Cách biện luận bài tập môn Các Phương pháp phân tích không hủy mẫu

Materials Science and Engineering B 145 (2007) 67–75

Supercritical carbon dioxide-assisted synthesis of silver
nano-particles in polyol process
Yu-Wen Chih, Wen-Tung Cheng ∗
Department of Chemical Engineering, National Chung Hsing University 250 Kuo-Kuang Rd.,
Taichung 402, Taiwan, ROC
Received 16 May 2007; received in revised form 1 October 2007; accepted 6 October 2007

Abstract
Silver nano-particles have been synthesized by the polyol process with the assistance of supercritical carbon dioxide (SCCO2 ), with silver
nitrate used as the base material, polyvinyl pyrrolidone (PVP) as the stabilizer for the silver clusters, and ethylene glycol as the reducing agent
and solvent. Polyvinyl pyrrolidone not only protected nano-size silver particles from aggregation, but it also promoted nucleation. The silver nanoparticles synthesized by SCCO2 were smaller and had a more uniform dispersion than those made under the same conditions by the conventional
heating process. The superior fluidity and diffusivity of SCCO2 reduced the viscosity of the ethylene glycol and penetrated the entire solution
to help increase the contact frequency of silver ions and electrons and in doing so, nucleation from silver ion to seed crystal was increased. The
as-synthesized silver nano-particles were analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning
electron microscope (FESEM) and UV–vis spectrophotometer. The UV–vis spectrum of the silver nano-particles was sensitive to particle size. In
particular, high dispersion stability synthesized silver nano-particles could be obtained by binding PVP on their surface in this work.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Silver nano-particles; Supercritical carbon dioxide; Uniform dispersion

1. Introduction
Noble metal nano-particles are of great interest because of
their application in microelectronics and optical devices [1–4].
Silver plays an important role in the electronic and photonics
industries and, in recent years, the preparation of silver particles with supercritical fluids (SCFs) has received increasing
attention [5–11]. SCFs exhibit gas-like mass transfer properties
but liquid-like salvation capabilities and, because of their high
diffusivity and low viscosity, are capable of penetrating solutions or materials to quicken reactions. The density of SCFs
can be altered continuously by manipulating pressure and temperature, thus making the solution strength of the fluid tunable
[12]. In most cases, SCFs are used to obtain particles within a

narrow size distribution, i.e. micron, sub-micron and nanometer [13–16]. Moreover, SCFs can be used as processing media
for nanostructure devices [17–19]. The increasing number of
scientific and industrial research groups worldwide that are con-

∗

Corresponding author. Tel.: +886 4 22857325; fax: +886 4 22854734.
E-mail address: (W.-T. Cheng).

0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.mseb.2007.10.006

ducting research in SCFs technology attest to its importance for
nanotechnology development.
Of the many possible supercritical fluids, carbon dioxide
(CO2 ) is the one most frequently used as an alternative solvent for materials synthesis and processing. Researchers have
promoted CO2 as a sustainable and green solvent because it is
inexpensive, non-toxic, non-flammable, non-polluting, and has
a moderate critical temperature and pressure (Tc = 31.1 ◦ C and
Pc = 7.38 MPa). However, as CO2 has a zero dipole moment
and a low dielectric constant, its charge-separated molecular
system results in low polarity and high electrostatic interactions. Thus, hydrocarbon-based surfactants are not suitable for
a CO2 /water interface [20–22]. Nevertheless, supercritical CO2
has significant potential for future applications in CO2 solventbased systems that could lead to a wide range of particle
synthesis.
For this research, supercritical CO2 was employed in the synthesis of nano-size silver particles by the polyol process and, in
order to prevent coalescence, polyvinyl pyrrolidone (PVP) was
used as a stabilizer. PVP can also promote the nucleation of
metallic silver because silver ions are easily reduced by PVP
[23]. PVP has a polyvinyl skeleton with polar groups. The

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Fig. 1. Supercritical CO2 process apparatus in this study.

donated lone pairs of both nitrogen and oxygen atoms in the
polar groups of the PVP repeated unit may occupy two sp orbital
of the silver ions to form a complex compound to decrease their
chemical potential [24]. The temperature and pressure of super-

critical carbon dioxide, the molar ratio of PVP/AgNO3 , and the
molecular weight of PVP were analyzed to determine the size
of the silver nano-particles synthesized by the polyol process
through UV–vis spectroscopy, XRD, TEM, and FESEM.

