Pt nanocrystals: Shape control and Langmuir-Blodgett monolayer formation

Article abstract of DOI:10.1021/jp0464775

We report the synthesis of monodisperse Pt nanocrystals with three different shapes-cubes, cuboctahedra, and octahedra, selectively, with similar sizes of 9-10 nm by a modified polyol process. We found that addition of silver ion enhances the crystal growth rate along (100), and essentially determines the shape and surface structure of the Pt nanocrystals. After the reaction, the silver species can be easily removed by repetitive precipitation giving pure Pt nanoparticles. Two-dimensional arrays of the Pt nanocrystals were assembled by using the Langmuir-Blodgett (LB) method. The particles were evenly distributed on the entire substrate, and their surface coverage and density can be precisely controlled by tuning the surface pressure. The resulting Pt LB layers are potential candidates for 2-D model catalysts as a result of their high surface area and the structural uniformity of the metal nanocrystals.

Full text of DOI:10.1021/jp0464775

188
J. Phys. Chem. B 2005, 109, 188-193
Pt Nanocrystals: Shape Control and Langmuir-Blodgett Monolayer Formation
Hyunjoon Song, Franklin Kim, Stephen Connor, Gabor A. Somorjai, and Peidong Yang*
Department of Chemistry, UniVersity of California, Berkeley, Materials Science DiVision, Lawrence Berkeley
National Laboratory, Berkeley, California 94720
ReceiVed: August 5, 2004; In Final Form: September 28, 2004
We report the synthesis of monodisperse Pt nanocrystals with three different shapesscubes, cuboctahedra,
and octahedra, selectively, with similar sizes of 9-10 nm by a modified polyol process. We found that addition
of silver ion enhances the crystal growth rate along 100 , and essentially determines the shape and surface
structure of the Pt nanocrystals. After the reaction, the silver species can be easily removed by repetitive
precipitation giving pure Pt nanoparticles. Two-dimensional arrays of the Pt nanocrystals were assembled by
using the Langmuir-Blodgett (LB) method. The particles were evenly distributed on the entire substrate,
and their surface coverage and density can be precisely controlled by tuning the surface pressure. The resulting
Pt LB layers are potential candidates for 2-D model catalysts as a result of their high surface area and the
structural uniformity of the metal nanocrystals.
Introduction
morphology control of nanocrystals. We also demonstrated two-
dimensional organization of the Pt nanocrystals by the Lang-
muir-Blodgett (LB) technique. The resulting Pt layer structures
will be exploited as ideal 2-D model catalysts as a result of
their high surface area, the tunable interparticle spacing, and
the structural uniformity of the metal nanocrystals.
Metal nanocrystals with precisely controlled shape exhibit
unique optical, magnetic, and catalytic properties.1 There have
been extensive studies on approaches to control size and shape
of the metal nanoparticles, and most of the methods developed
thus far can be categorized as either reduction or decomposition
of the metal precursors in the presence of organic capping
reagents in solution. Cetyltrimethylammonium bromide (CTAB)
and poly(vinylpyrrolidone) (PVP) have been widely used as
regulating agents for the selective growth of nanocrystals with
well-defined shapes such as rods,2,3 prisms,4 and cubes.5
Recently, Yang et al. reported the synthesis of gold nanocrystals
with various shapes by refluxing gold precursor in ethylene
glycol (EG) in the presence of PVP.6 Murphy et al. have also
prepared a variety of gold colloids from the cubes to the
branched particles on the basis of seed-mediated growth with
CTAB.7 However, the exact binding nature between these
capping reagents and the specific crystallographic planes is still
elusive, and there are no generalized mechanisms interpreting
various metal nanocrystal shape-control experiments.
