Site-specific growth of Au-Pd alloy horns on Au nanorods: A platform for highly sensitive monitoring of catalytic reactions by surface enhancement raman spectroscopy

Article abstract of DOI:10.1021/ja4004602

Surface-enhanced Raman scattering (SERS) is a highly sensitive probe for molecular detection. The aim of this study was to develop an efficient platform for investigating the kinetics of catalytic reactions with SERS. To achieve this, we synthesized a novel Au-Pd bimetallic nanostructure (HIF-AuNR@AuPd) through site-specific epitaxial growth of Au-Pd alloy horns as catalytic sites at the ends of Au nanorods. Using high-resolution electron microscopy and tomography, we successfully reconstructed the complex three-dimensional morphology of HIF-AuNR@AuPd and identified that the horns are bound with high-index {11l} (0.25 < l < 0.43) facets. With an electron beam probe, we visualized the distribution of surface plasmon over the HIF-AuNR@AuPd nanorods, finding that strong longitudinal surface plasmon resonance concentrated at the rod ends. This unique crystal morphology led to the coupling of high catalytic activity with a strong SERS effect at the rod ends, making HIF-AuNR@AuPd an excellent bifunctional platform for in situ monitoring of surface catalytic reactions. Using the hydrogenation of 4-nitrothiophenol as a model reaction, we demonstrated that its first-order reaction kinetics could be accurately determined from this platform. Moreover, we clearly identified the superior catalytic activity of the rod ends relative to that of the rod bodies, owing to the different SERS activities at the two positions. In comparison with other reported Au-Pd bimetallic nanostructures, HIF-AuNR@AuPd offered both higher catalytic activity and greater detection sensitivity.

Full text of DOI:10.1021/ja4004602

Article
pubs.acs.org/JACS
Site-Specific Growth of Au−Pd Alloy Horns on Au Nanorods: A
Platform for Highly Sensitive Monitoring of Catalytic Reactions by
Surface Enhancement Raman Spectroscopy
Jianfeng Huang,†,§ Yihan Zhu,†,§ Ming Lin,‡ Qingxiao Wang,⊥ Lan Zhao,⊥ Yang Yang,⊥ Ke Xin Yao,†
and Yu Han*,†
†Advanced Membranes and Porous Materials Center, Physical Sciences and Engineering Division, and ⊥Imaging and Characterization
Core Lab, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia
‡Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link,
Singapore 117602
W
S
* Web-Enhanced Feature * Supporting Information
ABSTRACT: Surface-enhanced Raman scattering (SERS) is a
highly sensitive probe for molecular detection. The aim of this study
was to develop an efficient platform for investigating the kinetics of
catalytic reactions with SERS. To achieve this, we synthesized a
novel Au−Pd bimetallic nanostructure (HIF-AuNR@AuPd)
through site-specific epitaxial growth of Au−Pd alloy horns as
catalytic sites at the ends of Au nanorods. Using high-resolution
electron microscopy and tomography, we successfully reconstructed
the complex three-dimensional morphology of HIF-AuNR@AuPd
and identified that the horns are bound with high-index {11l} (0.25 < l < 0.43) facets. With an electron beam probe, we
visualized the distribution of surface plasmon over the HIF-AuNR@AuPd nanorods, finding that strong longitudinal surface
plasmon resonance concentrated at the rod ends. This unique crystal morphology led to the coupling of high catalytic activity
with a strong SERS effect at the rod ends, making HIF-AuNR@AuPd an excellent bifunctional platform for in situ monitoring of
surface catalytic reactions. Using the hydrogenation of 4-nitrothiophenol as a model reaction, we demonstrated that its first-order
reaction kinetics could be accurately determined from this platform. Moreover, we clearly identified the superior catalytic activity
of the rod ends relative to that of the rod bodies, owing to the different SERS activities at the two positions. In comparison with
other reported Au−Pd bimetallic nanostructures, HIF-AuNR@AuPd offered both higher catalytic activity and greater detection
sensitivity.
developed, in which the performance (SERS or catalytic
activity) of the core-metal was concealed by the shell
component especially when the shell was compact and thick.3
Better results were achieved in nanoparticles with hierarchical
structures that were prepared by depositing one metal in the
form of islands on a thin shell of another metal formed on top
of an inner core (e.g., SiO2@Au@Pd-islands by Wong et al.1a
and Au@Pt@Au-islands by Schlucker et al.1b) to expose both
plasmonic and catalytic sites. The obtained materials showed
adequate catalytic and SERS-responsive functionalities and
allowed catalysis to be probed via SERS. However, the
complicated multistep preparation processes for these materials
may result in inhomogeneous particle morphologies and hinder
in-depth research on these systems as a consequence.
INTRODUCTION
■
In situ monitoring of a heterogeneous reaction on a catalyst
surface is fundamentally crucial for understanding the reaction
mechanisms and kinetics.1 Conventional methods, such as
Fourier transform infrared spectroscopy, mass spectroscopy,
and gas chromatography, suffer from slow response, low
sensitivity, or incapability to detect surface species and are
therefore not able to reveal surface reactions in a real-time
manner. In contrast, surface-enhanced Raman spectroscopy,
which takes advantage of the plasmonic resonances in metallic
nanostructures to obtain significantly enhanced Raman signals
of the adsorbed molecules, allows fast-responsive and surface-
selective detection with high sensitivity down to the single
molecule level.2
Applying SERS to in situ monitoring of catalytic reactions
requires developing an appropriate bifunctional platform that is
both plasmonically and catalytically active. To this end,
colloidal metallic nanocrystals (NCs) have been extensively
investigated due to their localized surface plasmon resonance
(SPR) effect, inherent activity for catalysis and compositional
flexibility. Some core@shell bimetallic structures were
Integrating high SERS and catalytic activity into a simple and
well-defined structure through facile synthesis, which requires a
rational design that takes particle size, shape, composition, and
nanoscale architecture into account, remains challenging.
Received: January 25, 2013
Published: May 15, 2013
© 2013 American Chemical Society
8552
dx.doi.org/10.1021/ja4004602 | J. Am. Chem. Soc. 2013, 135, 8552−8561

Journal of the American Chemical Society
Article
with a shell thickness of ∼1 and ∼5 nm (denoted as AuNR@Pd-1 nm
and AuNR@Pd-5 nm, respectively), were also synthesized. Their
synthetic procedures are presented in the Supporting Information
(SI).