Fig. 2. TEM images of as-synthesized Ag nano-particles at 100 ◦ C for different synthesis process in the molar ratio of PVP (MW = 10,000)/AgNO3 = 1: (a)
conventional heating method; (b) SCCO2 -assisted at 25 MPa.


Y.-W. Chih, W.-T. Cheng / Materials Science and Engineering B 145 (2007) 67–75

Fig. 3. UV–vis spectra of as-synthesized Ag nano-particles at 100 ◦ C for different synthesis process in the molar ratio of PVP (MW = 10,000)/AgNO3 = 1.

69

Fig. 6. XRD pattern of as-synthesized Ag nano-particles with a SCCO2 assisted process at 25 MPa and 100 ◦ C for the molar ratio of PVP
(MW = 10,000)/AgNO3 = 1.

2. Experiment
2.1. Materials
Silver nitrate (J & J Materials Incorporated), used as the
precursor for the preparation of silver nano-particles; polyvinyl
pyrrolidone with molecular weight-averages of 10,000, 40,000,
and 55,000 was purchased from SIGMA, MP Biomedicals Inc.,
and Aldrich, respectively; ethylene glycol (SHOWA), used as
both reducing agent and solvent; 99.5% purity of liquid CO2
(pressurized to 75 kg/cm2 ) was purchased from TOYO Gas company; and ethanol (ECHO Chemical Co. LTD), used as a thinner
for the TEM sample.
Fig. 4. FESEM morphology of as-synthesized Ag nano-particles with a
SCCO2 -assisted process at 25 MPa and 100 ◦ C for the molar ratio of PVP
(MW = 10,000)/AgNO3 = 1.

2.2. Preparation of silver nano-particles
The schematic representation of the experimental set-up
used in this study is shown in Fig. 1. The 5 × 10−3 M of
AgNO3 in ethylene glycol and 2.5–7.5 × 10−3 M of PVP
reactant were charged into a reaction vessel with a maxi-

Fig. 5. SAED pattern of as-synthesized Ag nano-particles with a SCCO2 assisted process at 25 MPa and 100 ◦ C for the molar ratio of PVP
(MW = 10,000)/AgNO3 = 1.

Fig. 7. UV–vis spectra of as-synthesized Ag nano-particles at 25 MPa for various temperature in the molar ratio of PVP (MW = 10,000)/AgNO3 = 1.


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Fig. 8. TEM images of as-synthesized Ag nano-particles at 25 MPa for various temperature in the molar ratio of PVP (MW = 10,000)/AgNO3 = 1: (a) 50 ◦ C; (b)
70 ◦ C; and (c) 100 ◦ C.

mum volume of 300 ml with a peristaltic pump (Cole-Parmer
Masterflex® L/S 07518-00). For each experiment, the reactor was operated according to the following procedure: after
introducing 20 ml of reactant, the reactor was closed and
CO2 was then injected up to 6.5–30 MPa by air driven liquid pump (Haskel ALG-60). The temperature, controlled by
thermostat (NewLab KD-2), and pressure for the process were
maintained for 3 h. Supercritical CO2 was then vented at atmosphere and the as-synthesized silver particles were collected for
characterization.
2.3. Instrumentation
Fig. 9. UV–vis spectra of as-synthesized Ag nano-particles at 100 ◦ C for various
pressure in the molar ratio of PVP (MW = 10,000)/AgNO3 = 1.

For UV–vis spectroscopy, a 1 cm quartz cuvette with
SHIMADZU UVmini-1240 spectrophotometer was used.


Y.-W. Chih, W.-T. Cheng / Materials Science and Engineering B 145 (2007) 67–75

71

Fig. 10. TEM images of as-synthesized Ag nano-particles at 100 ◦ C for various pressure in the molar ratio of PVP (MW = 10,000)/AgNO3 = 1: (a) 8 MPa; (b) 15 MPa;
and (c) 25 MPa.

