Morphosynthesis of rhombododecahedral silver cages by self-assembly coupled with precursor crystal templating

Article abstract of DOI:10.1002/anie.200461859

Through the reduction of micrometer-sized silver phosphate crystals with a perfect rhombododecahedral shape by a reaction/diffusion process to hierarchical, rhombododecahedral silver cages (see picture): The controlled self-assembly of silver particles around the precursor crystal surfaces leads to the formation of morphology-preserved single- or double-walled silver cages constructed from different building units.

Full text of DOI:10.1002/anie.200461859

Communications
Crystal Engineering
Morphosynthesis of Rhombododecahedral Silver
Cages by Self-Assembly Coupled with Precursor
Crystal Templating**
Jinhu Yang, Limin Qi,* Conghua Lu, Jiming Ma, and
Humin Cheng
The self-assembly and higher-order organization of micro-
and nanostructured building blocks into complex architec-
tures with hierarchy across extended length scales is a key
challenge in current materials synthesis and device fabrica-
tion.[1–8] In particular, many recent efforts have been devoted
to the controlled organization of primary building blocks into
hollow spheres or cages,[9–14] as these hollow structures with
nanometer-to-micrometer dimensions have widespread
potential applications, including in catalysis, in drug delivery,
[*] J. Yang, Prof. L. Qi, Dr. C. Lu, Prof. J. Ma, Prof. H. Cheng
State Key Laboratory for Structural Chemistry
of Unstable and Stable Species
College of Chemistry, Peking University
Beijing 100871 (China)
Fax: (+86)10-6275-1708
E-mail: liminqi@chem.pku.edu.cn
[**] This work was supported by NSFC (20325312, 20473003,
20233010), and FANEDD (200020).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200461859
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as artificial cells, as light fillers, and as photonic crystals.[15] In
most cases, however, only spherical hollow structures have
been obtained either by using colloidal particles[9] or emulsion
droplets[10] as templates, or by direct self-assembly of the
primary particles without using external templates.[11–13] It is
expected that tailoring the external morphology of hollow
structures would endow them with unique properties as a
consequence of the importance of shape and texture in
determining the properties of the materials.[16] It is note-
worthy that hexagon-based drums formed by textured self-
assembly of ZnO nanocrystals were recently produced in a
solid–vapor deposition process involving epitaxial surface
oxidation;[14] however, the geometrical
lowed by reduction of the Ag3PO4 precursor crystals with
different reducing agents in aqueous solution. It is well known
that soluble macromolecules can considerably influence the
form and appearance (crystal habit) by specific interactions
with inorganic crystal surfaces in solution, and the effect of
macromolecular additives can be further modified by varying
the solvent composition.[2] Here we chose the anionic
derivative of dextran (dextran sulfate) as an effective
crystal-growth modifier for the crystallization of Ag3PO4 in
a water/formamide mixture, as our preliminary experiments
suggested that usually only irregular Ag3PO4 polyhedrons are
obtained in pure water. Figure 1a presents a typical scanning
shapes of the polyhedral cages are, in
general, not well-defined, and such a proc-
ess could be largely limited to specific oxide
systems. Thus, it is desirable to explore
feasible methods to organize primary build-
ing blocks into hollow cages with well-
defined, novel geometrical morphologies.
The controlled synthesis of metal par-
ticles has attracted much attention because
of their important roles in many areas of
modern science and technology.[17–23] Con-
siderable progress has been made in recent
years in the shape-controlled synthesis of
metal nanoparticles; examples include
silver nanorods,[17] nanoprisms,[18] and nano-
cubes.[19] Hollow spheres of a variety of
metals, such as Pd,[20] Pt,[21] and Ag,[22] have
also been successfully produced by employ-
Figure 1. a, b) SEM images of Ag3PO4 rhombododecahedral crystals formed in a mixed water/form-
amide solvent, and c) index of a rhombododecahedral crystal model. SEM images of single-walled Ag
rhombododecahedral cages: d) before and e, f) after sonication. The Ag cages were produced by
reducing the Ag3PO4 precursor crystals shown in (a) with ascorbic acid at 208C.