Metallic NCs with high-index facets usually show high catalytic
activity owing to the high density of their low-coordinated
atomic steps and kinks.4 The synergetic effect between two
metals often leads to enhanced catalytic activity for alloy NCs
in comparison with their monometallic counterparts.5 On the
other hand, SERS activity is strongly dependent on the
composition and shape of the NCs.6 For example, gold and
silver are the best candidates for SERS among various metals,
and Au nanorods (AuNRs) with proper aspect ratios exhibit
localized SPR concentrated on the rod ends, where the SERS
effect is exceptionally strong.7
Characterizations. Extinction spectra were taken on a Varian Cary
5000 UV−vis−NIR spectrophotometer. X-ray photoelectron spec-
troscopy (XPS) spectra were obtained using the Axis Ultra DLD
system with Al Kα X-ray radiation (1486.6 eV). Scanning electron
microscopy (SEM) was run on a high-resolution FEI Magellan 400L
scanning electron microscope operated at 2 kV. Low-magnification
transmission electron microscopy (TEM) images were acquired on a
FEI-Tecnai T12 microscope operated at 120 kV. Ultramicrotomy was
employed using a Leica EM UC6 ultramicrotome. Aberration-
corrected high-angle-annular-dark-field (HAADF) scanning (S)TEM
imaging, energy dispersive X-ray spectroscopy (EDX) mapping, and
electron energy loss spectroscopy (EELS) mapping were carried out
on a cubed FEI Titan G2 electron microscope equipped with both a
probe-corrector and a monochromator operated at 300 kV. The probe
convergence angle was 24.9 mrad, and the inner detector angle was 76
mrad. A probe size of 0.8 Å and energy-resolution of 0.17 eV as
measured from the full-width-at-half-maximum (fwhm) of the zero-
loss peak (ZLP) were achieved. High-resolution TEM imaging and
STEM tomography were carried out on a FEI-Titan ST electron
microscope operated at 300 kV. An electron tomography tilt series
from −75° to 75° at 1° intervals was first aligned and then
reconstructed to a 3-D volume using the SIRT function in the FEI
Inspect 3D software. The 3-D volume rendering, density segmentation
and isosurface construction were then achieved by the Avizo software.
The Raman spectra were recorded on a Horiba Jobin Yvon LabRAM
HR-800 spectrophotometer coupled to an Olympus confocal micro-
scope (BX41) with a ×50 objective (NA = 0.50) in the backscattering
configuration with 532 and 785 nm laser excitations. Optical density
filters and a constant acquisition time of 10 s were used for the
measurements.
Colloidal SERS Measurements. AuNPs, HIF-AuNP@Pd, AuPd-
Oct, AuNRs, AuNR@Pd-1 nm, AuNR@Pd-5 nm, and HIF-AuNR@
AuPd were each incubated with an ethanol solution of 4-NTP (10−4
M) overnight to form colloidal suspensions with roughly identical
particle concentrations. Enhancement factors (EFs) were calculated on
the basis of the Raman spectrum of a pure 4-NTP solution (0.1 M)
collected under the same experimental conditions (see SI for
calculation details).
In Situ Monitoring of the Hydrogenation of 4-NTP with
SERS. The reactions were performed in a vertically oriented reaction
chamber equipped with the Raman spectroscope under 785 nm laser
excitation (laser power 2 mW at sample). The planar detection
platforms mounted in the chamber were made by drop-casting a film
of desired NCs on Si substrates (5 mm ×5 mm). The platforms were
immersed into an ethanol solution of 4-NTP (10−2 M) for 12 h,
followed by repeated rinsing with ethanol. These treatments yielded a
monolayer of 4-NTP chemisorbed on the platform surface, as
evidenced by the disappearance of the characteristic S−H stretching
Raman band at 2550 cm−1.12 The reactions were performed at room
temperature with 10 sccm of H2/N2 flow continuously introduced to
the chamber. Successive SERS spectra were collected during the
reaction until there were no noticeable changes between adjacent
spectra.
Here, we report the design and synthesis of a novel Au−Pd
bimetallic nanostructure through selective growth of Au−Pd
alloy horns on the ends of single-crystal AuNRs (denoted as
HIF-AuNR@AuPd). We intentionally grew Au−Pd alloy rather
than monometallic Pd on AuNRs for three reasons: (i) the
Au−Pd alloy is more active than Pd alone in a large variety of
catalytic reactions;8 (ii) Au−Pd bimetallic NCs prepared via the
seed-mediated coreduction9 of metal ions tend to show exotic
crystal morphologies with high-index facets (HIF) and thus
superior catalytic activity;4g and (iii) in comparison with Pd, the
Au−Pd alloy may have a smaller damping effect on the SPR of
AuNRs, which allows them to maintain their SERS function.10
Using high-resolution electron microscopy (HREM) and
tomography, we successfully reconstructed the three-dimen-
sional (3-D) morphology of the obtained material. The results
showed that the Au−Pd alloy horns grew along the ⟨111⟩
direction with exposed high-index (approximately {11l}, 0.25 <
l < 0.43) side facets. Electron energy loss spectroscopy mapping
conducted with HREM indicated the presence of intensive SPR
on the two ends of the nanorods exactly where the horns were
located. Such a configuration unites highly active catalytic sites
with strong SERS sites in a single entity and is ideal for in situ
monitoring of catalytic reactions by SERS. We successfully
demonstrated this concept with a model reaction, i.e., the Pd-
catalyzed hydrogenation of 4-nitrothiophenol (4-NTP) to 4-
aminothiophenol (4-ATP), and found that HIF-AuNR@AuPd
was superior to other Au−Pd nanostructures in both catalytic
activity and SERS detection sensitivity. It is worth noting that
such a bifunctional platform offers a unique opportunity to
investigate the intrinsic reaction kinetics on the catalyst surface
by excluding the influence of adsorption/desorption of
reactants and products. In the model reaction, for example,
the hydrogenation of a monolayer of 4-NTP molecules on the
HIF-AuNR@AuPd surface was found to follow first-order
kinetics. Interestingly, the catalytic activity of the HIF-AuNR@
AuPd rod end was explicitly distinguished from that of the rod
body, owing to the very different SERS activities at the two
positions.
EXPERIMENTAL SECTION
■
Materials Synthesis. To synthesize HIF-AuNR@AuPd, AuNRs
were first prepared using the well-established seed-mediated method.11
Then, HAuCl4 (0.01 M, 0.03 mL), Na2PdCl4 (0.01 M, 0.02 mL), and
ascorbic acid (0.1 M, 0.2 mL) were sequentially added into a purified
AuNR solution (2 mL, ∼0.4 nM). The obtained mixture was gently
shaken for 10 s and then left under static conditions at room
temperature for 3 h. The resulting hydrosol was washed by repeated
centrifugation (5000 rpm, 10 min) and redispersed in H2O three
times. Several other nanostructures, including Au nanopolyhedron
(AuNP), AuNP coated with a high-index {12 5 3} faceted Pd shell
(HIF-AuNP@Pd), AuPd alloy nanooctahedron with {111} facets
exposed (AuPd-Oct), Pd-coated Au nanocrystals with a nearly
spherical morphology (Spherical Au@Pd), and Pd-coated AuNR
RESULTS AND DISCUSSION
■
We chose single-crystalline AuNRs with a moderate aspect ratio
(∼4) as seeds to prepare HIF-AuNR@AuPd because at this
ratio, the longitudinal SPR are localized on the rod ends
without antinodes formed in the body, which was important for
our platform design, as discussed later. As shown in Figure 1a,
the synthesized AuNRs had uniform sizes and aspect ratios
(length = (78.6
6.7) nm, width = (17.9
1.6) nm, and
aspect ratio = 4.3
0.5) (see Figure S1 for the statistics).
