Preparation and characterization of stable dispersed
and hydrophobic silver nanoparticles
Dan Li *
Department of Chemistry and Chemical Engineering,
Weifang University,
Weifang, China
E-mail: danli830109@163.com
Abstract—The
were
medium. Oleic acid is used as surface coating reagent and
successfully fabricated by stabilization in the mixed system of
alkylamines as solvent. The effects of n-butylamine or
water, alkylamine and oleic acid and reduction with hydrazine
cyclohexylamine in the reaction solvents for the morphology
hydrate solution. The surface coated silver nanoparticles can
and composition of prepared silver nanoparticles were
stably disperse in nonpolar solvent such as alkane, chloroform or
investigated. The silver nanoparticles dispersed well in
kerosene and form a silver colloid. The effects of n-butylamine or
nonpolar solvent formed silver colloid. The thermal stability
cyclohexylamine in the reaction solvents for the morphology and
of the silver colloids was invesgatied by monitored their
composition of prepared silver nanoparticles were investigated.
Uv-vis spectra changing.
hydrophobic
silver
nanoparticles
Keywords: silver nanoparticles; hydrophobic; alkylamine
1. INTRODUCTION
2. EXPERIMENTS
a.
The increased interest in small clusters of metal particles is
related to the unique properties exhibited by these systems
which are the result of their size, high surface area and
exceptional surface activity providing excellent catalytic, and
optical properties. Silver nanoparticle based colloids or
nanofluids were studied after they were found to have
Materials
All chemicals used were of analytical grade, obtained
commercially and used without further purification. Silver
nitrate was supplied by Shanghai Chemical Corporation,
China. Oleic acid, n-butylamine, cyclohexylamine, ethanol,
and hydrazine hydrate solution were purchased from
Sinopharm Chemical Reagent Co., Ltd., Shanghai, China.
significantly higher thermal conductivities. Recently, the
effects were not just focused on the water based nanofluids,
b.
Preparation of silver nanoparticles
and more and more studies have been focused on preparation
The hydrophobic silver nanoparticles with well dispersion
and evaluation of oil based nanofluids. Transformer oil,
were prepared by reducing AgNO3 with hydrazine hydrate
mineral oil, silicon oil, and some organic solution are used as
solution at 80 °C). In a typical synthesis, 20 mL 0.05 M
the base fluid for studying nanofluids. [1-5] However, metal
AgNO3 solution was mixed with a solution of oleic acid (2.0 g,
nanoparticles tend to form agglomerates because of strong
as a surface modifie) and amine (10 mL) with stirring at a
interparticle interactions, often leading to precipitation of the
temperature. The pH of solution was maintained at about
particles in the fluid phase. [6, 7] Therefore, a challenge of
11~12. Excess hydrazine hydrate solution was added to the
nanofluids is to produce well-dispersed, stable oil based
mixture. The color of the reaction mixture turned red and
nanofluids with a feasible method. It is strongly needed that
eventually brown. The reaction was allowed to proceed at the
the metal particles have well dispersible in oil and nonpolar
last temperature for 0.5 h with continuous stirring. The
medium.
reaction mixtures were left to cool down and stayed at room
This work provides a method to prepare hydrophobic silver
temperature. After cooling, the mixtures were separated from
nanoparticles with oleic acid coating in different alkylamine
the brown reaction mixture by centrifugation at 8000 rpm and
washed three times with excessive ethanol, and finally were
morphology are significant. In this process, oleic acid was
dried in a vacuum oven at 40 °C. The powders were obtained.
used to coat the surface of the silver nanoparticles due to the
c.
Measurements
weak coordination, so the prepared silver nanoparticles were
oil soluble. Oleic acid has also been extensively used in this
The transmission electron micrographs (TEM) were
obtained by employing HRTEM CM-200 (PHILIPS, USA)
instrument using an operating voltage of 160 kV. UV-vis
spectroscopy measurements (200-700 nm) were performed
using a UV-2550 spectrophotometer (Shimadzu, Japan)
equipped with 1.0 cm quartz cells at 25 C. The X-ray
diffraction patterns of Ag nanoparticles were recorded by
employing a Bruker D8 Advance X-ray diffractometer
(Bruker, Germany) with monochromatized Cu Kα radiation
(λ=1.5405 Å). The differential scanning calorimeter (DSC/TG,
NETZSCH STA 409 PC/PG) was used to analyze the thermal
decomposition process of the particles with a heating rate of
10 K min-1 in N2 with a flow rate of 20 ml/min. The metal
content of silver nanoparticles was determined by TG
analysis.
process as the surfactant to control the growth of metal
particles and prevent their agglomeration. Thus the silver core
growth was not effectively suppressed by oleic acid in the
cyclohexylamine solution. The structure of cyclohexylamine
in reaction system affected oleic acid coating on the surface of
the silver nanoparticles.