Transmission electron microscopy (TEM) was employed to
characterize the as-synthesized silver nano-particles. That is the
solution with as-synthesized Ag nano-particles is diluted five
times by absolute ethanol, and placed two to three drops onto

carbon-coated Cu grid. The sample was then photographed by

two photos at 50,000× and 100,000×, respectively. The image
of 50,000× is used to estimate average size of as-synthesized Ag
particle by FUJIFILM Image Gauge V4.0 software. The number of Ag particles is about 100–500 granules at 50,000×, which
is depended on different experiment condition. TEM measure-

Fig. 11. The reaction mechanism for PVP and silver ions [23].


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ment was carried at 120 KV using JEOL JEM-1200 CX II. Field
emission scanning electron microscopy (FESEM) was employed
to observe the morphology of Ag nano-particles (JEOL JSM6700F). Crystalline phases were determined by X-ray powder
diffraction (XRD) (MAC SCIENCE MXT III).
3. Results and discussion
3.1. Performance of SCCO2 -assisted polyol synthesis

Fig. 12. UV–vis spectra of as-synthesized Ag nano-particles at 100 ◦ C and
25 MPa for various molar ratio of PVP (MW = 10,000)/AgNO3 .

Fig. 2 compares a conventional heating process at 100 ◦ C
with a supercritical CO2 -assisted process at 100 ◦ C and 25 MPa
for synthesizing silver nano-particles from a ration of PVP
(MW = 10,000)/AgNO3 = 1 in the presence of ethylene glycol.
The Ag nano-particles synthesized by supercritical CO2 are
clearly smaller and have a more uniform dispersion than those

made by the conventional heating process through increasing

Fig. 13. TEM images of as-synthesized Ag nano-particles at 100 ◦ C and 25 MPa for various molar ratio of PVP (MW = 10,000)/AgNO3 : (a) 0.5; (b) 1.0; and (c) 1.5.


Y.-W. Chih, W.-T. Cheng / Materials Science and Engineering B 145 (2007) 67–75

the solubility of CO2 to ethylene glycol by increasing pressure
[25,26]. The superior mass transfer of supercritical CO2 permeated more effectively into the reaction solution to promote the
synthesis of particles and enhance the reaction rate [5,27,28].
Therefore, the superior fluidity and diffusivity of supercritical
CO2 can also reduce the viscosity of the ethylene glycol and
penetrate the entire solution to promote the contact frequency
of silver ions (Ag+ ) and electrons (e− ). Evidence of this effect
is displayed in Fig. 3 by UV–vis absorption spectra of the particles synthesized by the conventional heating method and the
supercritical CO2 method, respectively. The figure reveals that
the supercritical CO2 -assisted synthesis has higher absorption
intensity than the conventional heating method due to the higher
concentration of as-synthesized silver nano-particles of granular
structure and uniform size, as shown in Fig. 4.
The electron diffraction pattern of five strong fringes of Ag
(1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2), as pictured in Fig. 5,
exhibits the characteristic peaks of face central cubic crystalline
structure. Corresponding to Fig. 5, the powder XRD pattern in
Fig. 6 shows a similar diffraction peak feature, and. The value
of the lattice constant is calculated from its corresponding XRD
˚ The referable JCPDS file NO. 87-0597 is
pattern: a = 4.068 A.
˚
4.0862 A, and the silver peak feature is similar. In addition, the

structure of the synthesized Ag nano-particles by polyol process
generally appears as multiply twinned particles (MTP) because
of the surfaces bounded by the lowest energy (1 1 1) face [29].
3.2. Effect of temperature on SCCO2 -assisted polyol
synthesis
The particles synthesized by chemical reduction may be created by the nucleation and growth of grain: nucleation formats
new particles and growth increases the particles’ characteristic
length. Thus, the steps complete each other. Faster nucleation
relative to the growth of grain makes for a smaller particle
size [30], which is more sensitive to temperature during the
process.
Fig. 7 illustrates the relationship between the reaction temperature and UV–vis absorption spectra of the synthesized Ag
nano-particles. The surface plasma absorption band of silver is
about 420 nm. This band is broader at low temperatures than at
higher temperatures, and the absorption intensity can be raised
with a slight red shift as the temperature increases because the
particles are larger at higher temperatures. As presented in Fig. 8,
the as-synthesized Ag nano-particles are spherical, 5–25 nm in
diameter at 50, 70, and 100 ◦ C, and 25 MPa. It may be that
at higher temperatures the rate of particle growth increases,
thus making possible the formation of larger Ag particles. A
lower temperature, however, does not produce enough energy
for ethylene glycol to transfer aldehydes into ketones, producing electrons to form Ag nano-particles via the supercritical
CO2 -assisted process.