ing hard or soft sacrificial templates. Inter-
estingly, unique polyhedral Au–Ag alloy
nanoboxes have been synthesized by the
replacement reaction between silver nano-
cubes and chloroauric acid in a refluxing
aqueous medium.[19,23] However, to the best of our knowledge,
there have been no reports on the hierarchical assembly of
hollow structures from nonspherical metal particles. Inspired
by a promising recent study on the formation of hollow
nanocrystals of cobalt oxide and chalcogenides by cobalt
electron microscopy (SEM) image of the Ag3PO4 crystals
obtained in the presence of 10 gLꢀ1 dextran sulfate, and
shows that all of the obtained Ag3PO4 crystals exhibit a
uniform rhombododecahedral morphology. The size of the
rhombododecahedra, as estimated from the distance between
the two parallel surfaces, typically ranges from 4 to 8 mm, with
an average size of about 6 mm. The related X-ray diffraction
(XRD) pattern (see the Supporting Information) shows sharp
reflections characteristic of simple cubic Ag3PO4 (JCPDS
No. 6-0505), thus suggesting that the rhombododecahedra are
pure Ag3PO4 crystals. An enlarged SEM image (Figure 1b)
clearly shows that the ideal rhombododecahedron consists of
12 well-defined crystal faces and exhibits cubic symmetry,
thus confirming that each rhombododecahedron is a single
crystal of cubic Ag3PO4. The 12 faces can be indexed to the
{110} planes (Figure 1c).
As Ag3PO4 can be readily reduced by a variety of
reductants to metallic Ag, Ag3PO4 crystals with well-defined
morphologies may be used as precursors for the morphology-
preserved synthesis of silver particles. Ascorbic acid, hydra-
zine, and NaBH4 are three widely used reducing agents with
an increasing reducing ability; moreover, they are expected to
show increasing diffusion ability in water, as their molecular
nanocrystal templating through
a nanoscale Kirkendall
effect,[24] we extend this strategy here to the facile synthesis
of well-defined, rhombododecahedral silver cages by the
reduction of micrometer-sized silver phosphate crystals with a
perfect rhombododecahedral shape through a microscale
Kirkendall effect. We demonstrate that the controlled self-
assembly of silver particles produced in situ around the
precursor crystal surfaces leads to the formation of morphol-
ogy-preserved, single- or double-walled silver cages con-
structed from different building units, such as particles,
nanoplates, and quasispherical assemblies of nanoplates.
This method of self-assembly coupled with templating of
the precursor crystals has been further used for the synthesis
of unique hollow silver cones consisting of silver nanoplate
assemblies.
Our synthesis of hierarchical silver hollow structures
involves the sulfated dextran-directed crystallization of
Ag3PO4 with specific morphologies in mixed solvents, fol-
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volume decreases from ascorbic acid through hydrazine
hydrate to the BH4 ion. It was therefore interesting to
preliminary thermogravimetric analysis (TGA) suggested
that the Ag3PO4 crystals do not contain significant amounts
of polymer after being washed thoroughly with water.
The formation of single-walled silver cages when using
ascorbic acid as the reductant indicates that the outward
diffusion of Ag3PO4 is most likely faster than the inward
diffusion of ascorbic acid. Hydrazine is a stronger reductant
than ascorbic acid, and has a higher diffusion ability. As
shown in Figure 2a, well-defined, rhombododecahedral silver
ꢀ
explore the synthesis of silver particles by reduction of the
rhombododecahedral Ag3PO4 precursor crystals with these
three reductants. Figure 1d shows a typical SEM image of the
silver product obtained by the reduction of the precursor
crystals in 0.02m ascorbic acid solution at 208C; it suggests the
formation of silver particles with a well-preserved rhombo-
dodecahedral morphology. The rhombododecahedra have an
average size of about 7 mm, which is
slightly larger than the precursor
rhombododecahedra
(ca. 6 mm),
and exhibit rather rough surfaces,
in contrast to the smooth surfaces of
the precursor rhombododecahedra.