Since El-Sayed and co-workers reported the Pt cube synthesis
using polymer capping agents,8 many researchers tried to make
various shapes of the Pt particles9 in order to investigate their
influence on catalytic activity.10 El-Sayed et al. have synthesized
three different shapes of the Pt nanocrystals and have run the
electron-transfer reaction between hexacyanoferrate(III) and
thiosulfate ions.11 While this study gives intriguing correlations
between the kinetic parameters of the reaction and surface atom
fraction in each shape, more uniform Pt nanoparticles in terms
of the size and shape are required to precisely quantify the
catalytic influences. Herein we report the synthesis of mono-
disperse Pt nanocrystals with various shapesscubes, cubocta-
hedra, and octahedra, selectively, in the same range of the
particle size (9-10 nm). We found that silver ion enhances the
crystal growth rate along 100 , and essentially determines the
shape and surface structure of the Pt nanocrystals. This process
may be applicable to other metal and semiconductor nanostruc-
tures, and may provide insights for a general mechanism on
Experimental Section
Materials. Dihydrogen hexachloroplatinate (H2PtCl6‚6H2O,
99.9%, metals basis) was purchased from Alfa Aesar. Silver
nitrate (AgNO3, 99+%) and poly(vinylpyrrolidone) (PVP, Mw
) 55 000) were obtained from Sigma-Aldrich. Ethylene glycol
(EG) and all other solvents were of analytical grade and were
used without further purification.
Synthesis of Pt Nanocrystals. A volume of 2.5 mL of EG
was refluxed for 5 min. Then, 93.8 µL of 0.375 M PVP (total
3 mL, PVP/Pt salt ) 12:1) and 46.9 µL of 0.0625 M H2PtCl6‚
6H2O (total 1.5 mL) solutions in EG were added to the boiling
EG every 30 s over a 16-min period. The resulting mixture was
refluxed for an additional 5 min. The product was centrifuged
at 5000 rpm for 15 min. The supernatant was separated and
precipitated by adding a triple volume of acetone, followed by
centrifugation at 3000 rpm for 5 min. The precipitate was
collected and redispersed in 3 mL of ethanol with sonication.
A volume of 9 mL of hexane was added to the dispersion, and
the solution was centrifuged at 3000 rpm for 5 min. The
precipitate was washed twice with the same solvent mixture.
Finally, the precipitate was dispersed in 3 mL of ethanol.
For Pt cube synthesis, 0.5 mL of 2 × 10-3 M AgNO3 solution
in EG (Ag salt/Pt salt ) 1.1 mol %) was added to the boiling
EG prior to the addition of PVP and Pt precursors. For Pt
cuboctahedron and octahedron samples, 0.5 mL of 2 × 10-2
M
(Ag/Pt ) 11 mol %) and 6 × 10-2 M (Ag/Pt ) 32 mol %)
AgNO3 solutions in EG were added to the boiling EG,
respectively. PVP and Pt salt solutions were immediately added
to the reaction mixture.
For mechanism studies, several aliquots of the reaction
mixture were collected in an appropriate time during the
reaction, and checked by UV-Vis spectroscopy.
* Corresponding author. E-mail: p_yang@uclink.berkeley.edu.
10.1021/jp0464775 CCC: $30.25 © 2005 American Chemical Society
Published on Web 12/01/2004

Pt Nanocrystals: Shape and Surface Structure
J. Phys. Chem. B, Vol. 109, No. 1, 2005 189
Figure 1. TEM image of the Pt nanocrystals synthesized without
AgNO3 addition.
Figure 3. (A) TEM image of the Pt cuboctahedra. (B) HRTEM image
of the Pt cuboctahedron along the [110] zone axis. (C) An ideal
cuboctahedron structure and its 2-D projection along the [110] direction.
Figure 2. (A) TEM image of the Pt cubes. (B) HRTEM image of the
Pt cube along the [001] zone axis. (C) An ideal cube structure. (D)
HRTEM image of the Pt tetrahedron along the [111] zone axis.
Figure 4. (A) TEM image of the Pt octahedra. (B) HRTEM image of
the Pt octahedron along the [110] zone axis. (C) HRTEM image of the
Pt octahedron along the [001] zone axis. (D) An ideal octahedron
structure.
Langmuir-Blodgett Layer Formation. The Pt colloidal
solution in ethanol was precipitated by adding a triple volume
of hexane. The precipitate was redispersed in 3 mL of
chloroform with sonication. LB experiments were done with
deionized water on a LB trough (Nima Technology, M611) at
room temperature. The surface pressure was monitored with a
Wilhelmy plate, and was adjusted to zero before spreading the
particles. The chloroform solution of Pt cubes was slowly spread
on the water surface of the trough. The resulting surface layer
was compressed by moving the mobile barrier at a rate of 15
cm2/min. At the different stages of compression, the Pt layers
at the water-air interface were carefully transferred onto
continuous carbon TEM grids using the Langmuir-Scha¨ffer
horizontal liftoff method.
