Synthesis and characterization of silica-silver core-shell composite particles with uniform thin silver layers

Article abstract of DOI:10.1016/j.jssc.2007.08.022

Silica-silver core-shell composite particles with uniform thin silver layers were successfully synthesized by a facile and one-step ultrasonic electrodeposition method. By electrolysis of the slurry consisting of preformed silica spheres and silver perchlorate without any additives, the homogenous composite particles can be prepared. The average size of single silver crystals in the composite is about 12 nm and the thickness of silver layer is 14±2 nm. Moreover, the continuity of Ag distribution, the surface roughness and the thickness of silver layer are controllable by adjusting the current density (I), the concentration of electrolyte (C) and the reaction time (t). Optical properties of the composite particles with different silver content were also investigated.

Full text of DOI:10.1016/j.jssc.2007.08.022

ARTICLE IN PRESS
Journal of Solid State Chemistry 180 (2007) 2871–2876
www.elsevier.com/locate/jssc
Synthesis and characterization of silica–silver core–shell composite
particles with uniform thin silver layers
Ã
Shaochun Tang, Yuefeng Tang, Shaopeng Zhu, Haiming Lu, Xiangkang Meng
Department of Materials Science and Engineering, National Laboratory of Solid State Microstructures, Nanjing University,
Nanjing 210093, People’s Republic of China
Received 19 May 2007; received in revised form 22 July 2007; accepted 20 August 2007
Available online 4 September 2007
Abstract
Silica–silver core–shell composite particles with uniform thin silver layers were successfully synthesized by a facile and one-step
ultrasonic electrodeposition method. By electrolysis of the slurry consisting of preformed silica spheres and silver perchlorate without any
additives, the homogenous composite particles can be prepared. The average size of single silver crystals in the composite is about 12 nm
and the thickness of silver layer is 1472 nm. Moreover, the continuity of Ag distribution, the surface roughness and the thickness of
silver layer are controllable by adjusting the current density (I), the concentration of electrolyte (C) and the reaction time (t). Optical
properties of the composite particles with different silver content were also investigated.
r 2007 Elsevier Inc. All rights reserved.
Keywords: Thin silver layer; Ultrasonic electrodeposition method; Silica–silver composite particles; Optical properties
1. Introduction
layer process [13]. However, it is relatively complex because
these silica-coating procedures usually involved multistep
processes and it is quite difficult to obtain a dense and
uniform nanoscaled silver layer with high purity. In the
synthesis of nanoshells, there are often some problems to
be solved such as incomplete coverage, rough surfaces,
nonuniformity in shell thickness, and poorly controlled
composition [14]. Besides, present study focused on coating
the silica spheres with dense and uniform silver layer in the
thickness range of 20–100 nm, however, the uniform and
complete coverage of each particle in a sample by a defined
silver coating below 20 nm thick, which is the basis for
several fundamental investigations involving optical phe-
nomena, still remains a challenge. In addition, simplicity
and controllability of the process are necessary for
industrial applications. Therefore, further exploration and
evaluation of deposition methods are necessary for the
preparation of optically interesting materials.
In recent years, dielectric-metal core–shell building
structures (nanoshells) represent a new type of construc-
tional unit and have attracted much attention. These
hybrid particles have greatly potential application in
various fields such as surface-enhanced Raman scattering
[1], modulation of optical properties [2], photonics [3],
biological detection [4], magnetics [5], and catalysis [6], etc.
Especially, mono-dispersive nanoshells are promising
building blocks of photonic crystals (PCs) with large
dielectric contrast, which has been demonstrated to have
complete photonic band gap (CPBG) even in the optical
wavelengths [7,8]. In general, this kind of core–shell
structure was fabricated by depositing nanoscaled metal
particles on dielectric spheres with various methods. Silver-
coated silica nanoshells, for example, have been fabricated
by a variety of approaches such as pretreatment of
electroless deposition [9], seeding plating [10], polyol
process [11], surface functionalization [12] and layer-by-
In this paper, a facile and one-step ultrasonic electro-
deposition method, which has recently been used for
deposition of monodisperse Ag nanoparticles on silica
substrate [15], is extended to entirely and evenly coat the
surfaces of dielectric silica spheres with uniform thin silver
Ã
Corresponding author. Fax: +86 25 8359 5535.
E-mail address: mengxk@nju.edu.cn (X. Meng).
0022-4596/$ - see front matter r 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2007.08.022