The corresponding XRD pattern
(see the Supporting Information)
exhibits sharp reflections attributed
to fcc silver (JCPDS No. 04-0783),
thus confirming a complete trans-
formation of the Ag3PO4 precursor
crystals to silver crystals. After
10 min of ultrasonic treatment in
water, the rhombododecahedral par-
ticles can be broken and the hollow
structure of the rhombododecahedra
is clearly revealed by the SEM
images (Figure 1e, f). It can be seen
that the rhombododecahedral cages
have a shell consisting of intercon-
nected, irregularly shaped, primary
particles about 200–400 nm in size,
which results in an average shell
Figure 2. SEM images of double-walled Ag rhombododecahedral cages produced by reducing the
Ag3PO4 precursor crystals shown in Figure 1a with hydrazine at: a–c) 208C and d–f) 48C. The
images shown in (c) and (f) correspond to the broken cages after sonication. The insets show the
double-walled structures of the broken rhombododecahedral cages.
thickness of about 300–400 nm. Although there is a clear
difference in the length scale, this formation of micrometer-
sized, rhombododecahedral cages around rhombododecahe-
dral precursor crystals is analogous to the reported formation
of hollow nanospheres around spherical precursor nano-
particles,[24] which was attributed to a fundamental solid-state
phenomenon, known as the Kirkendall effect[25] that deals
with the movement of the interface between a diffusion
couple. In the present case, ascorbic acid and Ag3PO4 can
react with each other and form a diffusion pair. The coupled
reaction/diffusion at the crystal/solution interface could lead
to the quick formation of an interconnected silver shell
around the external surfaces of the Ag3PO4 crystals. This
process is followed by a continuous outward flow of Ag3PO4
from the solution to and through the porous silver shell to
form a hollow interior and a relatively compact shell, as well
as some small, individual silver particles precipitated outside
the shell. This result demonstrates that, similar to the
nanoscale Kirkendall effect used for the fabrication of
hollow nanoparticles, a microscale Kirkendall effect can be
used for the fabrication of micrometer-sized hollow cages
with well-preserved morphologies if a suitable reaction-
diffusion process exists at the solid/liquid interface. It may
be argued that the polymer could play a role in the self-
assembly process, for example, by gluing together the primary
particles; however, this role is likely to be negligible, as a
particles of about 6–7 mm were again obtained when hydra-
zine was used to reduce the rhombododecahedral Ag3PO4
crystals at 208C instead of ascorbic acid. A high-magnification
image (Figure 2b) reveals that the shell mainly consists of
nanoplates ranging from 200 to 400 nm in size and less than
100 nm in thickness. The TEM image and electron diffraction
(ED) pattern of separated silver nanoplates obtained by
ultrasonically breaking the rhombododecahedral particles
suggest that the nanoplates normally exhibit a hexagonal
shape and that they are single crystals of fcc silver with the top
face bounded by the {111} planes (see the Supporting
Information); this arrangement is typical for platelike silver
nanocrystals.[18] The representative SEM images of the
broken rhombododecahedral particles shown in Figure 2c
suggest that most of the rhombododecahedra are actually
double-walled hollow cages. It should be noted that single-
and triple-walled cages can be occasionally observed in the
product, although the double-walled cages form the over-
whelming majority. The primary platelike particles and the
double-walled structure are unique for the rhombododeca-
hedral cages synthesized under these conditions. Platelike
silver nanocrystals with a (111) top face are usually obtained
in the presence of an appropriate surface-capping reagent;[18]
therefore, it can be reasonably assumed that the reductant
(hydrazine) may also act as an effective stabilizer for the
silver nanoplates here. The formation of the double-walled
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structure is surprising. However, it is still in accordance with
the Kirkendall effect because significant inward hydrazine
transport could occur after the initial formation of the shell
around the rhombododecahedral precursor crystals, as hy-
drazine diffuses considerably faster than ascorbic acid. This
leads to the formation of a second shell inside the first as a
result of the coupled reaction/diffusion process. The double-
walled pattern is reminiscent of the well-known Liesegang
patterns[26] that typically form by a periodic precipitation
through the moving reaction front when two coprecipitating
reactants interdiffuse in a gel medium. The Liesegang
phenomenon observed in reaction/diffusion systems has
been studied for a long time because of its similarities with
natural, pattern-forming systems and potential applications in
materials patterning; however, the mechanisms responsible
for the patterns are still under discussion because of the rich,
nonequilibrium dynamics involved.[27] Nevertheless, the for-
mation of multiwalled hollow cages in a coupled reaction/
diffusion process is unprecedented, and the exact mechanism
is worthy of further study.