Characterization. Transmission electron microscopy (TEM),
high-resolution transmission microscopy (HRTEM), and energy-
dispersive X-ray spectroscopy (EDX) experiments were made
on a Philips CM200 microscope operated at 200 kV at the
National Center for Electron Microscopy at Lawrence Berkeley
National Laboratory. The Pt colloidal solutions in EG were
deposited on carbon-film-coated copper grids (Ted Pella). X-ray
TABLE 1: Yield and Average Size of the Pt Nanocrystals
amount of
Ag addeda
major shape
yield, average size
minor shape
yield, average size
1.1 mol %
11 mol %
32 mol %
cubes
tetrahedra
∼10%, 9.8 ( 0.7 nm
∼80%, 9.4 ( 0.6 nm
cuboctahedra
∼100%, 9.1 ( 0.6 nm
octahedra
∼80%, 9.8 ( 0.6 nm
tetrahedra
∼10%, 9.9 ( 0.7 nm
a With respect to the Pt salt concentration.
diffraction (XRD) patterns were measured on a Bruker D8
GADDS diffractometer using Co KR radiation (1.79 Å). UV-
Vis absorption spectroscopy experiments were carried out on
an Agilent 8453 UV-Vis system.
Results and Discussion
Synthesis and Morphology Control of Pt Nanocrystals.
In a typical synthesis, EG solutions of PVP (93.8 µL of 0.375

190 J. Phys. Chem. B, Vol. 109, No. 1, 2005
Song et al.
Figure 5. (A) UV-Vis absorption spectra of the Pt octahedron sample
during the reaction. The aliquots were collected (a) 0, (b) 0.5, (c) 5,
(d) 16, (e) 21, and (f) 76 min after the addition of AgNO3. (B) UV-
Vis spectra of (a) the supernatant after the second centrifugation in
EG, (b) the purified Pt octahedra, and (c) as-prepared mixture without
purification.
Figure 6. (A) EDX spectrum of the pure Pt octahedra. (B) XRD spectra
of (a) the cubes, (b) the cuboctahedra, and (c) the octahedra.
obtained (∼100%), including hexagons as the majority after
purification (Figure 3A). The Pt nanocrystals are monodisperse
with the largest vertex-to-vertex distance of 9.1 ( 0.6 nm. Figure
3B is a representative HRTEM image of the hexagon, and
clearly shows the lattice fringe image of {111} planes with the
interplanar distance of 2.26 Å and the separation angle of 70°,
consistent with the hexagonal projection of an ideal cubocta-
hedron along the [110] zone axis (Figure 3C). In this projection,
four {111} and two {100} facets are placed on the edges of the
hexagonal shape. Other facets represent the projections from
the cuboctahedral structure along different orientations.
At higher concentrations of AgNO3 up to 32 mol %, the
resulting Pt nanocrystals are dominated by diamond- and square-
shaped particles (∼80%) with tetrahedra (∼10%) (Figure 4A).
The average vertex-to-vertex distance of the major particles is
9.8 ( 0.6 nm. Figure 4B shows HRTEM image of a diamond-
shaped particle, which turns out to be the [110] oriented Pt
octahedron. The two ends of the octahedron are slightly
truncated. The square shapes are not from the Pt cubes, but
from the same octahedra oriented along the [001] zone axis.12
Figure 4C exhibits four {111} facets edged on the Pt octahedron,
while four {100} planes are located on the edges of a Pt cube
along the same direction. The ideal model of the octahedron
(Figure 4D) indicates that the surface is covered with {111}
faces.
M) and dihydrogen hexachloroplatinate (46.9 µL of 0.0625 M)
were added to the boiling EG every 30 s over a 16-min period.
