Photoinduced Shape Evolution: From Triangular to Hexagonal Silver Nanoplates

Abstract

Successfully using the solution phase, we have prepared, in large quantities, uniform hexagonal silver nanoplates developed from silver triangular nanoprims by employing a photoinduced technique. The growth process was characterized by ultraviolet−visible (UV−vis) spectroscopy, transmission electron microscope (TEM), and high-resolution transmission electron microscope (HRTEM). The UV−vis spectra showed that three bands of hexagonal silver nanoplates appear at 341 (weak), 368 (medium), and 498 (strong) nm. TEM images showed that hexagonal silver nanoplates had an average edge size of 25.9 nm and thickness of 15.7 ± 1.0 nm. The mechanism of the conversion from triangular to hexagonal nanoplates has also been studied. Triangular silver nanoplates were at first fabricated through seed-mediated growth of silver particles in the presence of trisodium citrate. Subsequently, the truncation of triangular nanoplates led to the formation of hexagonal nanoplates.

Full text

Photoinduced Shape Evolution: From Triangular to Hexagonal Silver Nanoplates
Jing An,†Bin Tang,†Xiaohua Ning,†Ji Zhou,†Shuping Xu,†Bing Zhao,†Weiqing Xu,*,†
Charlie Corredor,‡and John R. Lombardi*,‡
Key Laboratory for Supramolecular Structure and Material of Ministry of Education of China, Jilin UniVersity,
Changchun, 130012, P. R. China, and Department of Chemistry, The City College of New York,
New York, New York 10031
ReceiVed: June 11, 2007; In Final Form: September 14, 2007
Successfully using the solution phase, we have prepared, in large quantities, uniform hexagonal silver nanoplates
developed from silver triangular nanoprims by employing a photoinduced technique. The growth process
was characterized by ultraviolet-visible (UV-vis) spectroscopy, transmission electron microscope (TEM),
and high-resolution transmission electron microscope (HRTEM). The UV-vis spectra showed that three bands
of hexagonal silver nanoplates appear at 341 (weak), 368 (medium), and 498 (strong) nm. TEM images
showed that hexagonal silver nanoplates had an average edge size of 25.9 nm and thickness of 15.7 (1.0
nm. The mechanism of the conversion from triangular to hexagonal nanoplates has also been studied. Triangular
silver nanoplates were at first fabricated through seed-mediated growth of silver particles in the presence of
trisodium citrate. Subsequently, the truncation of triangular nanoplates led to the formation of hexagonal
nanoplates.
Introduction
Research in the growing field of nanomaterials has attracted
special attention in material science. Nanomaterials, such as
silver and gold nanoparticles, have unique chemical and physical
properties including optical, electronic, magnetic, and catalytic
properties.1Both the size and shape of anisotropic nanomaterials
provide useful control over the properties mentioned above.
These nanoparticles exhibit anisotropic optical absorption
properties associated with collective oscillations of conduction
electrons, which are also known as surface plasmon resonances
(SPR).2For example, Schultz and co-workers has shown that
pentagonal nanoparticles display a peak plasmon resonance
wavelength in the range 500-560 nm while triangular nano-
prisms display resonances in the range 530-700 nm.3Recently,
Mirkin et al. has also successfully prepared triangular nano-
prisms that showed a strong in-plane dipole plasmon resonance
at 670 nm.4
Over the last two decades, many distinctively shaped nano-
structures have been observed or synthesized using various
chemical approaches. Due to the potential applications of
nanoparticles in optics and interconnection in nanoelectronics,5
many chemical techniques have been employed to synthesize
anisotropic nanoparticles. However, further investigations in 1D
and 2D nanomaterials, such as nanobeams6and nanoprisms,7
should be expanded to better understand their behavior. Many
synthetic methods have been used in a practical and versatile
way8for production of nanoparticles in the shape of triangular,9
hexagonal,10 square,11 and circular plates.12 However, the
preparation of hexagonal silver nanoplates by controlling kinetic
growth in liquid solution is very limited and different mixtures
of shapes were often obtained from this mixture.13 Thus,
successful synthetic strategies are still a formidable challenge
for the preparation of uniform hexagonal silver nanoplates. In
this paper, we have successfully prepared uniform hexagonal
silver nanoplates using a solution-phase chemical approach,
which are generated in large quantities by choosing light with
a selected wavelength used to drive the photochemical growth.
