Synthesis and characterization of Ag nanoshells by a facile sacrificial template route through in situ replacement reaction

Article abstract of DOI:10.1021/ic060539j

A facile in situ replacement reaction route was successfully introduced for synthesizing Ag nanoshells with outer diameters of 40-50 nm and inner diameters of 20-30 nm using Co nanoparticles as sacrificial templates. The products were characterized by XRD, TEM, SAED, and UV-vis absorption spectra. The formation mechanism was also discussed. The reaction driving force comes from the large reduction potential gap between the Ag⁺/Ag and Co²⁺/Co redox couples, which results in the consumption of Co cores and the formation of a hollow cavity of Ag nanoshells. The UV-vis spectrum of this nanostructure exhibits a distinct difference from that of solid nanoparticles, which makes it a good candidate for application in photothermal materials.

Full text of DOI:10.1021/ic060539j

Inorg. Chem. 2006, 45, 5145−5149
Synthesis and Characterization of Ag Nanoshells by a Facile Sacrificial
Template Route through in situ Replacement Reaction
Minghai Chen and Lian Gao*
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai
Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China
Received March 30, 2006
A facile in situ replacement reaction route was successfully introduced for synthesizing Ag nanoshells with outer
diameters of 40 50 nm and inner diameters of 20 30 nm using Co nanoparticles as sacrificial templates. The
products were characterized by XRD, TEM, SAED, and UV vis absorption spectra. The formation mechanism was
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−
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also discussed. The reaction driving force comes from the large reduction potential gap between the Ag+/Ag and
Co2+/Co redox couples, which results in the consumption of Co cores and the formation of a hollow cavity of Ag
nanoshells. The UV−vis spectrum of this nanostructure exhibits a distinct difference from that of solid nanoparticles,
which makes it a good candidate for application in photothermal materials.
Introduction
nanoprisms.7,8 However, hollow nanostuctures have rarely
been reported because of the difficulty in preparing them.
Metallic nanoshells are one of the most interesting and
possibly useful materials of the recently developed nano-
structures. Generally, most of the preparations of the noble
metal nanoshell are focused on the template strategy, in
which noble nanoparticles are attached to the surfaces of
template spheres. The commonly used templates include
silica9 and polystyrene spheres.10,11 But because of the large
size of the present sphere, both the outer diameter and the
cavity size of the obtained nanoshells are in the range of
hundreds of nanometers, which limits the practical applica-
tions and the basic study. Furthermore, the removal of the
core will always bring a negative effect to the nanoshell
structure. It is of great importance to reduce the size to
nanoscale. So, selecting a smaller sacrificial core will
practically resolve this problem. Xia et al.12-17 successfully
prepared Au and Pt hollow nanostructures using a Ag
In the past decade, the synthesis of noble metal nano-
structures has been an active research area because of the
importance of these materials to catalysis, photography,
electronics, photonics, information storage, optoelectronics,
biological labeling, imaging, and sensing.1,2 As noble metals
are reduced in size to tens of nanometers, a very strong new
absorption is observed that results from the collective
oscillation of the electrons in the conduction band from one
surface of the particle to the other. This is called the surface
plasmon resonance (SPR), which has become the focus of
research work on noble metal nanostructures.3-6 As the most
important noble metal, silver’s nanostructure has attracted
great interest. The intrinsic properties of nanostructures can
be tailored by controlling their size, shape, composition,
crystallinity, and structure (e.g., solid or hollow). Great effort
has been paid to control the shape of the nanostructure, and
all kinds of morphologies have been successfully prepared,
such as nanowires, nanorods, nanocubes, nanoplates,1 and
(7) Sun, Y.; Mayers, B.; Xia, Y. Nano Lett. 2003, 3, 675.
(8) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J.
G. Science 2001, 294, 1901.
* To whom correspondence should be addressed. E-mail:
liangaoc@online.sh.cn. Tel.: 0086-21-52412718. Fax: 0086-21-52413122.