EG serves both as a solvent for dissolving the reactants and as
a reducing agent. The color of the solution immediately changed
from yellow to dark brown, indicating the fast reduction of Pt-
(IV) to Pt(0) species. The solution was refluxed for an additional
5 min. Without adding Ag ions, the particles were obtained as
a mixture of different shapes. Figure 1 exhibits various shapes
of the Pt particles synthesized in this reaction including cubes
with round edges (∼40%), tetrahedra (∼10%), spheres and
facets (∼30%), and irregular rods (∼20%). However, when 1.1
mol % of AgNO3 (with respect to the Pt concentration) was
introduced into the solution, Pt cubes (∼80%) were preferen-
tially obtained with a small amount of tetrahedra (∼10%) after
purification. A TEM image (Figure 2A) shows that the Pt cubes
are homogeneous in shape with a narrow size distribution (face-
to-face: 7.1 ( 0.6 nm, vertex-to-vertex: 9.4 ( 0.6 nm). An
HRTEM image (Figure 2B) of a Pt cube demonstrates the
exposed {100} surface of the cube oriented along the [001] zone
axis. The distance between the adjacent lattice fringes is 1.96
Å, in good agreement with the interplanar distance of the (200)
plain in the face-centered cubic (fcc) Pt structure. The minor
tetrahedral particles are in the average size of 9.8 ( 0.7 nm.
Figure 2D shows a triangular projection of the tetrahedral
particles along the [111] direction, in which all side faces are
covered with {111} planes.
All three Pt particles were carefully purified by repetitive
centrifugation and dispersion. Table 1 shows that all particles
generated in this reaction were in the similar sizes of 9-10 nm
with narrow distributions less than σ ∼ 7%. It means that the
shape is the decisive factor for the surface structure of each
nanocrystal. Considering the ideal models, the cube has {100}
Increasing the AgNO3 concentration to 11 mol % changes
the morphology of the Pt particles. Faceted particles were mostly

Pt Nanocrystals: Shape and Surface Structure
J. Phys. Chem. B, Vol. 109, No. 1, 2005 191
Figure 7. LB layers of the Pt cubes deposited at the surface pressure of (A) 3.6 mN/m, (B) 4.0 mN/m, (C) 6.0 mN/m, (D) 10.6 mN/m, and (E)
16.9 mN/m. (F) Pt cube nanocrystal coverage (estimated from TEM data) as a function of the surface pressure during the LB process.
faces (Figure 1C), and the octahedron and tetrahedron display
{111} surfaces (Figure 4D). In the cuboctahedron, the surface
is composed of six {100} and eight {111} planes with an area
ratio of 1:0.577 (Figure 3C). Accordingly, surface-dependent
properties such as catalytic reactivity13 can be modified
rationally by manipulating the shape of the particles with a
variation of the added silver ion. Several shapes of Pt nano-
particles have been successfully synthesized previously,8,9 but
there were few reports to tune both the size and shape of
nanocrystals simultaneously under the same reaction conditions.
Addition rate of the reactants is important to control the size
of Pt nanocrystals. If the PVP and Pt salts were added all at
once, small and irregular Pt particles were generated with the
average size less than 4 nm. On the other hand, slow addition
of the PVP and Pt salt solutions over a period of 30 min led to
polycrystalline particles larger than 13 nm. These experiments
indicate that the particle size can be adjusted by controlling
concentration of seeds formed at the early stages of the reaction.
Plausible Mechanism and Purification of Pt Nanocrystals.
It is commonly believed that the final morphology of the fcc
nanocrystals is dependent upon the R value, defined as the
relative growth rate along the 100 direction with respect to
that of the 111 .12,14 As the concentration of Ag ion increases
in the reaction mixture, the majority of the Pt particles changes
from the cubes (R ) 0.58) to the cuboctahedra (R ) 0.87), and
eventually to the octahedra (R ) 1.73). It reveals that introduc-
tion of the Ag ion enhances the growth along 100 , and/or
suppresses the growth along 111 . The reaction progress of Pt
octahedron synthesis was monitored by UV-Vis absorption to
support this mechanism. At early stages of the reaction (Figure
5A, (b) and (c)), a clear absorption band at ∼270 nm was
observed. The strong absorption at 270 nm was also observed
under the same reaction in the absence of the Pt salts. It is
2+
probably attributed to the reduced silver species such as Ag4
,
which is the most stable intermediate formed in the reduction
of silver nitrate by γ-irradiation and sodium borohydride.15 This
silver species seems to be preferentially adsorbed on more active
{100} surfaces of the Pt nuclei than {111} facets during the
reaction. Stru¨ber et al. reported that the desorption energy of
Ag on the Pt(100) single crystalline surface is higher than that
on Pt(111) in the Ag monolayer film growth, indicating the
relative stability of Ag(0) on the Pt{100} surface.16 When the
Pt precursors were continuously added, the Pt salts were reduced
spontaneously with the oxidation of adsorbed Ag species on

192 J. Phys. Chem. B, Vol. 109, No. 1, 2005
Song et al.