Traditional morphological models describe a single-crystalline
8553
dx.doi.org/10.1021/ja4004602 | J. Am. Chem. Soc. 2013, 135, 8552−8561

Journal of the American Chemical Society
Article
Figure 1. (a) TEM image of the AuNR seeds. (b) TEM image and (c) SEM image of the HIF-AuNR@AuPd nanorods. (d) HAADF-STEM images
of HIF-AuNR@AuPd nanorods taken along the [100], [110], and [001] axes (the first column, from the top down), and the corresponding
elemental mappings for Au (the second column) and Pd (the third column); the last column shows combined mappings, in which the simultaneous
presence of Au and Pd appears yellowish. (e) HRTEM image of HIF-AuNR@AuPd taken at a rod end along the [110] axis. The inset is a fast
Fourier transform diffractogram of the marked area, showing the perfect epitaxial growth of the horn from the rod body.
Figure 2. (a,b) SAED patterns (insets) and high-resolution HAADF-STEM images of a HIF-AuNR@AuPd nanorod taken along (a) the [100] and
(b) the [110] axes. The corresponding low-magnification images are also provided as insets, in which the marked regions correspond to the high-
resolution images. (c) SAED pattern (upper) and HAADF-STEM image (lower) of a HIF-AuNR@AuPd nanorod taken along the [001] axis (i.e.,
along the rod axis). The scale bars represent 20 nm in (a,b) and 5 nm in (c).
AuNR as an octagonal prism enclosed by four {100} and four
{110} side facets.13 However, two recent independent studies14
demonstrated that contrary to this long-held model, AuNRs are
actually enclosed by eight symmetry-equivalent high-index side
facets of the {5 12 0} family. One of these studies further
determined by using electron tomography that the rod ends
terminated in a pyramid with {130} facets, and each pyramid
was connected to the rod sides by four small {5 12 0}
“bridging” facets (Figure S2).14a Here, we investigated the
morphology of AuNR seeds by simply using ultramicrotomy for
HRTEM sample preparation, which allowed direct imaging of
the octagonal cross section of the AuNRs to identify their side
facets (Figure S3). Our result accorded with the new model.
The design for HIF-AuNR@AuPd originated from the
attempt to prepare a plasmonic and catalytic bimetallic
nanostructure by depositing Pd onto AuNR seeds. We found
that, provided with sufficient Pd precursors, a continuous Pd
shell can be formed on individual AuNRs (Figure S4a), whereas
a small amount of precursor only gives rise to discrete Pd
particles that are selectively anchored on the two ends of
AuNRs, specifically at the rod corners where two side facets and
one bridging facet meet (Figure S4b). HRTEM showed that
these Pd particles had the same crystalline orientations as
AuNR had, suggesting an epitaxial growth mode (Figure S4c−
e). The observed preferential deposition of Pd on AuNR ends
is thermodynamically sensible, as low-coordinated sites (e.g.,
crystal corners) are favorable for heterogeneous nucleation.
Inspired by this result, we supposed that it would be possible to
grow Au−Pd alloy site-specifically on the ends of AuNR
through the codeposition of Au and Pd.
As revealed in the TEM image, the as-prepared HIF-AuNR@
AuPd NCs remained rod-shaped without significant changes in
the aspect ratio (despite being slightly fatter) relative to the
AuNR seeds, but they featured newly grown “horns” on the rod
8554
dx.doi.org/10.1021/ja4004602 | J. Am. Chem. Soc. 2013, 135, 8552−8561

Journal of the American Chemical Society
Article
Figure 3. Surface-rendered visualization of the HIF-AuNR@AuPd nanorod morphology reconstructed by HAADF-STEM tomography, viewed
along (a) the [100], (c) the [110], and (e) the [001] axes. The corresponding schematic morphology models are given for reference (b, d, and f).
Note that the viewing angle of (e) and (f) deviates slightly from the exact [001] axis to show the surface features more clearly, and a top-end pyramid
is labeled with blue color in (f).
ends (Figures 1b andS5). The SEM image further indicated
that there were four horns at each end of each nanorod (Figure
1c). Selected-area electron diffraction (SAED) demonstrated
that each individual HIF-AuNR@AuPd nanorod was a single
crystal with its rod axis parallel to the [001] zone axis, and the
HRTEM image accordingly showed perfect epitaxial growth of
the horns from the AuNR (Figure 1e). HAADF-STEM images
of the [100] and [110] projections (Figure 2a,b), which were
taken from a same nanorod by tilting the specimen along its
rod axis by 45°, confirmed the single-crystal nature of HIF-
AuNR@AuPd. They also showed that the “horn” feature was
more remarkable in the [110] projection. To clarify the 3-D
structure of HIF-AuNR@AuPd, it is crucial to acquire images
along the rod axis, i.e. along the [001] direction, which requires
a specimen with “standing” nanorods. To achieve this, we
employed ultramicrotomy for specimen preparation. Figure 2c
shows a HAADF-STEM image of a HIF-AuNR@AuPd
nanorod taken along the rod axis, in which four horns at the
corners can be easily distinguished from the rod body in the
center region by the image contrast arising from different
specimen thicknesses. The center region with brighter contrast
appears as a square with the edges normal to the four ⟨110⟩
directions, as determined by SAED (Figure 2c), suggesting that
the rod body of HIF-AuNR@AuPd is bound by four {110}
lateral low-index facets (LIFs). Furthermore, by analyzing the
HRTEM images of different orientations, we identified that the
horns grow along the ⟨111⟩ directions (see Figure S6 for
details).