Our particular interest is that hexagonal silver nanoplates are
developed from triangular silver nanoplates by the photoinduced
effect.
Experimental Section
Materials. AgNO3(99.5%) was obtained from Wako Pure
Chemical Industries, Ltd. NaBH4(98%) was obtained from
Sigma Chemical Co. Trisodium citrate (98%) was purchased
from Shanghai Chemical Reagent Co., Ltd. All chemicals used
were analytic grade reagents without further purification.
Instrumentation. Light for the photoinduced effect was
obtained by using a 70-W sodium lamp purchased from
Shanghai Yaming Co., Ltd. UV-vis spectra were recorded on
a Shimadzu UV-3100 spectrophotometer. Transmission electron
micrographs (TEM) were measured with a Hitachi H-8100 IV
operating at 200 kV. HRTEM images were measured with a
JEOL 3010 high-resolution transmission electron microscope
operating at 300 kV.
Preparation of Hexagonal Silver Nanoplates. A typical
experimental procedure was carried out by the following steps:
Silver seeds were prepared by dropwise addition of NaBH4
solution (8.0 mM, 1.0 mL) to an aqueous solution of AgNO3
(0.1 mM, 100 mL) in the presence of trisodium citrate (0.1 mM)
under vigorous stirring. The yellow silver seeds were then
irradiated with a conventional 70-W sodium lamp. A set of color
changes for the preparation of hexagonal silver nanoplates was
observed during the course of the reaction. Initially, the solution
turned green after being irradiated 3.5 h. After 8 h the solution
turned purple and finally pink over 10 h (Figure 1). The final
pink products consisted of two different size distributions of
* To whom correspondence should be addressed. E-mail: wqxu@
jlu.edu.cn (W.X.); lombardi@sci.ccny.cuny.edu (J.R.L.). Fax: +86-431-
85193421 (W.X.).
†Jilin University.
‡The City College of New York.
18055J. Phys. Chem. C 2007, 111, 18055-18059
10.1021/jp0745081 CCC: $37.00 © 2007 American Chemical Society
Published on Web 11/10/2007

nanoparticles, small nanodisks and large hexagonal nanoplates.
To separate the hexagonal Ag nanoparticles, we used a
centrifugation process at 4000 rpm for a period of 3 min in the
presence of trisodium citrate.
Results and Discussion
Figure 2a shows TEM images of hexagonal silver nanoplates
after centrifugation. In Figure 2b clearly we define three curves.
The first one, curve a, shows the UV-vis spectrum of silver
nanoparticles before centrifuging. The second one, curve b, is
the UV-vis spectrum of silver nanoparticles corresponding to
Figure 2a, which are located at the bottom layer of the sample
after centrifuging. The last one, curve c, shows the UV-vis
spectrum of the silver nanoparticles that are located at the top
layer of the sample after centrifugation. Three bands of curve
b appeared at 341 (weak), 368 (medium), and 498 (strong) nm,
which can be assigned to quadrupole, out-of-plane dipole, and
in-plane dipole plasmon resonances of hexagonal silver nano-
plates, respectively.4Curve c shows a single peak at 395 nm,
indicating an individual presence of silver nanodisks.14 Figure
S1 shows the TEM image of the small silver nanodisks. From
this image, we can conclude that the small nanoparticles were
disklike and not spherical in shape. In the spectrum of the
mixture of small silver nanodisks and hexagonal nanoplates,
curve a shows three peaks at 341 (weak), 393 (medium), and
509 (strong) nm, respectively. The peak of hexagonal nanoplates
at 368 nm which appeares in curve b cannot be observed in
curve a because the band of nanodisks at 395 nm is so strong
that it covers the neighboring weak peaks. Meanwhile, the band
at 395 nm exhibited a blue-shift to 393 nm owing to the
influence of the band at 368 nm. As well, the band at 509 nm
exhibited a blue-shift to 498 nm, which might be due to the
change of environment during the redispersion of silver nano-
particles located at the bottom layer of the sample after
centrifuging. In the experiment, we observed that the longest
wavelength band of silver nanoparticles was sensitive to the
environment of silver nanoparticles. Though the slight change
of environment caused the band at 509 nm to blue-shift, we
think it was reasonable to assign the band at 509 nm to
hexagonal silver nanoplates in the centrifugal process.
