Controlled synthesis of monodisperse silver nanocubes in water

Article abstract of DOI:10.1021/ja046037r

Monodisperse silver nanocubes with edge length of 55 ± 5 nm were, for the first time, synthesized in water on the basis of HTAB-modified silver mirror reaction at 120 °C (HTAB, n-hexadecyltrimethylammonium bromide). The individual nanocube was crystallographically well defined with a single crystal bonded by six {200} facets. The nanocubes were soluble to form stable aqueous solutions and had a strong tendency to assemble into two-dimensional arrays with regular checked pattern on substrate. Copyright

Full text of DOI:10.1021/ja046037r

Published on Web 09/25/2004
Controlled Synthesis of Monodisperse Silver Nanocubes in Water
Dabin Yu and Vivian Wing-Wah Yam*
Center for Carbon-Rich Molecular and Nano-Scale Metal-Based Materials Research and Department of Chemistry,
The UniVersity of Hong Kong, Pokfulam Road, Hong Kong, P. R. China
Received July 3, 2004; E-mail: wwyam@hku.hk
Controlled synthesis of monodisperse inorganic nanoparticles
(NPs) in terms of size and shape has been strongly motivated by
the requirements to uncover and map their size- and shape-
dependent properties and to achieve their practical applications
ranging from biosensing to catalysis, optics, and data storage.1
Among various NPs, silver NPs are of particular interest because
of their wide applications resulting from the unusual properties that
depend on their shape and size.2 The face-centered cubic (fcc)
structure of silver metal confers its tendency to nucleate and grow
into NPs with their surfaces bounded by the lowest-energy {111}
facets, with most of the previous methods, in particular the wet
Figure 1. A typical XRD pattern of the Ag nanocubes.
chemical synthesis, being mainly confined to those of the prepara-
tion of nanowires, rods, or spheres.3 Nevertheless, a few of its novel
nanostructures, such as the reported disks,4 prisms, and plates,2a,5
have attracted great attention. Recently, Xia and co-worker syn-
thesized silver nanocubes with edge length of 115 ( 9 and 95 (
7 nm by reducing silver nitrate in ethylene glycol.6 Currently, such
kinds of cubic nanostructures have attracted considerable interest
because of their important applications. For example, there have
been reports on silver nanocubes employed to template formation
of gold nanoboxes and iron nanocubes used as the building blocks
of magnetic superlattices.6,7
Herein we report an alternative new approach to monodisperse
silver nanocubes with a mean edge length of 55 ( 5 nm in water
by a HTAB-modified silver mirror reaction at 120 °C (HTAB )
n-hexadecyltrimethylammonium bromide). The silver mirror reac-
Figure 2. TEM images of Ag nanocubes: (a) with low magnifications;
tion is an “old” chemical route, which was used to generate
reflective mirrors on solid supports.8 Controlled synthesis of silver
nanocubes could be accomplished by such an “old” reaction by
the introduction of HTAB. The synthetic reactions were as follows.
(b-d) with higher magnifications. Images c and d were recorded almost
from the same several cubes, but image d was recorded after rotating the
cubes by an angle of 30°.
agreement with that found in the literature (PCPDF No. 040783).
Interestingly, the (200) diffraction peak showed the strongest
intensity in the pattern and its intensity was nearly three times that
of the commonly observed strongest (111) diffraction peak (the
intensity ratio of (200) to (111) peaks was 40:100, PCPDF No.
040783), and other peaks corresponding to (220), (331), and (222)
diffraction were not observed. This indicated that the product had
the structure with a preferential [100] orientation. The unusual
intensity of the (200) diffraction peak is a result of the strong
tendency of the Ag NPs to assemble into two-dimensional (2D)
arrays on the solid surface with the c-axis perpendicular to the
substrate upon the evaporation of the water.11
Figure 2a shows a typical transmission electron microscopy
(TEM) image of the Ag NPs, indicating that the products consisted
of a large quantity of uniform nanocubes. Figure 2b-d shows the
TEM images with higher magnifications. Compared to Figure 2c,
Figure 2d was recorded after rotating the sample by an angle of
30°, and its remarkable contrast clearly shows a cubic shape of the
sample. The TEM analysis indicates that the cubes with smooth
faces have a mean edge length of 55 ( 5 nm. The cubes had a
strong tendency to assemble into 2D arrays with regular checked
pattern on the TEM grid, and the spacings between adjacent cubes
[Ag(NH3)2]+ (aq) + Br- (aq) h AgBr(s) + 2NH3 (aq) (1)
[Ag(NH3)2]+ (aq) + RCHO (glucose) (aq) f
Ag (NPs) + RCOO- (aq) + NH4+ (aq) (2)
Upon addition of HTAB into the solution containing [Ag-
(NH3)2]+, the Br- from HTAB reacted with [Ag(NH3)2]+ to form
AgBr (eq 1). This is a reversible reaction with an equilibrium
constant of ca. 1.63 × 105 at 120 °C; thus Ag(I) mainly existed in
the form of AgBr, resulting in the cathodic shift of the reduction
potential of [Ag(NH3)2]+/Ag.9 Consequently, unlike the silver mirror
reaction that was normally performed at around room temperature,10
reaction 2 proceeded in a more controllable manner at such a high
temperature to form the nanocubes. With the consumption of