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2872
layers. The resulted silver-coated silica nanoshells were
demonstrated by X-ray diffraction (XRD), transmission
electron microscopy (TEM), selected area electron diffrac-
tion (SAED), high-resolution transmission electron micro-
scopy (HRTEM) and energy dispersive X-ray (EDX)
microanalysis. The reaction parameters such as the current
density (I), the concentration of electrolyte (C) and the
reaction time (t) for the formation of uniform silver layers
were studied. Optical properties of the composite particles
with different silver content were also investigated.
analyzed by EDX, SAED and HRTEM, which were also
performed with the same microscope. The UV–visible
absorption spectroscopy was carried out on an America
VARIAN CARY 50 Probe UV–visible spectrophotometer
with wavelength in range of 300–800 nm.
3. Results and discussion
Fig. 1 displays the XRD pattern of the sample obtained
when I, C and t were 2 mA/cm2, 3.25 ꢀ 10ꢁ6 mol/l and
30 min, respectively. Several main diffraction peaks are
observed at 38.121, 44.341, 64.521, 77.311 (2y), which
correspond to the (111), (200), (220) and (311) reflections of
fcc phase silver (JCPDS card No 4-783). This result
indicates the presence of silver in the sample and the good
crystallinity of the silver product. The XRD peaks are
relatively broad due to the small size of the nanocrystals.
The FWHM of the peaks corresponding to (111), (200),
(220) and (311) reflections are 0.61, 0.65, 0.68 and 0.70,
respectively. According to the well-known Scherrer diffrac-
tion formula (d ¼ kl/b cos y), the average single crystal size
is calculated to be 12 nm (the FWHM datum of four
reflections was all used in the Scherrer equation). A very
broad XRD reflection peak at 221 (2y) is also observed,
which is attributed to the amorphous silica spheres.
Fig. 2a display a TEM image of as-prepared silica
spheres, the individual silica substrates in these images are
spherical in shape, with smooth edges and bare surface.
The silica substrates have a narrow size distribution and an
average diameter of the substrates are about 647 nm.
Further details of the silver–silica nanocomposites, includ-
ing the morphology, the size and the composition, can be
obtained from TEM and HRTEM observations, which are
shown in Figs. 2b–g with gradually increscent resolution of
the sample analyzed in Fig. 1. The TEM image with a
relatively low magnification (Fig. 2b), shows that the
product is very homogeneous. It is observed that the silica
spheres have been coated and the dispersive composite
particles in these images remain spherical in shape with
2. Experimental
The silica spheres used as the substrates were pre-
¨
prepared by the modified StOber method [16]. Typically,
12 ml of ammonia solution was diluted by 64 ml of
anhydrous ethanol in a conical flask and placed in a water
bath at a constant temperature (35 1C). Then another
mixture solution of 12 ml Si(OC2H5)4 and 66 ml anhydrous
ethanol was added to the conical flask drop by drop under
vigorous magnetic stir. Gentle stir continued for 24 h to
ensure that the reaction was complete. A milk suspension
with silica spheres was then separated by means of
centrifuging at 3000 rpm for 5 min, washed with deionized
water and anhydrous ethanol for at least five times.
Subsequently, take these silica spheres as substrates to
prepare Ag/SiO2 composite particles, note that the silica
spheres were used for silver deposition without any further
treatment. Two identical silver slices (the effective surface
area was 2 cm ꢀ 2 cm) were used as the anode and the
cathode, respectively, which were 4 cm apart from each
other. A quadrate insulating electrochemical cell consisting
of two-electrode setup was put inside an ultrasonic cleaner
with its volume of 2 l, in which there was 400 ml of water.
A direct current power supply was an electrophoresis
apparatus trophoresis that maintain a constant current or
voltage. Electrolysis of the 40 ml of slurry consisting of
silica submicrospheres (200–300 mg) and silver perchlorate
(3.25 ꢀ 10ꢁ6 mol/l) was carried out with the current density
of 2 mA/cm2 for 30 min under continuous ultrasonic
radiation with the frequency of 40 kHz and the power of
50 W. The starting pH of the electrolyte solution was 9.01.
Finally, the suspension was centrifuged at 3000 rpm for
5 min. The product was purified successively by five more
centrifugation/rinsing/redispersion steps with deionized
water and anhydrous ethanol and dried at 40 1C for 12 h
in an oven.
Silica
(111)
XRD measurements were performed to investigate the
crystallinity, the average crystal size and the crystal
structure of silver in the resulted products. XRD patterns
of the samples were measured with a D/Max-RA XRD
(200)
(220)
(311)
˚
(using CuKa ¼ 1.5418 A radiation). The size, distribution,
and morphology of silver particles on silica spheres were
studied by TEM) that was performed with a FEI TECNAI
F20 microscope, operating at 200 kV accelerating voltage
and equipped with an EDX detector. The elemental
composition and structural analysis of the products were
20
30
40
50
60
70
80
2θ (degrees)
Fig. 1. XRD pattern of the sample synthesized when I, C and t were
2 mA/cm2, 3.25 ꢀ 10ꢁ6 mol/l, and 30 min, respectively.