the {100} surface by favorable electrochemical reaction (4Ag
+ H2PtCl6 f 4AgCl + Pt(0) + 2HCl), and subsequently the
growth rate along the 100 direction was enhanced with the
dissolution of AgCl into the solvent. In Figure 5A (c)-(f), the
peak at 270 nm is diminished, and the broad shoulder appears
at ∼290 nm as the reaction proceeds, indicative of consumption
of the reduced silver species with the concomitant formation
of Pt(0) nanoparticles.
Importantly, silver byproducts can be effectively removed by
repetitive precipitation. After the reaction, the product consisted
of Pt particles as well as AgCl, reduced silver species, and silver
particles (Figure 5B (c)). The first centrifugation precipitated
AgCl as a white powder as a result of its low solubility in EG
at room temperature. Adding acetone into the mixture precipi-
tated the Pt particles by centrifugation, while the reduced silver
species and free-standing PVP stayed in the supernatant as seen
in Figure 5B (a), exhibiting strong absorption at 270 nm. The
clean Pt particles were obtained by precipitation and redispersion
in an ethanol/hexane (1:3) mixture several times, and were
characterized by UV absorption, EDX, and XRD. Figure 5B
(b) shows a very broad signal at ∼290 nm, but no distinguish-
able peaks at ∼270 nm (for reduced silver species) and ∼400
nm (for Ag particles). There were no detectable Ag signals in
EDX nor in XRD spectra (Figure 6) within the detection limit.
These results suggest that Ag ions may act as a shape-control
reagent during the reaction, and can be easily removed to
produce pure Pt particles without significant alloying between
Ag and Pt. This is opposite to the Galvanic replacement reaction
of Ag nanoparticles and gold salts giving the Au/Ag alloy wall
structure.17
Figure 8. LB monolayers of (A) Pt cuboctahedra and (B) Pt octahedra.
packed 2-D phases were formed only for Ve < 350 Å, where
short alkyl chain ligands were bound to the large particles. In
our experiment, the chains of PVP are quite long, and they are
likely entangled. The hydrophilic nature of the pyrrolidone ring
in PVP also prevents the effective packing of particles through
the hydrophobic interaction of long alkyl chain ligands.19 The
surface pressure increased suddenly when the Pt layer was
further compressed. The monolayers started to collapse and form
multilayer structures at the pressure of 10.6 mN/m (Figure 7D).
Figure 7F shows the relationship between surface coverage and
pressure of the particle layers. In the monolayer region, the
coverage rapidly increases as the surface pressure increases.
It is interesting to point out that previously reported gold
nanorod synthesis by photochemical and electrochemical meth-
ods3 may follow the analogous mechanism. Introducing Ag ions
enhances the 100 directional growth, and subsequently controls
the aspect ratio of the nanorods. The resulting rods contain no
Ag composition in their lattice as well. We believe that this
process can be applicable to other metal and semiconductor
systems using various foreign ions as shape-control agents.
Langmuir-Blodgett Layer Formation of Pt Nanocrystals.