The growth process of hexagonal silver nanoplates was
monitored by UV-vis spectra proceeding in time (Figure 3a).
For the silver seeds (curve a in Figure 3a), a single absorption
band due to the plasmon resonance of small spherical particles
of the seed solution appeared at 392 nm. After 3.5 h of
irradiation with sodium light, two distinctive peaks appeared at
333 (weak, as a shoulder peak) and 738 (medium) nm,
respectively (curve b in Figure 3a). According to Mie’s theory,
only a single surface plasmon resonance band could be observed
in the absorption spectra of spherical nanoparticles, and aniso-
tropic particles could give rise to two or more surface plasmon
resonance bands depending on the shape of the particles.15 These
two distinctive peaks which appeared at 333 and 738 nm can
be assigned to quadrupole and in-plane dipole plasmon reso-
nances of triangular nanoplates, respectively. During this period,
the peak at 392 nm showed a slight red shift to 395 nm while
its intensity decreased. As the irradiation continued, the band
located at 738 nm showed a significant blue shift to 509 nm
(shown in curve d), concomitant with the red shift of the 333-
nm band to 341 nm (shown in inset of Figure 3a). The 341-
and 509-nm bands can be assigned to quadrupole and in-plane
dipole plasmon resonances of hexagonal nanoplates, respec-
tively. Meanwhile, the location of the peak at 395 nm remained
basically unchanged but its intensity increased progressively
(shown in curves b-d). We have also noticed the color of the
silver seeds turned lighter gradually under room light presence
before the irradiation of sodium light, and we observed a series
of changes of silver seeds using the UV-vis spectrometer to
monitor the process. The 392-nm band showed a progressive
decrease in intensity (shown in Figure 3b), which might be due
to the fragmentation of silver seeds. This phenomenon was the
most obvious when the molar ratio of NaBH4to AgNO3was
Figure 1. Photograph of the reaction solution at different stages: (I)
0 h; (II) 3.5 h; (III) 8 h; (IV) 10 h.
Figure 2. (a) TEM image of hexagonal silver nanoplates. (b) UV-vis spectra of silver nanoparticles before centrifuging (curve a), the bottom after
centrifuging (curve b), and the top after centrifuging (curve c), respectively.
18056 J. Phys. Chem. C, Vol. 111, No. 49, 2007 An et al.

1:1. After irradiation with a sodium lamp, its intensity increased
until triangular nanoplates began to appear (Figure 3c).
Correlated with the UV-vis spectroscopic observations, the
TEM images showed that the initial silver seeds (Figure 4a)
were first converted into triangular silver nanoplates (Figure
4b) and, subsequently, they turn to hexagonal plates under
irradiation of sodium light (Figure 4d). During the initial stages
of growth (after 3.5 h, Figures 4b and 5a), silver nanoprisms
with edge length 76.2 nm (designated as type 1) were the
dominant shape of the solution, while some small silver
nanoparticles (designated as type 2) with diameter of about 7.6
nm were also observed adjacent to the triangular particles. With
increase of the irradiation time (Figure 4c), the type 1 particle
changed from a prism to a hexagon shape and the diameter of
the type 2 particle increased corresponding to the enhancement
of the peak at 395 nm (curves b-d in Figure 3a). After 10 h,
all of the silver prisms were completely converted into hexagonal
silver nanoplates with edge length 25.9 nm (Figures 4d and 5b).
Meanwhile, the particles of type 2 changed to nanodisks with
the diameter of 11.2 nm. Although, the average edge lengths
for triangular and hexagonal nanoplates are different, their
thickness is essentially unchangeable (14.1 (1.7 nm at 3.5 h
vs 15.7 (1.0 nm at 10 h, Figure 4e,f). Figure S3 shows the
electron diffraction pattern taken from an individual nanoplate
by directing the electron beam perpendicular to one of its flat
faces. The electron diffraction pattern indicates that each
nanoplate was a single crystal. The hexagonal symmetry of these
patterned spots implied that the two flat faces of each nanoplate
were bounded by the (111) planes and the (111) basal planes
were not changed during this shape evolution process. Further-
more, Figure S4 shows the HR-TEM images of the triangular
and hexagonal nanoplates recorded perpendicular to flat faces
of an individual nanoplate. The fringes of the triangular silver
nanoplate are separated by 2.49 Å, which can be ascribed to
the (1/3) {422}reflection that is generally forbidden for an fcc
lattice. The fringe spacing of the hexagonal nanoplate is also
measured as 2.49 Å, which is the same as the triangular
nanoplate.