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Fig. 2. (a) TEM image of the silica spheres before the coating; (b)–(e) TEM images with different magnifications; (f) SAED pattern of the entire SiO2/Ag
composite particle corresponding to (e); and (g) HRTEM image of the sample obtained when I, C and t were 2 mA/cm2, 3.25 ꢀ 10ꢁ6 mol/l and 30 min,
respectively.
average diameter of 673 nm. TEM images with higher
magnification shown as Figs. 2c–d indicate that silver
nanocrystals are darker in contrast and distribute homo-
geneously and consecutively on the silica spheres. As
observed in Fig. 2e, a single silica sphere has been
completely covered by consecutive silver layer. The
thickness of silver consecutive layer is measured to be
1472 nm. The silver layer is deposited on the surface of the
silica sphere by the ultrasonic electrodeposition process;
therefore, it is composed of many silver nanoparticles. The
continuously distributed silver particles cover the silica
surface completely to form a silver shell due to their quite
high filling factor on the silica surface. The corresponding
SAED pattern of an entire Ag/SiO2 composite sphere
displays several diffraction rings, which emanate from
multiple nanocrystals on the surface of a silica sphere, as
shown in Fig. 2f. The d spaces between lattice planes are
calculated to be respectively 0.2360, 0.2037, 0.1441 and
0.1230 nm, corresponding to (111), (200), (220) and (311)
planes of fcc structural silver (JCPDS No. 4-783). A
HRTEM image shown in Fig. 2g indicates high crystal-
linity of silver in the product. The lattice space is measured
to be 0.2351 nm, which is close to that of (111) planes for
fcc silver (0.2359 nm). The insets in the right upper corner
of Fig. 2g are, respectively, local magnification image of the
selected section of the square and computer-generated fast

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Fourier transform diffraction pattern, which also confirm
the fcc structure of silver nanocrystals.
diameter can be successfully deposited onto the surface
of preformed colloidal silica spheres, [15] as is shown in
Fig. 4a. However, when I is increased to 2.5 mA/cm2 while
the other conditions remain the same, it is observed that a
consecutive silver layer consisting of nonuniform silver
particles has formed and the surfaces of silver-coated silica
spheres look rough, as shown in Fig. 4b. This may result
from high growth rate of silver at a high current density.
Moreover, when C is increased to 3.25 ꢀ 10ꢁ4 mol/l while
the other conditions remain the same, the thickness of
silver layer is up to about 20 nm, as that shown in Fig. 4c,
the size distribution of silver nanoparticles gets wider and
surface roughness increases. This is attributed to high
growth rate of silver at a high concentration of the
electrolyte. In addition, reaction time is a very important
factor. For example, when reaction time is shortened to
15 min while other conditions remain the same, inhomo-
geneous silver nanoparticles formed on the surface of silica
spheres. Thus, Ag distribution on silica spheres, e.g. the
continuity, the surface roughness and the layer thickness, is
controllable.
Fig. 3 shows the pattern of EDX measurements of the
coated silica spheres with silver layers of 1472 nm. EDX
analysis displays strong Si, O and Ag peaks next to Cu and
C peaks and no other impurities can be detected. Note that
the existence of Cu and C results from the sample support.
High purity of silver nanocrystals can be confirmed by the
results. Moreover, a table as an inset in the upper right
corner in Fig. 3 shows the weight and atomic percentages
of every detected element, according to the quantification
results, a quantitative estimate of the Ag/Si atomic
percentage ratio is about 1:4.
Therefore, it can be concluded from the above results
that a highly pure, uniform and consecutive silver
nanocoating on silica spheres can be obtained at the
appropriate reaction by this electrodeposition procedure
under ultrasonic conditions. Furthermore, this method is
controllable and by adjusting the experimental conditions,
such as I, C and t, dispersive silver nanoparticles or silver
layers with different surface roughness and different
thickness can also be obtained.
Some interesting information is obtained after compar-
ison between the products shown in Figs. 1 and 4a. When I
and C are relatively low, dispersive silver nanoparticles are
formed, while continuous distribution of silver is observed
when I and C increase to 2 mA/cm2 and 3.25 ꢀ 10ꢁ6 mol/l.
However, when I and C exceed certain values (e.g., when I
and C are, respectively, 3 mA/cm2 and 6 ꢀ 10ꢁ3 mol/l), no
silver particles but large silver conglomerations form in the
electrolyte. The effecting mechanisms of I and C are
discussed respectively as follows. When I increases, more
positive charges will aggregate on the silica surface and
facilitate the nucleation to form a continuous silver layer.
And the formation of the conglomerations in the electro-
lyte may result from the relatively strong eletroreductive
driving force for the ions at high I. And it is exactly the
driving force that makes it difficult for silver ions to
nucleate on silica spheres, because it makes the cathode
more attractive than silica substrates. As for C, there exists
a relatively larger radial concentration gradient near the
silica spheres due to the larger concentration difference
between the boundary layer and the electrolyte when C is
low, which can promote the radial growth of the nuclei on
the substrates. On the contrary, at high C, the concentra-
tion gradient becomes lower, which reduces the nuclei’s
For example, as we have recently reported that when I, C
and t were 0.75 mA/cm2, 3.25 ꢀ 10ꢁ7 mol/l and 30 min,
respectively, silver nanoparticles with sizes of 8–10 nm in
Elment Weight (%) Atomic(%)
C(K)
5.133
14.396
29.972
33.517
13.974
8.165
Si
O(K) 14.262
Si(K) 27.996
Cu(K) 26.411
Ag(K) 26.196
O
Cu
Ag
Ag
Cu
9
Cu
1
C
0
2
3
4
5
6
7
8
10
Energy / Kev
Fig. 3. EDX analysis of the silica spheres coated with a silver layer of
1472 nm in thickness.
Fig. 4. TEM images of the samples synthesized when I, C and t are, respectively (a) 0.75 mA/cm2, 3.25 ꢀ 10ꢁ7 mol/l and 30 min; (b) 2.5 mA/cm2,
3.25 ꢀ 10ꢁ6 mol/l and 30 min; (c) 2.0 mA/cm2, 3.25 ꢀ 10ꢁ4 mol/l and 30 min.

