a
relatively safe solvent with low toxicity and low volatility.
0.25
0.20
0.15
0.10
0.05
0.00
250
300
350
400
450
500
Wavelength / nm
The authors thank Dr. K. Nakamura and Dr. H. Aoyagi
for helpful discussions.
Figure 2. UV–vis absorption spectrum of silver particles in
PEG400 (1:0 ꢂ 10ꢃ3 mol dmꢃ3).
References and Notes
1
2
3
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O
H
C
OCH2CH2 OH
n
1
Chart 1.
small new signals appeared at ꢀ 4.33 (t, J ¼ 4:5 Hz) and 8.10 (s).
The IR spectrum showed that an new absorption appeared
at 1720 cmꢃ1. We suppose that these observations might be
attributed to the formation of a trace amount of the formate 1
(Chart 1) produced by the autoxidation of PEG400 via car-
bon–carbon bond cleavage. The signals at ꢀ 4.33 and 8.10 in
1
the H NMR spectrum were assigned to the methylene protons
adjacent to the ester and the proton of the formate, respectively.
The absorption at 1720 cmꢃ1 in the IR spectrum was attributed to
the existence of the ester group. Long standing of PEG400 in air
4
5
1
afforded the same peaks in H NMR and IR spectra. Although
Zhang et al. reported that these signals in the 1H NMR spectrum
were attributed to the formation of the aldehyde possessing the
interaction with the silver nanoparticle,8 the signals of aldehyde
protons should be observed at 9–10 ppm in our experiments,
which is the standard chemical shift of aldehyde protons.
In addition, silver was generated using Ag2O and AgO
(mirrors), Ag2CO3, and AgOAc (deposits) in polyethylene
glycol 500 dimethyl ether (PEG500DME). These facts indicate
that the silver coating reactions can proceed in the solvent
without hydroxy groups. Thus, the reactions may proceed via
the redox reactions between Ag2O and compound 1 or other
oxidized materials.
6
7
It has been reported that silver nanoparticles were formed
from the reduction of AgNO3 in PEG2000.8 In the case of
using PEG as a solvent, many oxygen atoms in the long PEG
chains provided coordinative stabilization on the surface of the
nanoparticles.8 PEG2000 might have great stabilization effect
to prevent the growth of the silver nucleus than PEG400.
Since PEG is a cheap and relatively safe compound with low
toxicity and low volatility, it is a great advantage that aqueous
ammonia, which is a basic and irritant solvent, is not employed.
After the silver coating reaction, excess silver and PEG400 ca be
easily separated by filtration.
8
9
C. Luo, Y. Zhang, X. Zeng, Y. Zeng, Y. Wang, J. Colloid
Interface Sci. 2005, 288, 444.
B. Wiley, T. Herricks, Y. Sun, Y. Xia, Nano Lett. 2004, 4,
1733.
´
10 L. M. Liz-Marzan, I. Lado-Tourin˜o, Langmuir 1996, 12,
3585; M. Andersson, V. Alfrerdsson, P. Kjellin, A. E. C.
Palmqvist, Nano Lett. 2002, 2, 1403; G. C. Lica, B. S.
Zelakiewicz, Y. Y. Tong, J. Electroanal. Chem. 2003, 554,
127.
Finally, we attempted the reactions of the other metal oxides
in PEG400. Metal oxides such as CuO, Cu2O, SnO, Co3O4,
Au2O3, and Ti2O3 were appreciably soluble in PEG400,6 but
Published on the web (Advance View) May 19, 2007; doi:10.1246/cl.2007.782