To gain insight into the mechanism of this unusual and
remarkably efficient conversion of triangular to hexagonal
nanoplates, a set of experiments were further carried out by
changing the amount of reagent. Triangular silver nanoplates
(type 1) and small silver nanoparticles (type 2) were obtained
when the molar ratio of Na3C6H5O7and AgNO3was at the range
0.8-2. The triangular silver nanoplates (type 1) could only be
observed when their molar ratio was at the range of 3-50. The
Figure 3. (a) UV-vis spectra showing the conversion of silver seeds to hexagonal nanoplates: before irradiation (curve a) and after 3.5 (curve b),
8 (curve c), and 10 (curve d) h of irradiation. Inset: Enlarged UV-vis spectra between 315 and 365 nm. (b) UV-vis spectra of silver seeds placed
before irradiation for different times: (1) 0 min; (2) 10 min; (3) 20 min; (4) 30 min; (5) 40 min; (6) 50 min. (c) UV-vis spectra of silver seeds
irradiated with a sodium lamp for different times: (7) 10 min; (8) 20 min; (9) 30 min; (10) 40 min.
Figure 4. TEM images showing the conversion of silver seeds to
hexagonal nanoplates: (a) before irradiation and after (b) 3.5, (c) 8,
and (d) 10 h of irradiation. Inset: Enlarged photos are showing
triangular (e) and hexagonal (f) silver nanoplates stacks.
Photoinduced Shape Evolution J. Phys. Chem. C, Vol. 111, No. 49, 2007 18057

growing process of triangular silver nanoplates can be attributed
to the transformation of small nanoparticles into other particle
geometries via the Ostwald ripening process.16 The results
indicate that, due to the photoinduced effect, the vertices of
triangular nanoplates gain higher energy and begin to be
truncated as reactive dots (shown in Figure 4c). In this process,
the effect of heat is obvious. The triangular silver nanoplates
will change to nanodisks rather than hexagonal nanoplates when
the reactive system is above 80 °C. Hexagonal nanoplates cannot
be changed from triangular nanoplates when the molar ratio of
Na3C6H5O7and AgNO3is at the range 5-50. A detailed
mechanism for this process has been suggested in previous
articles on the self-limiting photoinduced growth of Ag
nanoplates.4,9a We believe the mechanism here is similar.
Meanwhile, the effect of heat is also limited in this process.
The final products are triangular silver nanoplates (Figure S2).
The results suggest that the vertices of triangular nanoplates
are fixed by excess citrate ions in this case. The thickness values
of triangular and hexagonal silver nanoplates are essentially
similar (14.1 (1.7 nm vs 15.7 (1.0 nm), implying that the
(111) basal plane is not changed during the growth process.
The result shows that the anisotropic growth of silver mainly
occurrs at the edge (110), (101), and (011h) planes of the
triangular silver nanoparticles.17 It is reasonable to suppose that
the citrate molecule plays a key role in this anisotropic growth,
which acts as a capping agent for the silver particles and a
photoreducing agent for the silver ions.18 During the truncation
process, the (111) plane of the silver triangular particle was
protected by the well-defined self-assembled layer of citrate ions
and further change of the (111) plane was prevented. Combining
the information from the UV-vis spectra and TEM images,
we can further explore the mechanism of how light influences
Figure 5. Histograms of silver nanoparticles after 3.5 (a) and 10 (b) h of irradiation illustrating the particle size distributions (the radii for type 1
and the edge lengths for type 2) as bimodal.