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forms a spherical nucleus described by [31,32]
G∗ =

16πγ 3
16πγ 3
=
2
3 G2v
3(ρ| μ|)

(1)

where γ is the interfacial energy, ρ is the number density, μ is
the difference in chemical potential and Gv is the difference in
Gibb’s free energy per unit volume. The variation of the energy
barrier with pressure can be expressed as
∂ G∗
∂P

=
T

3 G∗
γ

∂γ
∂P

−
T

2 G∗
Gv

∂ Gv
∂P

T

(2)

Compared with Gv , the pressure variation of interfacial
energy is very small and can be ignored. The energy barrier
of nucleation lowers as pressure increases and this speeds up
the nucleation rate. According to the phase transition theory, the
ratio of growth rate to nucleation rate determines the crystalline
grains and the particle size is smaller at increased nucleation
rates. Therefore, the particle size decreases with increasing pressure.
The UV–vis absorption spectra of silver nano-particles are
shown in Fig. 9. The surface plasma absorption band, at
about 420 nm, not only strengthens as the pressure increases,
it also shifts to a shorter wavelength region as particle size
decreases. In addition, the results indicate that the absorption intensity increases along with the increasing pressure in
supercritical CO2 -assisted synthesis. Evidence of these phenomena can be obtained from Fig. 10 as to the particle size
and yields of as-synthesized silver nano-particles. The smallest particles are the Ag nuclei, and the increased pressure,
along with the connected increase in CO2 density, dilute the
ethylene glycol phase and thereby reduce the super-saturation.
The system of this study is formulated as follows, 20 ml ethylene glycol (Tc = 372 ◦ C and Pc = 7.7 MPa) plus 715 ml CO2
(Tc = 31.1 ◦ C and Pc = 7.38 MPa) for 735 ml (χEG = 0.027 and
χCO2 = 0.973) of mixture solution, which critical condition is
estimated by Pc = 7.7 × 0.027 + 7.38 × 0.973 = 7.389 MPa, and
Tc = 372 × 0.027 + 31.1 × 0.973 = 40.5 ◦ C.

3.3. Effect of pressure on SCCO2 -assisted polyol synthesis
The pressure variable is just as important to processing as
temperature and chemical composition. The energy barrier G*

Fig. 14. UV–vis spectra of as-synthesized Ag nano-particles at 100 ◦ C and
25 MPa for various molecule weight in the molar ratio of PVP/AgNO3 = 1.


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Y.-W. Chih, W.-T. Cheng / Materials Science and Engineering B 145 (2007) 67–75

Moreover, the pressure and temperature are more than 8 MPa
and 50 ◦ C, respectively, for the synthesis of Ag nano-particle
in ethylene glycol assisted with supercritical CO2 . Hence, the
system is a homogeneous phase before nucleating in this work.
Thus, the growth process encourages smaller and more
dispersed particles. Furthermore, higher densities provide the
enhanced ligand tail salvation necessary to suspend particles
[33]. Raising the pressure can increase the density and mass
transfer efficiency of supercritical CO2 and enhance the solubility of supercritical CO2 to ethylene glycol, making the reactive
solution rapidly reach super-saturation, and it can increase the
nucleation rate to effect instantaneous forming of Ag nuclei.
Clearly, particle size is smaller at high pressure and increas-

ing the pressure may promote the formation of small-size Ag
nano-particles in supercritical CO2 -assisted synthesis.
3.4. Effect of a stabilizer on SCCO2 -assisted polyol
synthesis
The use of a stabilizer has two purposes: to generate a complex compound with the initial material and to protect particles

from growth and agglomeration. The PVP protective mechanism permits the lone pair of electrons from the nitrogen and
oxygen atoms in the polar groups of the PVP molecules may
be donated into sp hybrid orbitals of silver ions to create complex compounds. PVP molecules can bond with Ag ions by

Fig. 15. TEM images of as-synthesized Ag nano-particles at 100 ◦ C and 25 MPa for various molecule weight in the molar ratio of PVP/AgNO3 = 1.