(1) Wiley, B.; Sun, Y.; Mayers, B.; Xia, Y. Chem.sEur. J. 2005, 11,
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Chem.sEur. J. 2005, 11, 440.
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Phys. Chem. B 2004, 108, 1224.
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B. Langmuir 2003, 19, 9490.
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(11) Zhang, J.; Liu, J.; Wang, S.; Zhan, P.; Wang, Z.; Ming, N. AdV. Funct.
Mater. 2004, 14, 1089.
(12) Sun, Y.; Mayers, B.; Xia, Y. AdV. Mater. 2003, 15, 641.
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Au, L.; Zhang, H.; Kimmey, M. B.; Li, X. D.; Xia, Y. Nano Lett.
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10.1021/ic060539j CCC: $33.50
Published on Web 05/18/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 13, 2006 5145

Chen and Gao
nanostructure as the sacrificial template. But how can the
Ag hollow structure be prepared? It is a precondition that
the core materials are more reactive than silver. Although
Ni,18 Co,19-21 and Pb22 nanoparticles have been reported as
serving as core materials to construct an M@N (M) Ni,
Co, Pb; N ) Ag, Au, Pt) core-shell nanostructure, there
are few successful reports on the synthesis of Ag nanoshells
through this facile route. Recently, Liang et al.23,24 success-
fully synthesized Au and Pt hollow nanospheres using newly
prepared cobalt nanoparticles as sacrificial templates, and
the SPR properties of Au nanoshells could be well-tuned by
controlling the interior cavity sizes. In the present work, Ag
hollow nanoshells were successfully synthesized by a similar
strategy using Co nanoparticles as sacrificial templates, which
were thoroughly depleted during the process. This unique
hollow Ag nanostructure is expected to be used in drug
delivery, biological labeling, catalysis, and high performance
SPR applications.
Figure 1. XRD pattern of the as-prepared Ag nanoshells
reduction reaction (eq 2).25 Considering the whole experiment
is conducted under ambient conditions, the dissolving of O2
in the solution is inevitable and provided the required O2
for reaction 2. The replacement reaction driving force comes
from the large reduction potential gap between the Ag+/Ag
and Co2+/Co redox couples. Because the reduction potential
of the Ag+/Ag (0.799 V vs SHE) redox couple is higher
than that of Co2+/Co (-0.377 V vs SHE), the following
replacement reaction (eq 3) can occur spontaneously as soon
as a Ag+ ion contacts with metallic Co.
Experimental Section
All chemicals were of analytical grade and used as received
without further purification. In a typical experimental procedure,
0.038 g of citric acid and 0.06 g of NaBH4 were dissolved in 200
mL of deionized water; 0.8 mL of a Co(NO3)2 solution (0.1 M)
was then rapidly injected into the solution to form a brown solution
(denoted as solution I). In the meantime, 1.6 mL of a AgNO3
solution (0.1 M) was diluted to 50 mL (denoted as solution II).
After several minutes, the bubble ceased to release from the reaction
between surplus NaBH4 with water in solution I. Solution II was
then mixed with solution I with rapid agitation. The color of the
mixture gradually changed from brown to deep green and formed
a stable colloidal solution. The products were collected by
centrifugation at 8000 rpm for 30 min.
2Co2+ + 4BH4- + 9H2O f Co2B + 12.5H2 + 3B(OH)3
(1)
4Co2B + 3O2 f 8Co + 2B2O3
Co + 2Ag+ f 2Ag + Co2+
(2)
(3)
The XRD pattern of the as-prepared sample is shown in
Figure 1. All the diffraction peaks can be well-indexed to
face-centered cubic silver with calculated lattice constants
of a ) 4.08 Å, which is in good agreement with the value
in the literature (JCPDS card No. 04-0783, a ) 4.086 Å).