The information gained from the SAEDs and 2-D HREM
images of different orientations is essentially sufficient for
delineating a basic 3-D structural model of a HIF-AuNR@
AuPd nanorod. Advancing a step, we employed HAADF-STEM
tomography to discern more details, such as the top-end fine
structures of the nanorod and the side facets of the horns. In
comparison with classic bright-field TEM tomography,
HAADF-STEM tomography gives much reduced diffraction
contrast (unwanted in tomography) along with the enhanced
Z-contrast and is therefore particularly advantageous for
reconstructing crystalline metallic nanostructures. Specifically,
a series of STEM images was continuously acquired by tilting
the specimen over a range of −75° to +75° with regular
intervals of 1°. The obtained image series was aligned and
processed to reconstruct a “volume” that was visualized through
surface rendering or as a set of slices to show the local
structures (Figures 3 and 4a and Movies 1 and 2). The
reconstructed 3-D tomographic images matched the 2-D
(S)TEM images of the same projections well (Figures 2 and
3) and clearly verified that a HIF-AuNR@AuPd nanorod has
four {110} side facets enclosing the rod body and four horns at
each rod end protruding along the ⟨111⟩ directions. More
importantly, tomography helps to identify local structural
features that cannot be easily recognized from 2-D images. For
example, the reconstructed 3-D image showed that along the
[00 1] directions (Figure 3e), the rod end terminated in a
flattened pyramid with four equivalent side faces that were
parallel to the [100] and [010] directions. Hence, these faces
became “edge-on” when the nanorod was projected along a
[100] zone axis and their Miller indices were readily
determined from the HR-STEM image. Figure 4b clearly
shows that the side surface of a top-end pyramid was
terminated by a {0 5 12} facet, which can be visualized as a
combination of two {001} terraces of three atomic widths,
three {001} terraces of two atomic widths, and four monatomic
{010} step risers. Moreover, the reconstructed image shows
that the “horns” are nearly tetrahedral with three exposed side
faces extending smoothly from the horn vertex to the body and
top-ends of the nanorod. Slicing at different positions along a
“horn” (the slices are perpendicular to the [111] axis) generates
a set of trigonal cross sections with each angle close to 60°,
suggesting that the three side faces of each “horn” are
topologically equivalent (Figure 4a). Because the [111] axis
of a cubic structure coincides with its threefold symmetry axis,
the three side faces are also crystallographically equivalent.
Since one of these three side faces is edge-on when the nanorod
is projected along a [110] zone axis, its index can be
determined by measuring the angle between its normal and a
known direction, e.g. the [1−10] direction. The statistical
results of horns from 50 rods show that this angle ranges from
10° to 17°, corresponding to the indices of {11l } (0.25 < l <
0.43). As an example, Figure 4c shows a HR-STEM image of a
“horn” taken along the [110] axis, in which the edge-on side
face is indexed as {441}. These results demonstrate the
presence of adequate high-index facets at the rod ends of HIF-
AuNR@AuPd. The complete model of HIF-AuNR@AuPd is
presented and compared with that of AuNR in Figure S2.
8555
dx.doi.org/10.1021/ja4004602 | J. Am. Chem. Soc. 2013, 135, 8552−8561

Journal of the American Chemical Society
Article
Figure 4. (a) Three slices of the “horn” along the [111] axis with an interval of 1 nm extracted from the HAADF-STEM tomography-reconstructed
volume. (b) HR-STEM image of a HIF-AuNR@AuPd nanorod taken along the [100] axis and the schematic atom arrangement, showing that the
top-end pyramid has {0 5 12} side facets. (c) HR-STEM image of a HIF-AuNR@AuPd nanorod taken along the [110] axis and the schematic atom
arrangement, showing that the horn has a {441} side facet. Low-magnification images are presented in the insets to show the regions (marked) from
where the high-resolution images were taken.
We combined EDX elemental analysis with STEM to
investigate the distribution of Pd and Au within HIF-AuNR@
AuPd. The EDX mappings of different orientations all
demonstrate that the horns are comprised of uniformly
distributed Pd and Au (Figure 1d). These results along with
the uniform contrast of the high-resolution HAADF-STEM
images (Figure 2a,b) indicate the alloy nature of the horns. The
EDX mappings also show that the four {110} lateral faces of
the nanorod are terminated with thin layers of Au−Pd alloy
(Figure 1d). The Pd/Au atomic ratio on the surface of HIF-
AuNR@AuPd is 53:47, as determined by XPS.
To investigate the influence of the growth of Au−Pd horns
on the SPR of AuNR, we first compared the UV−vis spectra of
different materials (Figure 5a). The original AuNRs exhibited
two extinction bands centered at 513 and 837 nm,
corresponding to a weak transverse surface plasmon (TSP)
resonance and a strong longitudinal surface plasmon (LSP)
resonance, respectively. We found that, in accordance with
literature,15 coating AuNRs with a thin shell (∼1 nm) of Pd
dramatically weakened and broadened the extinction bands and
that this effect became more severe with an increase in the Pd
shell thickness. In contrast, modest damping and a slight red-
shift of SPR was observed for HIF-AuNR@AuPd. This can be
explained by the fact that the dielectric function of Au−Pd alloy
is a linear combination of the dielectric functions of Au and Pd,
and it therefore has a smaller imaginary part throughout the
visible and near-infrared regime to render a weaker damping
effect, as compared with the case of pure Pd.
energy resolution, we employed a Cs-corrected and mono-
chromated electron beam (a sub-angstrom probe with an 0.17
eV energy spread) to excite all the SP modes in the
nanostructures.18 The spatially resolved EELS spectra clearly
show that in AuNR with an aspect ratio of ∼4, there is an
intense LSP excitation at 1.8 eV concentrated near the rod ends
and several overlapped weak SP excitations at ∼2.5 eV, which
are uniformly distributed near the rod body and possibly
associated with different transverse modes (Figure 5b). In HIF-
AuNR@AuPd, the two types of SP modes both experience a
red-shift of 0.3−0.4 eV, while the LSP mode remains dominant
(Figure 5b). We directly visualized the distributions of the two
types of different SP modes in AuNR and HIF-AuNR@AuPd
by EELS mapping with an energy filter at their respective
excitation positions with a 0.2 eV window. As shown in Figure
5c−e, typical dipolar SPR patterns, which show highly localized
strong fields at the two rod ends, were observed in the “bright”
LSP mode in both AuNR and HIF-AuNR@AuPd. In the latter
case, EELS mappings were performed along two major zone
axes (i.e., [100] and [110]) and the results were consistent
(Figure 5d,e). Also, the weak fields of those TSP modes
exhibited similar uniform distributions in both kinds of
nanorods (Figure 5c−e). These results confirmed that the
growth of Au−Pd alloy horns did not cause marked variations
in the intensity or spatial distribution of various SP modes of
the AuNRs. Since no “dark” SP modes with considerable field
intensities were observed, the SERS activity was mostly
contributed by the dipolar LSP mode. We thus concluded
that a sensitive SERS detection can be achieved in HIF-
AuNR@AuPd by exciting the LSP mode to render strong field
enhancement at the two rod ends.