SCHEME 1: Schematic Diagram of Photoinduced Shape
Evolution of Triangular to Hexagonal Silver Nanoplates
TABLE 1: Aspect Parameters (nm) of Silver Nanoparticles
Corresponding to Scheme 1
colloids
scheme part type 1 type 2
ba1)76.2 h1)14.1 (1.7 R1)22.0 2r1)7.6
da2)25.9 h2)15.7 (1.0 R2)22.4 2r2)11.2
18058 J. Phys. Chem. C, Vol. 111, No. 49, 2007 An et al.

the interconversion and growth of hexagonal silver nanoplates
(Scheme 1 and Table 1). The UV-vis spectra showed the 738-
nm peak was blue-shifted and the 333-nm peak was red-shifted,
which caused the in-plane shape transformation from triangle
to hexagon. TEM images exhibiting triangular and hexagonal
nanoplates possess the similar radii of inscribed circles (22.0
nm vs 22.4 nm), which indicates that hexagonal nanoplates were
derived from triangular nanoplates by truncation.
Conclusion
We have shown that uniform hexagonal silver nanoplates
developed from silver nanoprisms have been successfully
prepared by using a solution-phase chemical approach. These
are generated in large quantities by choosing a sodium lamp
with the appropriate wavelengths used to drive the photochemi-
cal growth. In this method, triangular silver nanoplates are
fabricated first through seed-mediated growth of silver particles
in the presence of trisodium citrate. Subsequently the truncation
of triangular nanoplates leads to the formation of hexagonal
nanoplates. The growth process has been characterized by UV-
vis spectroscopy, TEM, and HRTEM. TEM images show that
hexagonal silver nanoplates have an average edge size of 25.9
nm and thickness of 15.7 (1.0 nm (Scheme 1 and Table 1).
The UV-vis spectra show that three bands of hexagonal silver
nanoplates appear at 341 (weak), 368 (medium), and 498
(strong) nm.
Acknowledgment. This work is supported by the National
NaturalScienceFoundationofChina(NSFC20573041,-20773045,
and -20627002). We are also indebted to the National Institute
of Justice (Department of Justice Award No. 2006-DN-BX-
K034) and the City University Collaborative Incentive program
(No. 80209). This work was also supported by the National
Science Foundation under Cooperative Agreement No. RII-
9353488, Grant Nos. CHE-0091362, CHE-0345987, and
ECS0217646, and by the City University of New York PSC-
BHE Faculty Research Award Program.
Supporting Information Available: Additional TEM and
UV-vis images. This material is available free of charge via
the Internet at http://pubs.acs.org.
References and Notes
(1) (a) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem.
Rev. 2005,105, 1025. (b) Creighton, J. A.; Eadon, D. G. J. Chem. Soc.,
Faraday Trans. 1991,87, 3881. (c) Alivisatos, A. P. Science 1996,271,
933. (d) Shi, J.; Babcock, G. K.; Awachalon, A. A. Science 1996,271,
937. (e) Hu, J. T.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999,32,
435. (f) Wang, Z. L. J. Phys. Chem. B 2000,104, 1153. (g) Hoelderich, W.
F. Catal. Today 2000,62, 115.
(2) (a) Kreibig, U.; Vollmer, M. In Optical Properties of Metal Clusters;
Gonser, U., Osgood, R. M., Panish, M. B., Jr., Sakaki, H., Eds.; Springer:
New York, 1995; Chapter 2. (b) Bohren, C. F.; Huffman, D. R. In
Absorption and Scattering of Light by Small Particles; John Wiley &
Sons: New York, 1998; Chapter 12.
(3) Mock, J. J.; Barbic, M.; Smith, D. R.; Schultz, D. A.; Schultz, S.
J. Chem. Phys. 2002,116, 6755.
(4) Jin, R. C.; Cao, Y. W.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.;
Zheng, J. G. Science 2001,294, 1901.
(5) (a) Cui, Y.; Lieber, C. M. Science 2001,291, 851. (b) Huang, M.;
Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang,
P. Science 2001,292, 1897. (c) Hanrath, T.; Korgel, B. A. J. Am. Chem.
Soc. 2002,124, 1424. (d) Dumestre, F.; Chaudret, B.; Amiens, C.; Fromen,
M.-C.; Casanove, M.-J.; Respaud, M.; Zurcher, P. Angew. Chem., Int. Ed.