Y.-W. Chih, W.-T. Cheng / Materials Science and Engineering B 145 (2007) 67–75

intra- or inter-chain interactions. This reaction is shown in
Fig. 11 [23]. Subsequently, ethylene glycol can offer an electron
to the PVP–Ag+ complex, producing a colloidal Ag solution.
This effectively diminishes the chemical potential and makes
reduction of the PVP–Ag+ complex easier. Additionally, Facetselective capping agents can be used to promote a particular
shape by selectively interacting with a specific crystallographic
facet [29,34,35]. The advantage of PVP is its ability to specifically interact with Ag’s (1 1 1) planes to produce different
nanostructures [29,36]. The morphology and dimensions of the
particles are also dependent on the molar ratio of PVP to AgNO3 .
This study employed three molar ratios of PVP/AgNO3 to analyze the morphology and UV–vis absorption spectra of silver
nano-particles.
As presented in Fig. 12, the absorption peak is shaped and
slightly shifted to a shorter wavelength region that increases the
molar ratio of PVP/AgNO3 . This implies that higher concentrations of PVP can prevent as-synthesized Ag nano-particles from
aggregating, producing smaller particles. In addition, added
amounts of PVP promote nucleation of metallic ions because
Ag ions are easily reduced by the lone pair of electrons from
PVP molecules. PVP promotes silver nucleation and makes
for smaller nano-particles. Thus the surface plasma absorption
band of as-synthesized Ag nano-particles strengthens as PVP
increases, which is verified by TEM images in Fig. 13. Compared

with other methods, the dispersion in this study is favorable at
a low molar ratio of PVP/AgNO3 .
Fig. 14 shows UV–vis absorption spectra of as-synthesized
Ag nano-particles which vary with the molecular weight of PVP.
Larger molecular weight of PVP have longer chains that provide
superior physical barriers to protect as-synthesized Ag nanoparticles from aggregation, so the absorption peak shows a slight
blue shift due to the effect of smaller particles, as proved by
TEM images in Fig. 15. The smaller PVP molecules do not
have adequate time to surround the nano-particles and are consequently unable to stop aggregation. The viscosity of the liquid
increases with molecular weight of PVP, and thus it is more
difficult for supercritical CO2 to diffuse in the reaction solution. This suggests that the yield of silver nano-particles and
the molecular weight of the polymeric capping agent need to
optimize with supercritical CO2 -assisted synthesis by polyol
process.
4. Conclusions
In this research, supercritical CO2 -assisted synthesis by
polyol process for the preparation of silver nano-particles has
been demonstrated. As shown by the results, as-synthesized
Ag nano-particles have a central cubic structure with diameters ranging from 5 to 25 nm depending on the temperature
and pressure of supercritical carbon dioxide, the molar ratio

75

of PVP/AgNO3 , and the molecular weight of the polymeric
capping agent. Increasing both temperature and pressure promoted the production of smaller-diameter silver particles and
high-dispersion stability of silver nano-particles was obtained
by binding PVP on the surface of the particles.
References
[1] G. Schimd, Chem. Rev. 92 (1992) 1709.
[2] R.P. Andres, J.D. Bielefeld, J.I. Henderson, D.B. Janes, V.R. Kolagunta,