UV−vis spectroscopy uses a plane-wave incidence that only
excites the dipole-allowed SP modes (“bright” modes).16
However, the dipole-forbidden multipolar SP modes (“dark”
modes) may also contribute to the activity of a planar SERS
platform, owing to the symmetry-breaking of the charge density
distributions by interparticle or substrate-mediated SP
coupling.17 To investigate various SP-associated near-fields
and their distributions, which requires both high spatial and
The present discussion demonstrated that for an individual
HIF-AuNR@AuPd nanorod, much stronger SP fields exist at
the rod ends than at the rod body. However, the nanorods
agglomerate when used as a planar SERS platform. In that case,
whether or not the site-dependent SERS effect still holds when
8556
dx.doi.org/10.1021/ja4004602 | J. Am. Chem. Soc. 2013, 135, 8552−8561

Journal of the American Chemical Society
Article
Figure 5. (a) UV−vis extinction spectra of AuNR (red), HIF-AuNR@AuPd (black), AuNR@Pd-1 nm (green), and AuNR@Pd-5 nm (blue). (b)
EELS spectra of AuNR and HIF-AuNR@AuPd sampled from different positions (refer to (c−e), where filled and empty circles in different colors
labeled in the STEM images indicate the origin of each EELS spectrum with the same symbol). (c−e) STEM images (left), LSP mappings (middle),
and TSP mappings (right) of (c) AuNR and (d,e) HIF-AuNR@AuPd with different orientations: (d) [100] and (e) [110]. The energy window for
mapping is 0.2 eV and the filtered energy positions of LSP/TSP modes for AuNR and HIF-AuNR@AuPd are 1.8/2.5 and 1.5/2.2 eV, respectively.
Scale bars in (c−e) signify 20 nm.
“hot spots”, which could lead to further SERS enhancement, are
formed at particle junctions should be considered.19 It was
reported that under the circumstance of particle agglomeration,
the SERS enhancement mainly resulted from the dipolar
component of the hybridized plasmonic field.17a A study of
dipolar plasmonic coupling between AuNRs with different
geometries (e.g., end-to-end, side-by-side, L-type, or T-type)
further indicated that the couplings between LSP modes were
predominant over LSP-TSP or TSP-TSP coupling20 and the
most intensive field enhancement was still localized on the rod
ends that decayed rapidly with distance from the crystal surface.
We therefore deduced that when HIF-AuNR@AuPd is used as
a planar platform, the signal enhancement is primarily a
contribution by the rod ends.
The unique morphological and compositional design of HIF-
AuNR@AuPd makes it useful for in situ monitoring a catalytic
reaction via SERS. In a proof-of-concept experiment, we
investigated the Pd-catalyzed hydrogenation of 4-nitrothiophe-
nol (4-NTP) to 4-aminothilphenol (4-ATP) at room temper-
ature (inset of Figure 6a). Specifically, 4-NTP molecules first
formed a monomolecular layer on the surface of HIF-AuNR@
AuPd through chemisorption,12 where they were on-site
reduced to 4-ATP molecules by Pd upon the subsequent
introduction of a hydrogen flow. We note that Au has shown to
be catalytically inert for this reaction. The SERS spectrum of
the 4-NTP monolayer exhibited characteristic bands at 854,
1076, 1334, and 1571 cm−1 that were respectively assigned to
C−H wagging, C−S stretching, O−N−O stretching, and the
phenyl-ring mode (Figure 6a).12a The intensities of the R-NO2-
associated bands decreased with the conversion of 4-NTP to 4-
ATP, and concomitantly two characteristic bands of 4-ATP at
1486 and 1587 cm−1 emerged (Figure 6a).21 Since hydrogen
dissociative activation on Pd is a very fast process even at room
temperature,22 the subsequent reduction of 4-NTP by the
activated hydrogen species should be a kinetically relevant step.
Hence, the reaction kinetics can be quantified from the
intensity evolution of the SERS bands of 4-NTP, which
corresponds to its concentration variation.
We first evaluated the SERS activities of different
nanostructures in colloidal solution (Figure S7; see Exper-
8557
dx.doi.org/10.1021/ja4004602 | J. Am. Chem. Soc. 2013, 135, 8552−8561

Journal of the American Chemical Society
Article
Figure 6. (a) SERS spectra of the hydrogenation of chemisorbed 4-NTP to 4-ATP over a Pd catalyst at the initial stage with reactant 4-NTP
(upper), the final stage with product 4-ATP (bottom) and an intermediate stage with the coexistence of the two compounds (middle). (b)
Successive SERS spectra of the reduction of 4-NTP by H2 collected on a planar platform made of HIF-AuNR@AuPd NCs. (c) A plot of the
logarithm of the Raman intensity at 1334 cm−1 versus reaction time (ln(It/I0) ∼ t plot) for the catalytic hydrogenation of 4-NTP on HIF-AuNR@
AuPd. (d) Normalized rate constants and SERS EF values of AuNR, AuNR@Pd-1 nm, HIF-AuNP@Pd, HIF-AuNR@AuPd, and AuPd-Oct.
imental Section for the descriptions of various nanostructures
and SERS measurement details). The excitation wavelength for
each material was chosen based on its extinction spectra. The
results show that AuNR exhibits ∼40-fold higher SERS activity
than AuNP exhibits (enhancement factor (EF): 5.7 × 105 vs.
1.5 × 104). In accordance with the conclusion drawn earlier
from the extinction spectra (Figure 5a) that coating Au NCs
with Pd severely suppresses the SERS activity, the calculated EF
values for HIF-AuNP@Pd, AuNR@Pd-1 nm, and AuNR@Pd-5
nm were 3.6 × 103, 2.4 × 104, and 4.5 × 103, respectively. The
EF of AuPd-Oct was determined to be 5.8 × 103. Among all the
tested bimetallic nanostructures, HIF-AuNR@AuPd had the
highest EF of 2.5 × 105 (Figure 6d). These results demonstrate
that for the preparation of a bifunctional nanostructure,
incorporating AuPd alloy instead of pure Pd as a catalytic
component can effectively mitigate the plasmonic damping in
Au. As addressed earlier, when used in a solid form for reaction
monitoring, HIF-AuNR@AuPd would provide additional
sensitivity due to the “hot spot” effect. We attempted to
evaluate this effect and the results showed that the “hot spot”
involving SERS activity was approximately 1 order of
magnitude higher than that of a colloidal solution, giving an
EF of 1.6 × 106.