No impurities are detected, which shows the high purity of
the product. It also confirms the high efficiency of reaction
1. The morphologies of the sample were observed by TEM
and SEM, which are shown in Figure 2. In Figure 2a, the
low-magnification TEM image shows the well dispersed
nanoparticles with diameters of 40-50 nm. Close observation
shows a strong contrast difference between the center and
edge, which suggests a hollow nanostructure. The corre-
sponding SAED rings in Figure 2b indicate the good
crystallinity of the product, and all the rings can be indexed
to fcc Ag. As marked in Figure 2b, the centric rings can be
assigned to the (111), (200), (220), and (311) planes. The
high-magnification micrograph (Figure 2c) clearly shows the
hollow nanoshell, showing outer diameters of 40-50 nm and
inner diameter of 20-30 nm. The FE-SEM image in Figure
2d further shows the morphology of the nanosphere. The
HRTEM image of a single nanoshell is shown in Figure 3,
showing the high crystallinity. The stripes spacing are
measured to be 0.236 and 0.2 nm, which is close to the plane
spacing of (111) and (200), respectively. As marked in Figure
Powder X-ray diffraction (XRD) was carried out in a Japan
Rigaku D/max 2550V X-ray diffractometer using Cu KR (λ )
0.15406 nm) radiation at 40 kV and 60 mA. TEM and HRTEM
images were collected using a JEOL-2100F electron microscope.
FE-SEM images were collected on a JSM 6700F field-emission
scanning electron microscope. EDS was recorded on an OXFORD
ISIS spectroscope, which was attached to the JEOL-2100F electron
microscope. UV-vis absorption spectrum analysis was performed
on a Shimadzu UV-3101PC spectrophotometer.
Results and Discussion
In the present work, Co nanoparticles were synthesized
by the reduction of Co2+ by NaBH4. Although the major
products from this reduction reaction are always Co2B (eq
1), metallic Co can be formed during the sacrificial oxidation/
(17) Sun, Y.; Wiley, B.; Li, Z. Y.; Xia, Y. J. Am. Chem. Soc. 2004, 126,
9399.
(18) Bala, T.; Bhame, S. D.; Joy, P. A.; Prasad, B. L. V.; Sastry, M. J.
Mater. Chem. 2004, 14, 2941.
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M.; Radetic, T.; Farle, M. Nano Lett. 2002, 2, 621.
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2003, 107, 7351.
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J. Mater. Chem. 2004, 14, 1057.
(22) Wang, Y.; Cai, L.; Xia, Y. AdV. Mater. 2005, 17, 473.
(23) Liang, H. P.; Wan, L. J.; Bai, C. L.; Jiang, L. J. Phys. Chem. B 2005,
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Langmuir 1993, 9, 162.
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Synthesis of Ag Nanoshells by Facile in situ Replacement
Scheme 1. Sketch Map of the Formation Mechanism of the Ag
Nanoshell
into the Co nanoparticle suspension and formed a thin Ag
film on the surface of the Co nanoparticles. As the reaction
continued, Co2+ and Ag+ continuously diffused through the
film until the Co cores were completely consumed and a
hollow Ag nanoshell was formed. Yin et al. 26 have proposed
a strategy for constructing hollow nanocrystals through the
nanoscale Kirkendall effect, in which pores form because
of the difference in diffusion rates between two components
in a diffusion couple. In the present work, the formation of
Ag nanoshells can also be attributed to a similar Kirkendall
effect, which can explain why the products are hollow
nanoshells but not solid nanoparticles. At the interface of
Ag and Co, the diffusion rate of atomic Co is much faster
than that of atomic Ag, which results in a net directional
flow of matter outward. The outward-diffused Co atom will
be oxidized into Co2+ by Ag+ at once. So the cavities with
sizes similar to the sizes of Co nanoparticles are kept after
the Co cores are consumed thoroughly. Because of the small
size of Co cores, both the outer and inner diameters of the
as-prepared Ag nanoshell are in the nanometer scale.
Figure 2. Morphology of the as-prepared Ag nanoshell: (a) low-
magnification TEM image, (b) its corresponding SAED pattern, (c) high-
magnification TEM image, and (d) FE-SEM image
To prove this assumption, we carried out a series of
comparative experiments, which was similar to the process
used to prepare Ag nanoshells except for some modifications.