The LB technique is one of the powerful tools for fabricating
the monolayer of nanoscopic structures18 such as quantum dots,19
rods,20 and wires21 on various substrates. This method was
utilized for 2-D organization of the Pt nanocrystals here. The
Pt cubes were dispersed in chloroform, and the solution was
slowly deposited on the water surface of the trough. The surface
pressure increased up to 3.6 mN/m during the deposition,
presumably as a result of the presence of weakly bound PVP
molecules. The layers were collected frequently by TEM grids
with the horizontal liftoff method in order to examine the particle
distribution. Even at low surface pressure, the particles were
evenly distributed on the entire area of the grids, and no particle
aggregation was observed in Figure 7A. The surface coverage
was estimated by counting the particles on the same area of the
TEM grids. As the surface pressure increased up to 6.0 mN/m,
the surface coverage rapidly changed from 7.6% to 40%. The
particles formed a continuous monolayer, but they showed no
long-range ordered structures as seen in Figure 7C. This is
mainly attributed to the high-molecular-weight PVP coordinated
on the Pt surface. Heath et al. have reported that the size of the
passivating ligand plays important roles in determining the 2-D
structures.19c When the small Au particles (∼2 nm) were capped
with oleylamine (excess conical volume, Ve > 350 Å), the phase
diagrams were dominated by 1-D structures at low surface
pressures and 2-D formlike phases at high pressures. The close-
The different shaped particles were deposited on the water
surface in the same way. The PVP-capped cuboctahedral and
octahedral particles were randomly distributed on the entire area
of the TEM grids with tunable density (Figure 8). Even though
close-packed 2-D structures were not generated here, high
surface coverage of the Pt particles was obtained by increasing
the surface pressure. It is interesting to mention the surface
structure of the resulting layers. The {100} surfaces were
enriched in the LB layer of Pt cubes (Figure 7), and the {111}
faces were dominant in that of Pt octahedra. The surface of the
cuboctahedra layer was in between. Therefore, we can adjust
the surface structure of substrates by LB deposition using the
different shapes of Pt nanocrystals.
Conclusions
We demonstrated that monodisperse Pt nanocrystals with
various shapes including cubes, cuboctahedra, and octahedra
can be synthesized selectively by a modified polyol process.
The Ag ion plays a crucial role in controlling the shape and
surface structure of the Pt nanocrystals. The resulting particles
were deposited on the water surface to generate 2-D monolayers
by the LB technique. The Pt coverage was controlled by
changing the surface pressure, and the surface structure was
modified by using different shapes of the particles.

Pt Nanocrystals: Shape and Surface Structure
J. Phys. Chem. B, Vol. 109, No. 1, 2005 193
(5) Sun, Y.; Xia, Y. Science 2002, 298, 2176.
(6) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. Angew.
Chem., Int. Ed. 2004, 43, 3673.
The Pt nanocrystal monolayer can also be deposited on
various metal oxide substrates such as silica and alumina by
the same LB method. The resulting Pt nanocrystal/support
structures are expected to be utilized as an ideal 2-D model
catalyst as a result of their high surface area and the small/
tunable interspacing between the particles compared to the
lithography-based structures.22 The ease of density control on
the Pt layer is also beneficial for investigating the cooperative
roles of metal particles and supports in such bifunctional
catalysts. Studies on structure-dependent physical and chemical
properties of the Pt particles and LB monolayers are in progress.
(7) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648.
(8) (a) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-
Sayed, M. A. Science 1996, 272, 1924. (b) Ahmadi, T. S.; Wang, Z. L.;
Henglein, A.; El-Sayed, M. A. Chem. Mater. 1996, 8, 1161.
(9) (a) Chen, J.; Herricks, T.; Geissler, M.; Xia, Y. J. Am. Chem. Soc.
2004, 126, 10854. (b) Fu, X.; Wang, Y.; Wu, N.; Gui, L.; Tang, Y. Langmuir
2002, 18, 4619. (c) Zhao, S.-Y.; Chen, S.-H.; Wang, S.-Y.; Li, D.-G.; Ma,
H.-Y. Langmuir 2002, 18, 3315. (d) Miyazaki, A.; Nakano, Y. Langmuir
2000, 16, 7109. (e) Teranishi, T.; Kurita, R.; Miyake, M. J. Inorg.
Organomet. Polym. 2000, 10, 145.
(10) (a) Yoo, J. W.; Hathcock, D.; El-Sayed, M. A. J. Phys. Chem. A
2002, 106, 2049. (b) Balint, I.; Miyazaki, A.; Aika, K. Chem. Commun.
2002, 1044. (c) Zhu, J.; Ko´nya, Z.; Puntes, V. F.; Kiricsi, I.; Miao, C. X.;
Ager, J. W.; Alivisatos, A. P.; Somorjai, G. A. Langmuir 2003, 19, 4396.