was proportional to the surface coverage percentage, θ, of 4-
NTP (i.e., θ[4‑NTP](t) = It/I0, where I0 refers to the initial
intensity at full coverage). Approximating the average
coordination number of Pd/Au-4-NTP to be 1 for simplicity,
the equation of turnover frequency (TOF) normalized to the
fraction of Pd active sites at the catalyst surface, which was
derived from the XPS Au/Pd atomic ratio, can be expressed as
∂θ[4‐NTP](t)
∂t
1
η
TOF (s−1) = −
= k[H2]mθ[4‐NTP](t)n
(1)
where m, n, and η are the reaction order of [H2] and [4-NTP]
and the fraction of Pd, respectively. Since hydrogen activation is
facilitated on Pd and the diffusion limitation is negligible owing
to a large linear velocity of hydrogen flow, we expected the
reaction order of [H2] to be zero, which was confirmed by the
constant reaction rates under different hydrogen partial
pressures (>0.5 bar). Considering m = 0, the rate eq 1 can
be simplified as follows:
∂θ[4‐NTP](t)
∂t
1
η
TOF (s−1) = −
= kθ[4‐NTP](t)n
(2)
⎛
⎞
∂(I /I )
1
η ⎠
t
0
⇒ln −
= n ln(I /I ) + ln k
⎜
⎟
t
0
Owing to its high SERS activity, the evolution of 4-NTP to 4-
ATP over HIF-AuNR@AuPd was explicitly reflected in a series
of in situ Raman spectra collected during the reaction (Figure
6b). Since the chemisorbed 4-NTP molecules formed a
monolayer on the surface of the catalysts, the integrated
intensity of the strongest band at 1334 cm−1 (denoted as It)
⎝
∂t
(3)
Hence, both the reaction order of 4-NTP and the rate constant,
k, can be calculated on the basis of the (It/I0) evolution
diagram. A typical diagram (Figure 6b) shows an initial delay of
∼60 s before the reduction becomes evident. Such an induction
8558
dx.doi.org/10.1021/ja4004602 | J. Am. Chem. Soc. 2013, 135, 8552−8561

Journal of the American Chemical Society
Article
period could result from the filling-up of hydrogen in the
reaction chamber and/or catalyst activation by reducing the
pre-oxidized surface of Pd.23 A linear regression analysis of the
ln(−(∂(It/I0))/(∂t)(1/η)) versus ln(It/I0) plot gives n = 1.07
0.02, suggesting first-order kinetics (Figure S8). Hence, eq 3
becomes
of 4-NTP taking place at the rod ends was first captured by the
rapidly changed SERS spectra, to which the signals from the
rod body made negligible contributions due to the much
weaker enhancement effect of the TSP. When the 4-NTP
molecules at the rod ends were consumed, the slower
conversion of 4-NTP at the rod body became visible. Thus,
the catalytic activities at different positions of HIF-AuNR@
AuPd were clearly distinguished and accurately determined by
utilizing the anisotropic distribution of SPRs to achieve site-
dependent field enhancements. It should be noted that the
precondition for discriminating two kinds of catalytic sites on
HIF-AuNR@AuPd is that the rod ends and the rod body have
remarkable differences in both catalytic activity and plasmonic
activity. As a negative control, we prepared another
nanostructure that consists of Pd-coated Au nanocrystals with
nearly spherical morphologies (spherical Au@Pd)27 and
investigated its reaction kinetics using the in situ SERS method
described above. Because this structure has a nearly isotropic
SP field distribution and no remarkably distinct catalytically
active sites, only one reaction rate constant, k, was observed
(Figure S9). Actually, the nanostructures that we discussed
earlier, i.e. AuNR@Pd-1 nm and HIF-AuNP@Pd, can also be
regarded as “controls” in this sense. In these structures, the
exposed facets are approximately equivalent or one type of
crystal facets are predominant in term of surface area.
Accordingly, only one rate constant can be explicitly
determined for each of them despite their anisotropic SP
field distributions (Figure S9). Moreover, we repeated the
kinetic study using another two HIF-AuNR@AuPd platforms,
and found that the reaction rate constants of the HIFed sites
determined from three independent experiments accord closely
with each other (21.66 × 10−3, 18.11 × 10−3, and 19.12 × 10−3
s−1, respectively), despite the different initial absolute SERS
intensities (Figure S10). These results clearly demonstrate that
the kinetic parameters determined with this method are
intrinsic and unaffected by the laser probe position or hot
spot effect, and provide strong evidence for the reliability of our
conclusions on the reaction kinetics.
Finally, we would clarify three additional points regarding the
catalyst assessment method that we used in this study: (i) For
the first-order kinetics, the reaction rate constant k can actually
be determined from the absolute SERS intensities (It) as the
slope of the ln(It) ∼ t linear relationship. However, we prefer to
use relative intensities (It/I0), which are associated with the
surface coverage percentages of the reactant and thus in
principle allow the determination of TOF in addition to the
rate constant. (ii) For the HIF-AuNR@AuPd that contains two
kinds of catalytic active sites with different SERS activities, the
two k values can both be directly deduced from the slopes of
the two linear segments in the ln(It/I0) ∼ t plot. To determine
the TOF of the rod body, however, one should redefine the
surface coverage using a different initial SERS intensity (I′0),
which can be derived by extrapolating the second linear
segment to get the y-intercept. (iii) Due to the much stronger
enhancement at the rod ends, the SERS intensities used for
calculating the rate constant of the rod body (i.e., the second k)
inevitably involve the contribution from the residual 4-NTP
molecules at the rod ends despite their very low concentration.
This leads to a lower accuracy in k determination for the rod
body than for the rod ends.
ln(It/I0) = − kηt
(4)
A reaction rate constant of k = 21.66 × 10−3 s−1 can thus be
readily derived from the linearly fitted slope of the ln(It/I0) ∼ t
plot (Figure 6c). It is noteworthy that the validity of our
method is based on two prerequisites: (i) the SERS spectra
series are collected from exactly the same position, which was
satisfied by carefully operating a confocal Raman spectroscope;
and (ii) 4-NTP molecules form a uniform monolayer on the
metallic nanocrystal surface, which is well characterized and
generally accepted.12 It was reported that 4-NTP or 4-ATP
could undergo SPR-induced photocatalytic dimerization to
form dimercaptoazobenzene (DMAB) on silver and gold
surfaces under specific conditions.21,24 In our experiments,
however, we did not observe the formation of DMAB during
the reaction as indicated by the SERS spectra (Figure 6a). This
was possibly due to the different excitation conditions that we
used (785 nm laser, 2 mW power). Moreover, we found that
without introducing hydrogen, the SERS spectrum of 4-NTP
did not change over time. We therefore concluded that no
DMAB molecules were involved in the hydrogenation reaction
of 4-NTP and our above-discussed kinetic study was valid.