[Ag(NH3)2]+ by reaction 2, the equilibrium reaction 1 shifts to the
left until the reactions are completed.
A typical X-ray diffraction (XRD) pattern of the products is
shown in Figure 1, and the peaks were assigned to the diffraction
of (111), (200), and (400) planes of fcc silver accordingly. The
cell parameter calculated from this pattern was 4.083 Å, in
9
13200
J. AM. CHEM. SOC. 2004, 126, 13200-13201
10.1021/ja046037r CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. (a) A typical HRTEM image of a selected area of an individual
Ag nanocube with the SAED pattern shown in the inset. (b) The nanocube
used for the HRTEM and SAED studies with the area marked with a white
pane indicating where the HRTEM image was recorded. (c) Schematic
illustration of the facets of an individual cube.
Figure 4. (a-d) TEM images of Ag NPs synthesized with various molar
ratios of HTAB/[Ag(NH3)2]+: (a) 1, (b) 1.5, (c) 2.5, (d) 3. (e, f) photos
and UV/vis absorption spectra of the aqueous solutions of the Ag NPs,
marked with 1, 2, 3, and 4 corresponding to those of the Ag NPs shown in
panels a, b, c, and d, respectively.
are ca. 5 nm. Such a regular assembly of the cubes is due to the
facts that the HTAB adsorbed on the cube surfaces would form a
capping shell to generate a regular interparticle spacing and thus
to prevent the cubes from random aggregation,12 and that the
uniformity of the sample made it possible to manipulate the cubes
into a close-packed and ordered array caused by van der Waals
forces.13
In summary, monodisperse silver nanocubes with edge length
of 55 ( 5 nm were, for the first time, synthesized in water on the
basis of HTAB-modified silver mirror reaction. Their high stability
in water would be favorable for further chemical modification and
systematic investigations. Their 2D array with regular checked
pattern on solid substrate may open up the possibilities of fabricating
new nanodevices with novel physical properties, because the
coupling between proximal NPs may give rise to new collective
phenomena. Considering the unique properties of silver, its
nanocubes should be of importance for both theoretical investiga-
tions and practical applications.
Figure 3a shows the high-resolution TEM (HRTEM) image of
a cube as indicated in Figure 3b. The inset shows the corresponding
selected area electron diffraction (SAED) pattern, obtained by
directing the incident electron beam perpendicular to one of the
square facets of the cube. The square spot array was indexed to
[200] and [020] of the fcc silver. The 2D lattice fringes of the
HRTEM image were examined to be 0.204 nm, close to the {200}
lattice spacing of the fcc silver. Because the HRTEM image was
recorded from the area along one upright facet as marked by a
white pane in Figure 3b, the perpendicular fringes were either
parallel to or orthogonal to the upright facet of the cube, suggesting
that the six surfaces of the silver nanocubes consisted of {200}
lattice planes as schematically illustrated in Figure 3c. Note that
our HRTEM images clearly showed that the edges of the cubes
were not bounded by {110} as described by Xia,6 but actually they
consisted of small bent faces. On the basis of the TEM and HRTEM
analyses, it could be concluded that when these regular nanocubes
bounded by {200} facets assembled into a 2D array on a substrate,
they, of course, had an identical [001] zone direction, thus resulting
in the strongest diffraction of (200) as shown in the XRD pattern.
As an ionic surfactant, HTAB played multiple roles in the
formation of the nanocubes. It acted not only as the reaction
moderator as described above but also as the shape controller and
stabilizing agent. First of all, its micelles directed the silver metal
to nucleate and grow into NPs other than the usual silver mirror.
As shown in Figure 4a-d, an increase in the molar ratio of HTAB/
[Ag(NH3)2]+ led to an obvious shape evolution of silver NPs from
spheres to cubes, which was caused by the anisotropic adsorption
of the surfactant on the silver crystal faces, while such adsorption
characteristics depended on the molar ratio.14 When the molar ratio
reached ca. 1.5, anisotropic growth of the particles appeared as
indicated by the arrows in Figure 4b. When the molar ratio increased
further to 2.5, the product was dominated by nanocubes (Figure
4c). Second, it stabilized the silver NPs to form stable aqueous
solutions (Figure 4e), the color of which changed from orange to
yellow with the increase of the molar ratio. Note that the nanocubes
also showed an absorption band at ca. 420 nm (Figure 4f, curve
4), typical of the plasmon resonance of the spherical silver NPs,
since the nanocubes assumed bent edges rather than sharp ones
such as triangles or prisms.2a More detailed investigation on their
optical properties is still in progress.
Acknowledgment. V.W.-W.Y. acknowledges the financial
support from the University Development Fund (UDF) of The
University of Hong Kong and The University of Hong Kong
Foundation for Educational Development and Research Limited.
Supporting Information Available: Preparation and characteriza-
tion, calculation of the equilibrium constant, EDX spectrum, HRTEM
image and analysis. This material is available free of charge via the
Internet at http://pubs.acs.org.
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