(d) Miyazaki, A.; Balint, I.; Nakano, Y. J. Nanopart. Res. 2003, 5, 69.
(11) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343.
(12) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153.
Acknowledgment. This work was supported by the National
Science Foundation (CAREER) and the Department of Energy.
P.Y. is an Alfred P. Sloan Research Fellow. Work at the
Lawrence Berkeley National Laboratory was supported by the
Office of Science, Basic Energy Sciences, Division of Materials
Science of the U.S. Department of Energy. H.S. thanks the
Korea Science and Engineering Foundation (KOSEF) for
support under the Postdoctoral Fellowship program. We thank
the National Center for Electron Microscopy for the use of their
facilities.
(13) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis;
John Wiley & Sons: New York, 1994; Chapter 7.
(14) Petroski, J. M.; Wang, Z. L.; Green, T. C.; El-Sayed, M. A. J.
Phys. Chem. B 1998, 102, 3316.
(15) (a) Zhang, Z.; Patel, R. C.; Kothari, R.; Johnson, C. P.; Friberg, S.
E.; Aikens, P. A. J. Phys. Chem. B 2000, 104, 1176. (b) Petit, C.; Lixon,
P.; Pileni, M.-P. J. Phys. Chem. 1993, 97, 12974. (c) Linnert, T.; Mulvaney,
P.; Henglein, A.; Weller, H. J. Am. Chem. Soc. 1990, 112, 4657.
References and Notes
(16) Stru¨ber, U.; Kastner, A.; Ku¨ppers, J. Thin Solid Films 1994, 250,
101.
(1) (a) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (b) Daniel,
M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (c) Kelly, K. L.; Coronado,
E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (d) Jin, R.;
Cao, Y. C.; Hao, E.; Me´traux, G. S.; Schatz, G. C.; Mirkin, C. A. Nature
2003, 425, 487. (e) Mock, J. J.; Barbic, M.; Smith, D. R.; Schultz, D. A.;
Schultz, S. J. Chem. Phys. 2002, 116, 6755. (f) Puntes, V. F.; Krishnan, K.
M.; Alivisatos, A. P. Science 2001, 291, 2115. (g) Dumestre, F.; Chaudret,
B.; Amiens, C.; Renaud, P.; Fejes, P. Science 2004, 303, 821. (h) Somorjai,
G. A.; Borodko, Y. G. Catal. Lett. 2001, 76, 1.
(2) (a) Murphy, C. J.; Jana, N. R. AdV. Mater. 2002, 14, 80. (b) Sun,
Y.; Yin, Y.; Mayers, B. T.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14,
4736.
(3) (a) Kim, F.; Song, J. H.; Yang, P. J. Am. Chem. Soc. 2002, 124,
14316. (b) Yu, Y.-Y.; Chang, S.-S.; Lee, C.-L.; Wang, C. R. C. J. Phys.
Chem. B 1997, 101, 6661.
(17) (a) Sun, Y.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 3892. (b) Sun,
Y.; Xia, Y. Nano Lett. 2003, 3, 1569.
(18) Yang, P. Nature 2003, 425, 243.
(19) (a) Markovich, G.; Collier, C. P.; Henrichs, S. E.; Remacle, F.;
Levine, R. D.; Heath, J. R. Acc. Chem. Res. 1999, 32, 415. (b) Collier, C.
P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997,
277, 1978. (c) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B
1997, 101, 189.
(20) Kim, F.; Kwan, S.; Akana, J.; Yang, P. J. Am. Chem. Soc. 2001,
123, 4360.
(21) (a) Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R.; Sun, Y.;
Xia, Y.; Yang, P. Nano Lett. 2003, 3, 1229. (b) Whang, D.; Jin, S.; Wu,
Y.; Lieber, C. M. Nano Lett. 2003, 3, 1255.
(4) (a) Pastoriza-Sabtos, I.; Liz-Marza´n, L. M. Nano Lett. 2002, 2, 903.
(b) Chen, S.; Carroll, D. L. Nano Lett. 2002, 2, 1003.
(22) (a) Zhu, J.; Somorjai, G. A. Nano Lett. 2001, 1, 8. (b) Eppler, A.
S.; Zhu, J.; Anderson, E. A.; Somorjai, G. A. Top. Catal. 2000, 13, 33.