Using the same method, we also determined the rate
constants of some other metallic nanostructures for the
hydrogenation of 4-NTP (Figures 6d and S9). The results
show that AuNR was not able to catalyze this reaction. This is
consistent with previous studies,1b,24c,25 which showed that Au
NCs with sizes larger than 10 nm were catalytically inert for this
reaction even using stronger reducing agent sodium borohy-
dride. AuNR@Pd-1 nm was active because of the Pd shell,
giving a k value of (5.07 0.69) × 10−3 s−1; with a similar
core−shell structure, HIF-AuNP@Pd was more active [k =
(10.88 1.37) × 10−3 s−1] than AuNR@Pd-1 nm, likely due to
its high-index surfaces. Overall, HIF-ANR@AuPd exhibited the
highest catalytic activity among all the tested materials. In
addition to the high-index facets, the Au−Pd alloy nature of the
surface of HIF-ANR@AuPd is another reason for its superior
activity, because the 4d state of Pd is remarkably populated
through Pd4d−Au5d hybridization upon alloying,26 and this
favors the electrophilic hydrogenation of 4-NTP. AuPd-Oct
with alloyed (Au/Pd ≈ 1:1 in molar) but low-index {111}
surfaces was much less active [k = (2.74 0.32) × 10−3 s−1]
than HIF-ANR@AuPd, further illustrating the crystal-facet-
dependent catalytic activities.
It is very interesting to note that in the ln(It/I0) ∼ t plot of
HIF-AuNR@AuPd (Figure 6c), there is an inflection point at t
≈ 360 s, after which the reaction apparently becomes slower (k
= 4.23 × 10−3 s−1). This phenomenon implies the coexistence
of two kinds of catalytic sites with different activities in HIF-
AuNR@AuPd. According to its 3-D structure described earlier,
the high-index faceted horns at the rod ends are supposed to
have higher catalytic activity than the rod body that exposes
low-index {110} facets. More interestingly, the Raman signals
of the molecules residing on the rod ends are significantly
amplified by the LSP resonance, while the signals of the
molecules adsorbed by the rod body are only slightly enhanced
by the TSP under off-resonance conditions. The fast conversion
8559
dx.doi.org/10.1021/ja4004602 | J. Am. Chem. Soc. 2013, 135, 8552−8561

Journal of the American Chemical Society
Article
(4) (a) Tian, N.; Zhou, Z.-Y.; Sun, S. G.; Ding, Y.; Wang, Z. L. Science
2007, 316, 732. (b) Lim, B.; Jiang, M. j.; Camargo, P. H. C.; Cho, E.
C.; Tao, J.; Lu, X. M.; Zhu, Y. M.; Xia, Y. N. Science 2009, 324, 1302.
(c) Jin, M. S.; Zhang, H.; Xie, Z. X.; Xia, Y. N. Angew. Chem., Int. Ed.
2011, 50, 7850. (d) Quan, Z.; Wang, Y.; Fang, J. Acc. Chem. Res. 2013,
46, 191. (e) Li, C.; Sato, R.; Kanehara, M.; Zeng, H.; Bando, Y.;
Teranishi, T. Angew. Chem., Int. Ed. 2009, 48, 6883. (f) Yang, C. W.;
Chanda, K.; Lin, P. H.; Wang, Y. N.; Liao, C. W.; Huang, M. H. J. Am.
Chem. Soc. 2011, 133, 19993. (g) Kim, D.; Lee, Y. W.; Lee, S. B.; Han,
S. W. Angew. Chem., Int. Ed. 2012, 51, 159. (h) Zhang, J.; Langille, M.
R.; Personick, M. L.; Zhang, K.; Li, S. Y.; Mirkin, C. A. J. Am. Chem.
Soc. 2010, 132, 14012. (i) Yu, Y.; Zhang, Q.; Liu, B.; Lee, J. Y. J. Am.
Chem. Soc. 2010, 132, 18258. (j) Wang, F.; Li, C.; Sun, L. D.; Wu, H.;
Ming, T.; Wang, J. F.; Yu, J. C.; Yan, C. H. J. Am. Chem. Soc. 2011,
133, 1106.
(5) (a) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross,
P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493.
(b) Ferrando, R.; Jellinek, J.; Johnston, R. L. Chem. Rev. 2008, 108,
845. (c) Norskov, J. K.; Bligaard, T.; Rossmeisl, J.; Christensen, C. H.
Nat. Chem. 2009, 1, 37. (d) Liu, X. W.; Liu, X. G. Angew. Chem., Int.
Ed. 2012, 51, 3311.
(6) (a) Xia, X.; Zeng, J.; McDearmon, B.; Zheng, Y.; Li, Q.; Xia, Y.
Angew. Chem., Int. Ed. 2011, 50, 12542. (b) Hong, J. W.; Lee, S. U.;
Lee, Y. W.; Han, S. W. J. Am. Chem. Soc. 2012, 134, 4565. (c) Lee, Y.
W.; Kim, M.; Kang, S. W.; Han, S. W. Angew. Chem., Int. Ed. 2011, 50,
3466. (d) Kneipp, K.; Kneipp, H.; Kneipp, J. Acc. Chem. Res. 2006, 39,
443. (e) Huang, J. F.; Vongehr, S.; Tang, S. C.; Lu, H. M.; Shen, J. C.;
Meng, X. K. Langmuir 2009, 25, 11890.
(7) (a) Nicoletti, O.; Wubs, M.; Mortensen, N. A.; Sigle, W.; van
Aken, P. A.; Midgley, P. A. Opt. Exp. 2011, 19, 15371. (b) N’Gom, M.;
Li, S. A.; Schatz, G.; Erni, R.; Agarwal, A.; Kotov, N.; Norris, T. B.
Phys. Rev. B 2009, 80, 113411−1.
(8) (a) Chen, M.; Kumar, D.; Yi, C. W.; Goodman, D. W. Science
2005, 310, 291. (b) Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-
Espriu, B.; Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.;
Knight, D. W.; Hutchings, G. J. Science 2006, 311, 362. (c) Kesavan,
L.; Tiruvalam, R.; Rahim, M. H. A.; bin Saiman, M. I.; Enache, D. I.;
Jenkins, R. L.; Dimitratos, N.; Lopez-Sanchez, J. A.; Taylor, S. H.;
Knight, D. W.; Kiely, C. J.; Hutchings, G. J. Science 2011, 331, 195.
(d) Edwards, J. K.; Solsona, B.; N, E. N.; Carley, A. F.; Herzing, A. A.;
Kiely, C. J.; Hutchings, G. J. Science 2009, 323, 1037.
CONCLUSIONS
■
The integration of catalytic activity with the surface plasmonic
property enables HIF-AuNR@AuPd to serve as a bifunctional
platform to monitor catalytic reactions in situ with SERS. The
rod-like morphology of HIF-AuNR@AuPd leads to strong LSP
resonance localized at the rod ends, where there are high-index-
faceted horns providing highly active catalytic sites. The Au−Pd
alloy nature of the horns not only enhances the catalytic activity
but also circumvents plasmonic damping to a great extent.