When the reduction reaction was conducted at a lower
temperature (ca. 48C), the reaction rate and the reactant
diffusion decreased considerably, which led to the formation
of double-walled, rhombododecahedral cages with relatively
loose shells (Figure 2d–f). It is worth noting that the shell is
composed of quasispherical assemblies of interconnected
silver nanoplates about 50 nm thick and 200–300 nm wide. We
propose that a lower reaction and diffusion rate would favor
the formation of loosely packed nanoplate self-assemblies
rather than densely packed nanoplate aggregates, which
results in a change of the building units of the rhombodo-
decahedral shells at different temperatures. The unique silver
product shows a multilevel structure: nanoplates!quasi-
spherical assemblies!rhombododecahedral shell!double-
walled rhombododecahedral cage, which is similar to the
complex, hierarchical structures adopted by biological min-
erals, such as the shells of single-celled organisms.[16] This one-
pot, organic-free method based on self-assembly coupled with
templating of the precursor crystals may form the basis of a
new route to hierarchical inorganic hollow structures.
decahedra show a core–shell structure with a dense shell and a
loose core after having been broken by sonication (Fig-
ure 3b). A high-magnification image of the core (Figure 3c)
suggests that it consists of entangled nanowires ranging in
diameter from 60 to 100 nm. The formation of the core–shell
structure can be rationalized by considering that the very fast
inward diffusion of BH4ꢀ ions could bring about a quick
reduction of the Ag3PO4 inside the silver shell that is formed
immediately after mixing the Ag3PO4 rhombododecahedra
and NaBH4 in solution. The formation of the clewlike core
could be related to the specific reduction reaction between
Ag3PO4 and NaBH4, although the mechanism is still unclear.
All three reducing agents can therefore be used success-
fully for the synthesis of well-defined silver rhombododeca-
hedra by reducing Ag3PO4 crystals. Our preliminary experi-
ments have shown that a minimum reductant concentration is
necessary for the morphology-preserved transformation from
Ag3PO4 to silver. For example, only rhombododecahedral
Ag3PO4 skeletons along with small separated silver particles
were produced at a low NaBH4 concentration (see the
Supporting Information), which indicates that a quick for-
mation of extended silver networks is essential to preserve the
morphology. Accordingly, a tentative mechanism for the
formation of the three different types of silver rhombodode-
cahedra can be proposed (see the Supporting Information).