First, AgNO3 was not directly introduced to the newly formed
Co nanoparticle suspension, but the as-prepared Co nano-
particles were separated and washed by centrifugation
repeatedly before being introduced to AgNO3 solution
instead. So, the reaction between Co and AgNO3 occurred
immediately. The as-prepared sample was marked as sample
I, whose TEM micrographs are shown in panels a and b of
Figure 4. The low-magnification TEM image shown in
Figure 4a indicates that the products are composed of
nanospheres and nanoshells. In the high-magnification TEM
image shown in Figure 4b, a special yolk-shell nanostructure
inspires us to speculate that this structure resulted from
incomplete replacement reaction between Co and AgNO3.
Using EDS microprobe analysis, we can clearly prove the
element composition. Panels a and b of Figure 5 show the
EDS patterns of Ag nanoshells and Co-Ag yolk-shell
nanostructure, respectively. In Figure 5a, the EDS pattern
corresponds to the samples shown in Figure 2, indicating
that except for the peaks of C and Cu, which come from the
Cu grid, the peak of Ag is the only signal from the product.
There is no signal from Co, which indicates that the cobalt
nanoparticles have been depleted thoroughly during the
process. However, the EDS pattern in Figure 5b, which
corresponds to the yolk-shell nanostructure, clearly shows
Figure 3. Single Ag nanoshell selected to be undertaken for HRTEM
observation: (a) morphology micrograph, (b) HRTEM image, and (c) its
corresponding FFT pattern.
2b, the angle between the two stripes is 55°. Combined with
its corresponding FFT pattern (Figure 3c and the insert in
Figure 3b), these planes can be indexed to (-200), (-111),
and (-111h) of fcc Ag.
In the present work, citric acid served as a capping agent
to prevent the growth of Co nanoparticles through repulsive
forces between negatively charged cobalt nanoparticles,
which had the same use in preventing the resulting Ag
nanoshells from aggregating. Using NaBH4 as a reducing
agent has two advantages. First, the strong reduction ability
can reduce Co2+ rapidly, which presents a rapid nucleation
condition for forming small-sized metal nanoparticles;
second, superfluous NaBH4 in the system will not interfere
with the latter replacement reaction between Co and Ag+,
because NaBH4 can be consumed by the reaction with water.
Most other reducing agents cannot meet these requirements.
Scheme 1 shows the sketch map of the formation mechanism
of Ag nanoshells. Because of the strong reaction driving force
resulting from the difference in reduction potential between
the Ag+/Ag and Co2+/Co redox couples, the replacement
reaction occurred immediately when AgNO3 was introduced
(26) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G.
A.; Alivisatos, A. P. Science 2004, 304, 711.
Inorganic Chemistry, Vol. 45, No. 13, 2006 5147

Chen and Gao
of nanoparticles result from the reduction of NaBH4. In fact,
because citric acid was selected as the capping agent, the
-
whole solution was a weak acid containing H+ and NO3 .
So the reaction solution system had a strong oxidation ability
and the newly formed tender Co nanoparticles were easily
dissolved again. In the whole procedure, we found that after
being magnetically stirred beyond 30 min, the deep brown
Co nanoparticle suspension turns colorless. We wondered
why the core of Co nanoparticles was dissolved by the
solution but not by the reaction between Ag+ ions and Co
nanoparticles; we carried out the third comparative experi-
ment, in which the capping agent was replaced by trisodium
citrate while the other conditions were unchanged. The as-
prepared sample was marked as sample III, whose TEM
images are shown in Figure 4d. The TEM micrograph shows
the product is composed of well-dispersed nanoshells with
outer diameters of 40-60 nm and a wall thickness of ∼10
nm. The high-magnification TEM image inserted in Figure
4a shows further details that the cavities of these nanoshells
are smooth, although the outer surfaces are so irregular; this
indicates an advancing trace of the interface during the
replacement reaction. The above comparative experiments
well-confirm the assumption of the formation mechanism
for Ag nanoshells.