XRD was employed to examine the crystalline structures of
the as-prepared silver particles, as shown in Fig. 2. For all the
samples, the X-ray diffraction peaks can be well assigned to
the (111), (200), (220) and (311) crystallographic planes of
face-centered cubic (FCC) silver crystals, respectively. The
metal contents of silver nanoparticles were determined by TG
analysis. The metal contents of silver nanoparticles prepared
in n-butylamine solution or cyclohexylamine solution are
74 % and 93 % respectively, and the residual part is organic
3. RESULTS AND DISCUSSION
Butylamine and cyclohexylamine were used as a surfactant
for stabilizing Ag+ and oleic acid in the reaction system of
coexisting aqueous phase. Oleic acid was used for coating the
surface of the silver nanoparticles, which make the silver
nanoparticles are oil soluble.
stabilizer coating on the surface of the silver core. The metal
contents of coating layer coating on the surface of the silver
were affected by the reaction system, and the oleic acids
content coated on the silver nanoparticles prepared in
cyclohexylamine solution was significantly lower than those
prepared in n-butylamine solution.
The prepared silver nanoparticles can be separated from the
reaction medium and stable for storing. Temperature has
a
significant effect on the rate of reduction reaction. The silver
nanopartiles were prepared at different temperatures.
Samples for the TEM examination were prepared by
dissolving solid samples in dichloromethane or chloroform.
One or two drops of the sample solution were allowed to dry
on a copper grid coated with carbon film. Fig. 1 shows typical
TEM images and the corresponding size distribution of silver
nanoparticle obtained at different solvents. Fig. 1a showed
spherical and small nanoparticles which were prepared in
n-butylamine and oleic acid solution. The growth of silver
particles is effectively limited to an average size of 11~15 nm
by the well stabilization of oleic acids. Fig. 1b shows silver
nanoparticles prepared in the cyclohexylamin solution without
uniform morphology. Therefore, the influence of the different
alkylamine in the reaction system on the particle size and
b
Figure 1. TEM images of silver nanoparticles synthesized in different
Figure 4. UV-vis spectra of the as-prepared silver nanoparticles synthesized
alkylamine solutions: (a) butylamine; (b) cyclohexylamine.
in different solvents: (a) butylamine; (b) cyclohexylamine.
The solution for UV-vis measurement was obtained by
dispersing the silver nanoparticles in n-heptane. They have the
b
characteristic absorption with maxima at 410~420 nm for the
silver plasmon band similar to the literature values as shown
Intensity
in Fig. 4. The silver nanoparticle prepared in the mixture of
oleic acid and n-butylamine redissolved in n-heptane showed
a [110]
the λmax value of absorption at 410 nm (curve a). The other
[200]
silver
[311]
[220]
nanoparticles
prepared
in
oleic
acid
and
cyclohexylamine (curve b) show two absorption bands at 430
30
40
50
60
70
80
90
and 540 nm respectively, which is characteristic for increase
of the size. The peaks at 410 nm are conventional plasmon
2/
band for nearly spherical silver particles. However, compared
Figure 2. XRD patterns of silver nanoplates prepared in different alkylamine
with a, the band b is more broadened and the relative intensity
solutions: (a) butylamine; (b) cyclohexylamine.
of the plasma peak is reduced. The absorption peak of the
prepared silver nanoparticles in cyclohexylamine solution
appears red shifted and broadened. It is resulted from different
shape nanoparticles mixed in Fig.1b. The difference in the
100
shape of the plasma band suggested the change in particle size
b
95
under different laser wavelengths. The λmax values of these
TG/%
90
plasmon absorptions are different from the others because the
85
plasmon absorptions of nanoparticles rely on the particle size
and morphologies [8-10].
80
a
75
70
100
200
300
400
T/C
15
alkylamine solutions: (a) butylamine; (b) cyclohexylamine.
2.0
Thermal stability time/h
Figure 3. TG curves of the silver nanoparticles prepared in different
10
5
1.5
Abs.