Briefly, a layer of interconnected silver networks forms
around the exterior surface of the Ag3PO4 rhombododecahe-
dra as long as they are surrounded by a sufficient concen-
tration of the reductant. Then, the network layer develops
into a denser shell, which is accompanied by the formation of
a depletion layer near the shell as a result of the coupled
reaction/diffusion process. The final silver structure depends
on the inward diffusion rate of the reductant relative to the
outward diffusion rate of the Ag+ ions, namely, single-walled
rhombododecahedra form with ascorbic acid, which has a low
diffusion rate, whereas double-walled rhombododecahedra
form with hydrazine, which has a medium diffusion rate, and
core–shell structured rhombododecahedra form with NaBH4,
which has a high diffusion rate. Such a morphology-preserved
transformation process is not limited to the Ag3PO4/Ag
system. For example, hollow Ag octahedra have been readily
fabricated by reducing octahedral Ag2O crystals with ascorbic
acid following a similar strategy (see the Supporting Infor-
mation).
If NaBH4, which has an even higher reducing ability and
diffusion ability, is used as the reducing agent, novel core–
shell-structured silver rhombododecahedra can be produced.
As shown in Figure 3a, the silver product consists of
rhombododecahedral particles (ca. 7 mm) with some small
particles attached to the exterior surfaces. The rhombodo-
This method of self-assembly coupled with templating of
the precursor crystals can be extended to the morphology-
preserved synthesis of hierarchical
silver structures with morphologies
other than polyhedra. As shown in
Figure 4a and b, hollow Ag3PO4 cones
consisting of parallel fibers (longer
than 100 mm) were formed in a so-
lution of dextran sulfate in mixed
water/acetamide solvent. It should be
noted that similar hollow-cone struc-
tures have been obtained in the poly-
mer-controlled morphogenesis of
Figure 3. SEM images of: a) intact and b, c) broken Ag rhombododecahedral particles with a
core–shell structure that were produced by reducing the Ag3PO4 precursor crystals shown in Fig-
ure 1a with NaBH4 at 208C.
[28]
[29]
BaSO4
and bundles of BaCrO4
nanofibers. The hollow Ag3PO4 cones
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(0.5 mL), with vigorous stirring, to give a final phosphate ion
concentration of 1 mm and an excess of Ag+ ions. After stirring the
mixture for 1 min the slightly yellow, cloudy solution was aged for
1 day to give a yellow precipitate, which was collected, washed with
water thoroughly, and dried in air. A similar process was employed for
the synthesis of Ag3PO4 hollow cones, except that the solvent was a
mixture of water (4.5 mL) and acetamide (0.74 g) instead of the mixed
water/formamide solvent, and a longer aging time (4–5 days) was
adopted. The obtained Ag3PO4 crystals were subsequently used as
templates for the precursor crystals for the fabrication of the
corresponding silver hollow structures. In a typical synthesis proce-
dure, 0.004 g of the Ag3PO4 precursor crystals (ca. 0.01 mmol) was
dispersed in water (9 mL) and a 0.2m aqueous solution (1 mL) of the
reducing agent (ascorbic acid, hydrazine, or sodium brorohydride)
was added at room temperature (ca. 208C) to give a final reducing-
agent concentration of 20 mm. After aging the mixture for 10 h in the
solution, the resultant gray precipitate was collected by centrifugation
at 3000 rpm and washed with water repeatedly to remove any
separately precipitated small particles.
Received: September 1, 2004
Published online: December 15, 2004
Keywords: crystal engineering · nanostructures · polyhedral
.
cages · self-assembly · silver
Figure 4. a, b) SEM images of Ag3PO4 hollow cones formed in a mixed
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In summary, a variety of silver microstructures exhibiting
a well-defined, rhombododecahedral exterior morphology
and a remarkably complex hierarchy have been readily
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Experimental Section
In a typical synthesis of Ag3PO4 rhombododecahedra, Na3PO4
(0.005 mmol) and dextran sulfate (0.05 g, Fluka, MW 500000) were
dissolved in a solvent mixture containing water (3 mL) and forma-
mide (1.5 mL), followed by addition of 0.2m aqueous AgNO3 solution
602
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[30] S. Chan, S. Kwon, T.-W. Koo, L. P. Lee, A. A. Berlin, Adv. Mater.
2003, 15, 1595.
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