2002,41, 4286. (e) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science
2002,295, 2425. (f) Qian, C.; Kim, F.; Ma, L.; Tsui, F.; Yang, P.; Liu, J.
J. Am. Chem. Soc. 2004,126, 1195.
(6) Chen, J. G.; Wiley, B. J.; Xia, Y. N Langmuir 2007,23, 4120.
(7) Xue, C.; Mirkin, C. A. Angew. Chem., Int. Ed. 2007,46, 2036.
(8) Adair, J. H.; Suvaci, E. Curr. Opin. Colloid Interface Sci. 2000,5,
160.(9) (a) Callegari, A.; Tonti, D.; Chergui, M. Nano Lett. 2003,3, 1565.
(b) Chen, S.; D. Carroll, L. J. Phys. Chem. B 2004,108, 5500. (c) He, Y.;
Shi, G. J. Phys. Chem. B 2005,109, 17503. (d) Xiong, Y.; McLellan, J.
M.; Chen, J.; Yin, Y.; Li, Z. Y.; Xia, Y. J. Am. Chem. Soc. 2005,127,
17118. (e) Aslan, K.; Lakowicz, J. R.; Geddes, C. D. J. Phys. Chem. B
2005,109, 6247. (f) Liu, B.; Xie, J.; Lee, J. Y.; Ting, Y. P.; Chen, J. P. J.
Phys. Chem. B 2005,109, 15256. (g) Zhang, Y. W.; Sun, X.; Si, R.; You,
L. P.; Yan, C. H. J. Am. Chem. Soc. 2005,127, 3260.
(10) (a) Sigman, M. B., Jr.; Ghezelbash, A.; Hanrath, T.; Sanuders, A.
E.; Lee, F.; Korgel, B. A. J. Am. Chem. Soc. 2003,125, 16050. (b) Zhang,
H. T.; Wu, G.; Chen, X. H. Langmuir 2005,21, 4281. (c) Wang, W.; Poudel,
B.; Yang, J.; Wang, D. Z.; Ren, Z. F. J. Am. Chem. Soc. 2005,127, 13792.
(d) Hou, Y.; Kondoh, H.; Shimojo, M.; Kogure, T.; Ohta, T. J. Phys. Chem.
B2005,109, 19094. (e) Lu, W.; Ding, Y.; Chen, Y.; Wang, Z. L.; Fang, J.
J. Am. Chem. Soc. 2005,127, 10112. (f) Garje, S. S.; Eisler, D. J.; Ritch,
J. S.; Afzaal, M.; OMBrien, P.; Chivers, T. J. Am. Chem. Soc. 2006,128,
3120.
(11) (a) Cao, Y. C. J. Am. Chem. Soc. 2004,126, 7456. (b) Si, R.; Zhang,
Y. W.; You, L. P.; Yan, C. H. Angew. Chem., Int. Ed. 2005,44, 3256. (c)
Yu, T.; Joo, J.; Park, Y. I.; Hyeon, T. J. Am. Chem. Soc. 2006,128, 1786.
(12) (a) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001,
291, 2115. (b) Puntes, V. F.; Zanchet, D.; Erdonmez, C. K.; Alivisatos, A.
P. J. Am. Chem. Soc. 2002,124, 12874. (c) Chen, S.; Fan, Z.; Carroll, D.
L. J. Phys. Chem. B 2002,106, 10777.
(13) Lofton, C.; Sigmund, W. AdV. Funct. Mater. 2005,15, 1197.
(14) Chen, Y. B.; Chen, L.; Wu, L. M. Inorg. Chem. 2005,44, 9817.
(15) Roosen, A. R.; Carter, W. C. Physica A 1998,261, 232.
(16) Maillard, M.; Huang, P.; Brus, L. Nano Lett. 2003,3, 1611.
(17) Jin, R. C.; Cao, Y. C.; Hao, E. C.; Me´traux, G. S.; Schatz, G. C.;
Mirkin, C. A. Nature 2003,425, 487.
(18) (a) Henglein, A.; Giersig, M. J. Phys. Chem. B 1999,103, 9533.
(b) Redmond, P. L.; Wu, X. M.; Brus, L. J. Phys. Chem. C 2007,111,
8942.
Photoinduced Shape Evolution J. Phys. Chem. C, Vol. 111, No. 49, 2007 18059