0
110
b
1.0
120
130
140
150
160
Temperature/C
Figure 6. Thermal stability time of the silver-heptane colloids at different
0.5
temperatures.
a
0.0
300
400
500
/nm
600
700
The UV-vis spectral analysis was used to monitor the
thermal stability of silver prepared in a mixture of oleic acid
and n-butylamine. The silver nanoparticles dispersed in
n-heptane were stable at room temperature for months, and no
detectable deposits could be found. It follows that the
hydrophobic layers of oleic acid outside the silver cores truly
protect the particles from oxidation and agglomeration. To
further investigate the thermal stabilities of the silver
nanofluids, samples of 0.1 mass % silver nanoparticles
prepared at 80 °C with n-heptane as the base fluid were
prepared and monitored in the temperature range of
120-160 °C. The silver nanofluids were kept in an 50 mL
Teflon-lined stainless steel autoclave, sealed tightly, and
thermostatted at a certain temperature. The nanosilver colloids
were considered to be stable when the absorption value of the
maxima (λmax) at 418 nm did not show a visible change or
red-shift and the colloids were uniform without changing their
appearances. It indicated that the silver colloids were instable
when the absorption value decreased suddenly. Fig. 6 reports
the thermal stability times of the silver colloids as a function
of temperature. The silver colloids were stable with different
time limits at high temperatures. It is shown that the silver
nanofluid had a stable time of 15 h at 120 °C.
4. CONCLUSIONS
Hydrophobic silver nanoparticles surface-coated with oleic
acid were prepared in different in this work. Oleic acid as
surface coating reagent was essential for the production of
stable and small scale silver nanoparticles. n-butylamine and
cyclohexylamine were used for keeping the reaction medium
in one phase, respectively. Addition of n-butylamine and
cyclohexylamine had different effects on the morphology and
size of the silver nanoparticles. And the content of the organic
coating layer was also affected, which was reduced with
increasing temperature. All the analyses demonstrated that
n-butylamine was better than cyclohexylamine for adding in
the
reaction
system to
prepare
surface-coated
silver
nanoparticles. The thermal stability of surface-coated silver
prepared in n-butylamine solution was monitored by UV-vis
spectral analysis. The prepared silver nanoparticles dispersed
in heptane were stable with different time limits at high
temperatures
REFERENCE
[1]
Y. Xuan, Q. Li, Heat transfer enhancement of nanofuids, International
Journal of Heat and Fluid Flow 2000, 21, 58-64.
[2] R. A. Yetter, G.A. Risha, S. F. Son, Metal particle combustion and
nanotechnology, Proceedings of the Combustion Institute 2009, 32,
1819-1838.
[3] H. E. Patel, S. K. Das, T. Sundararajan, A. S. Nair, B. George, T.
Pradeep. Thermal conductivities of naked and monolayer protected
metal nanoparticle based nanofluids: Manifestation of anomalous
enhancement and chemical effects, APPLIED PHYSICS LETTERS
2003, 83, 14, 2931-2933.
[4] X. Wang, A. S. Mujumdar. Heat transfer characteristics of nanofluids: a
review. International Journal of Thermal Sciences 2007, 46, 1-19
[5] W. Yu, D. M. France, J. L. Routbort, S. U. S Choi. Review and
Comparison of Nanofluid Thermal Conductivity and Heat Transfer
Enhancements. Heat Transfer Eng. 2008, 29, 432-460.
[6] C. Choi, H. S. Yoo, J. M. Oh. Preparation and heat transfer properties of
nanoparticle-in-transformer oil dispersions as advanced energy-efficient
coolants. Curr. Appl. Phys. 2008, 8, 710-712.
[7] I. Sondi, D. V. Goia, E. Matijević, Preparation of highly concentrated
stable dispersions of uniform silver nanoparticles, Journal of Colloid
and Interface Science 2003, 260, 75-81.
[8] M. V. Roldán, L. B. Scaffardi, O. de Sanctis, Nora Pellegri. Optical
properties and extinction spectroscopy to characterize the synthesis of
amine capped silver nanoparticles. Materials Chemistry and Physics
2008, 112, 984-990.
[9] G. Lee, S. Shin, Y. Kim, S. Oh, Preparation of silver nanorods through
the control of temperature and pH of reaction medium, Materials
Chemistry and Physics, 2004, 84, 197-204.
[10] S. Link, M.A. El-Sayed, Spectral Properties and Relaxation Dynamics
of Surface Plasmon Electronic Oscillations in Gold and Silver Nanodots
and Nanorods, J. Phys. Chem. B 1999, 103, 8410-8426.