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tendency to grow radially, so the transverse growth becomes
more apparent. The possible formation mechanism of the
silica–silver core–shell composite particles is similar to that of
ultrasonic electrodeposition of Ag nanoparticles on silica
substrates [15]. That is silver ions in the electrolyte are bound
to the surface of colloidal silica spheres to form a silver ion
layer, due to the electrostatic interaction or possibly a
moderately strong chemical bond (Si–O–Agd+) [17], then the
silver ions bonded to the silica surface are reduced to form
nucleating sites which then evolve into small nuclei. The
nuclei act as suspensive nanoelectrodes and grow into silver
nanocrystals with controllable continuity, surface roughness
and layer thickness.
The modulation of the continuity of Ag distribution, the
surface roughness and the thickness of silver layer makes it
possible to improve the properties of the products.
UV–visible spectroscopy was employed for further quali-
tative characterization of the optical properties of the
materials. No visible peak appears in the spectrum of
uncoated silica colloids (Fig. 5 curve a) because the
dielectric function of silica is constant in the present
wavelength range (300–800 nm). However, a weak broad
peak appears at about 480 nm (Fig. 5 curve b) after the
deposition of discrete silver nanoparticles of 8–10 nm
(Fig. 4a). With increasing coating content (Fig. 2), an
obvious and broad absorption peak appears at near 500 nm
(Fig. 5 curve c), which is resulted from the coupling of the
neighboring nanoparticles because the silver nanocrystals
consecutively deposited on the silica surface may be
considered as the ‘‘silver aggregates’’ [18,19]. This is
consistent with the previous reports that the strong
dipole–dipole interactions between neighboring nanoparti-
cles and Mie scattering of silver shell would promote
redshift and broadening of the plasmon bands for silver
clusters attached on silica spheres [9,20]. With an increase
of the surface roughness and thickness of silver layer
(Fig. 4c), the extinction band is broadened greatly and
almost covers the wavelength range from 320 to 620 nm. In
addition, one resonance band maximum and a minimum
appears respectively at the wavelength of about 445 and
320 nm (Fig. 5 curve d), the minimum corresponds to a
minimum in the imaginary part of the refractive index
(about 0.4) for bulk silver [21]. The maximum band shifts
to shorter wavelength comparing with curve b, which is
consistent with previous literature reported by Halas’s
group [22]. They have proposed that once the shell is
complete, the peak absorbance is shifted to shorter
wavelengths with the growth of gold shell.
4. Conclusions
In summary, silica–silver core–shell structure with a
consecutive and uniform thin silver layer was successfully
fabricated through a one-step ultrasonic electrodeposition
method. The average single crystal size is about 12 nm and
the thickness of silver consecutive layer is 1472 nm.
Moreover, the continuity of Ag distribution, the surface
roughness and the thickness of silver layer can be adjusted
with the reaction conditions. With increasing coating
content, the extinction band is broadened and red-shifted
greatly. However, with an increase of the surface roughness
and the thickness of silver layer, the more broadened
extinction band is shifted to shorter wavelength. Such
spheres provide promising applications, for example, in
fabrication of metallic–dielectric PCs and providing new
basic units for the construction of nanodevices for use in
biochemical detection.
Acknowledgments
d
This research was supported by a grant for the State Key
Program for Basic Research of China (Grant no.
2004CB619305), the National Natural Science Foundation
of China (Grant no. 50571044, 50602022), and the Natural
Science Foundation of Jiangsu Province (Grant no.
BK2006716).
c
b
a
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