Figure 4. TEM images of the Ag samples prepared in the comparative
experiments: (a,b) sample I, nanoshells and yolk-shell nanostructure; (c)
sample II, nanoparticles and nanoshells; (d) sample III, nanoshells.
a Co signal, indicating the presence of the Co core. In fact,
this yolk-shell nanostructure is the intermediate product,
which has a tendency to grow into a hollow Ag nanoshell
with continuing provision of Ag+. But why is the reaction
incomplete? It may be the relative low reaction activity
compared to that of the newly formed Co nanoparticles in
the in situ replacement reaction. This phenomenon was also
In another comparative experiment, Ag nanoparticles were
prepared through direct reduction of AgNO3 by NaBH4, also
using citric acid as the stabilization agent. TEM observation
indicates a particle size of ∼20 nm (not shown). The UV-
vis absorption spectra of Ag nanoshells and nanoparticles
are shown in Figure 6, which shows that the shape and
structure have great effect on the position and shape of the
absorption peak. The nanoparticles have a plasmon peak at
395 nm and a weak shoulder peak around 350 nm. The case
of the nanoshells is quite different, with a plasmon peak that
has red-shifted to 506 nm. Metal nanoshells with shell layers
consisting of metals with strong plasmon resonances exhibit
a strong, plasmon-derived optical resonance, typically shifted
to much longer wavelengths than the plasmon resonance of
the corresponding solid metal nanosphere.3,27 The above
spectral results, in combination with the TEM images,
strongly indicate the as-prepared nanostructure is indeed a
nanoshell with a hollow interior. Furthermore, there is still
a wide absorption in the visible region beyond 600 nm, which
found in the reported preparation of Ni@Ag and Co@Ag
18,21
core-shell nanostructures,
in which the surfactant
sustained the reaction at the interfaces.
Second, if the AgNO3 was introduced into a Co nanopar-
ticle suspension as soon as the Co nanoparticles were formed
by the reduction of NaBH4, the products took on a different
morphology. The as-prepared sample was marked as sample
II, whose TEM morphology is shown in Figure 4c, which
indicates that the products are composed of lots of nano-
particles and a small quantity of nanoshell. Because at this
stage large quantities of Ag+ ions were reduced directly by
NaBH4, only part of the Ag+ ions reacted with the newly
formed Co nanoparticles. It is obvious that these quantities
Figure 5. EDS patterns of the as-prepared Ag nanoshells and Co-Ag yolk-shell nanostructure.
5148 Inorganic Chemistry, Vol. 45, No. 13, 2006

Synthesis of Ag Nanoshells by Facile in situ Replacement
infrared region through controlling the wall thickness and
core diameter. This interesting property makes these Ag
nanoshells good candidates for application in photothermal
biomaterials, such as photothermally triggered drug release,
photothermal cancer therapy, and contrast-enhanced imaging.
Conclusion
In summary, Ag nanoshells with outer diameters of 40-
50 nm and inner diameters of 20-30 nm were successfully
synthesized by a facile in situ replacement reaction using
Co nanoparticles as sacrificial templates, which can be easily
enlarged to the gram scale. The reaction driving force comes
from the large reduction potential gap between the Ag+/Ag
and Co2+/Co redox couples, which results in the consumption
of Co cores and the formation of a hollow cavity of Ag
nanoshells. The UV-vis spectrum of this nanostructure
exhibits a distinct difference from that of solid nanoparticles.
Figure 6. UV-vis absorption spectra of the as-prepared Ag nanoshells
and nanoparticles.
is similar to the reported results by Halas and co-workers,27
who also revealed that the plasmon peak of nanoshells could
be conveniently tuned from the visible region to the far-
(27) Jackson, J. B.; Halas, N. J. J. Phys. Chem. B 2001, 105, 2743.
IC060539J
Inorganic Chemistry, Vol. 45, No. 13, 2006 5149