C.P. Kubiak, W.J. Mahoney, R. G. Osifchin 273 (1996) 1690.
[3] D.L. Feldheim, C.A. Foss Jr., Metal Nanoparticles: Synthesis, Characterization, and Applications, Marcel Dekker, New York, 2002.
[4] P.V. Kamat, J. Phys. Chem. B 106 (2002) 7729.
[5] M. Ji, X. Chen, C.M. Wai, J.L. Fulton, J. Am. Chem. Soc. 121 (1999) 2631.
[6] H. Ohde, F. Hunt, C.M. Wai, Chem. Mater. 13 (2001) 4130.
[7] Y.P. Sun, P. Atorngitjawat, M.J. Meziani, Langmuir 17 (2001) 5707.
[8] P.S. Shah, S. Husain, K.P. Johnston, B.A. Korgel, J. Phys. B 106 (2002)
12178.
[9] K.S. Morley, P.C. Partricia, P.B. Webb, A.R. Berry, F.J. Allison, G.
Moldovan, P.D. Brown, S.M. Mowdle, J. Mater. Chem. 12 (2002) 1898.
[10] J. Liu, P. Raveendran, Z. Shervani, Y. Ikushima, Y. Hakuta, Chem.-Eur. J.
11 (2005) 1854.
[11] E. Reverchon, R. Adami, J. Supercrit. Fluids 37 (2006) 1.
[12] B. Chehroudi, Combust. Sci. Technol. 178 (2006) 555.
[13] J. Liu, P. Raveendran, Z. Shervani, Y. Ikushima, Chem. Commun. (2004)
2582.
[14] M.C. Mcleod, F.G. William, B.R. Christopher, Langmuir 20 (2004) 7078.
[15] H. Reve´ron, A. Cyril, L.S. Anne, E. Catherine, M. Mario, F. Cansell,
Nanotechnology 16 (2005) 1137.
[16] H. Wakayama, N. Setoyama, Y. Fukushima, Adv. Mater. 15 (2003) 742.
[17] D.P. Long, J.M. Blackburn, J.J. Watkins, Adv. Mater. 12 (2000) 913.
[18] A. O’Neil, J.J. Watkins, MRS Bull. 30 (2005) 967.
[19] H. Wakayama, Y. Fukushima, Ind. Eng. Chem. Res. 45 (2006) 3328.
[20] A. Fujii, T. Ebata, N. Mikami, J. Phys. Chem. A 106 (2002) 10590.
[21] P. Raveendran, S.L. Wallen, J. Phys. Chem. B 107 (2003) 1473.
[22] H.M. Woods, M.M.C.G. Silva, C. Nouvel, K.M. Shakesheff, S.M. Howdle,
J. Mater. Chem. 14 (2004) 1663.
[23] Z. Zhang, B. Zhao, L. Hu, J. Solid State Chem. 121 (1996) 105.
[24] W.J. Jin, H.K. Lee, E.H. Jeong, W.H. Park, J.H. Youk, Macromol. Rapid
Commun. 26 (2005) 1903.

[25] D.Q. Zheng, W.D. Ma, R. Wei, T.M. Guo, Fluid Phase Equilib. 155 (1999)
277.
[26] M. Saharay, S. Balasubramanian, Chem. Phys. Chem. 5 (2004) 1442.
[27] C.E. Bunker, H.W. Rollins, J.R. Gord, Y.P. Sun, J. Org. Chem. 67 (1997)
7324.
[28] Y.P. Sun, Supercritical Fluid Technology in Materials Science and Engineering, Marcel Dekker Inc, New York, 2002.
[29] Y. Sun, Y. Xia, Science 298 (2002) 2176.
[30] K.H. Lieser, Angew. Chem. Int. Ed. Engl. 8 (1969) 188.
[31] S. Auer, D. Frenkel, Nature 413 (2001) 711.
[32] S.C. Liao, W.E. Mayo, K.D. Pae, Acta Mater. 45 (1997) 4027.
[33] P.S. Shah, J.D. Holmes, K.P. Johnston, B.A. Korgel, J. Phys. Chem. B 106
(2002) 2545.
[34] C.J. Murphy, T.K. Sau, A. Gole, C.J. Orendorff, MRS Bull. 30 (2005) 349.
[35] B. Wiley, Y. Sun, J. Chen, H. Cang, Z.Y. Li, X. Li, Y. Xia, MRS Bull. 30
(2005) 356.
[36] R. Jin, Y.C. Cao, E. Hao, G.S. M´etraux, G.C. Schatz, C.A. Mirkin, Nature
425 (2003) 487.