Using HIF-AuNR@AuPd to catalyze and at the same time to
monitor the hydrogenation of 4-NTP, we directly determined
the kinetics of this reduction and distinguished two kinds of
catalytic active sites with different activities. These results
suggest exciting prospects for studying catalytic reaction
processes on catalyst surfaces with high sensitivity. As we
were finalizing this manuscript, we noticed a new publication by
Kneipp et al. that reported a SERS-assisted reaction kinetics
study using a physical mixture of separate gold and platinum
nanoparticles.25 In this work, we developed a well-defined
composite nanostructure that combines the plasmonic and
catalytic activities in one entity in the form of a single crystal
rather than separate particles, which can avoid the local
inhomogeneous distribution of two kinds of particles and
facilitate in-depth structure−property relationship studies.
ASSOCIATED CONTENT
* Supporting Information
Experimental details and supporting figures (Figures S1−S10).
This material is available free of charge via the Internet at
http://pubs.acs.org.
* Web-Enhanced Features
Movies 1 and 2, in .mpg format, are available online in the
HTML version.
■
S
W
AUTHOR INFORMATION
Corresponding Author
yu.han@kaust.edu.sa
■
(9) (a) DeSantis, C. J.; Peverly, A. A.; Peters, D. G.; Skrabalak, S. E.
Nano Lett. 2011, 11, 2164. (b) DeSantis, C. J.; Sue, A. C.; Bower, M.
M.; Skrabalak, S. E. ACS Nano 2012, 6, 2617.
Author Contributions
§J.H. and Y.Z. contributed equally to this paper.
(10) Kan, C.; Cai, W.; Li, C.; Zhang, L.; Hofmeister, H. J. Phys. D:
Appl. Phys. 2003, 36, 1609.
Notes
The authors declare no competing financial interest.
(11) (a) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B
2001, 105, 4065. (b) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater.
2003, 15, 1957. (c) Sun, Z. H.; Bao, Z. H.; Fang, C. H.; Wang, J. F.
Langmuir 2012, 28, 9082. (d) Sun, Z. H.; Yang, Z.; Zhou, J.; Yeung, M.
H.; Ni, W.; Wu, H.; Wang, J. F. Angew. Chem., Int. Ed. 2009, 48, 2881.
(e) Ni, W. H.; Kou, X. S.; Yang, Z.; Wang, J. F. ACS Nano 2008, 2,
677.
(12) (a) Yang, X. M.; Tryk, D. A.; Hasimoto, K.; Fujishima, A. Appl.
Phys. Lett. 1996, 69, 4020. (b) Rosario-Castro, B. I.; Fachini, E. R.;
́ ́
Hernandez, J.; Perez-Davis, M. E.; Cabrera, C. R. Langmuir 2006, 22,
6102. (c) Silien, C.; Dreesen, L.; Cecchet, F.; Thiry, P. A.; Peremans,
A. J. Phys. Chem. C 2007, 111, 6357.
(13) (a) Wang, Z. L.; Mohamed, M. B.; Link, S.; El-Sayed, M. A. Surf.
Sci. 1999, 440, L809. (b) Liu, M.; Guyot-Sionnest, P. J. Phys. Chem. B
2005, 109, 22192. (c) Keul, H. A.; Moller, M.; Bockstaller, M. R.
Langmuir 2007, 23, 10307. (d) Carbo-Argibay, E.; Rodriguez-
Gonzalez, B.; Pacifico, J.; Pastoriza-Santos, I.; Perez-Juste, J.; Liz-
Marzan, L. M. Angew. Chem., Int. Ed. 2007, 46, 8983.
ACKNOWLEDGMENTS
This research was supported by baseline research funds to Y.H.
from King Abdullah University of Science and Technology.
■
REFERENCES
■
(1) (a) Heck, K. N.; Janesko, B. G.; Scuseria, G. E.; Halas, N. J.;
Wong, M. S. J. Am. Chem. Soc. 2008, 130, 16592. (b) Xie, W.;
Herrmann, C.; Kompe, K.; Haase, M.; Schlucker, S. J. Am. Chem. Soc.
2011, 133, 19302. (c) Seo, D.; Park, G.; Song, H. J. Am. Chem. Soc.
2012, 134, 1221. (d) Larsson, E. M.; Langhammer, C.; Zoric, I.;
Kasemo, B. Science 2009, 326, 1091. (e) Zou, S.; Williams, C. T.;
Chen, E. K. Y.; Weaver, M. J. J. Am. Chem. Soc. 1998, 120, 3811.
(f) Kim, H.; Kosuda, K. M.; Van Duyne, R. P.; Stair, P. C. Chem. Soc.
Rev. 2010, 39, 4820.
(2) (a) Nie, S. M.; Emory, S. R. Science 1997, 275, 1102.
(b) Pieczonka, N. P.; Aroca, R. F. Chem. Soc. Rev. 2008, 37, 946.
(c) Kneipp, J.; Kneipp, H.; Kneipp, K. Chem. Soc. Rev. 2008, 37, 1052.
(d) Fang, Y.; Seong, N. H.; Dlott, D. D. Science 2008, 321, 388.
(3) (a) Mazumder, V.; Chi, M.; More, K. L.; Sun, S. Angew. Chem.,
Int. Ed. 2010, 49, 9368. (b) Wang, L.; Yamauchi, Y. J. Am. Chem. Soc.
2010, 132, 13636.
(14) (a) Katz-Boon, H.; Rossouw, C. J.; Weyland, M.; Funston, A.
M.; Mulvaney, P.; Etheridge, J. Nano Lett. 2011, 11, 273. (b) Carbo-
Argibay, E.; Rodríguez-Gonzalez, B.; Gomez-Grana, S.; Guerrero-
Martínez, A.; Pastoriza-Santos, I.; Perez-Juste, J.; Liz-Marzan
Angew. Chem., Int. Ed. 2010, 49, 9397.
́
́
́
̃
́
́
, L. M.
8560
dx.doi.org/10.1021/ja4004602 | J. Am. Chem. Soc. 2013, 135, 8552−8561

Journal of the American Chemical Society
Article
(15) (a) Zhang, K.; Xiang, Y.; Wu, X.; Feng, L.; He, W.; Liu, J.; Zhou,
W.; Xie, S. Langmuir 2009, 25, 1162. (b) Chen, H. J.; Wang, F.; Li, K.;
Woo, K. C.; Wang, J. F.; Li, Q.; Sun, L. D.; Zhang, X. X.; Lin, H. Q.;
Yan, C. H. ACS Nano 2012, 6, 7162.
(16) Zhang, W.; Gallinet, B.; Martin, O. Phys. Rev. B 2010, 81,
233407.
(17) (a) Dreaden, E. C.; Near, R. D.; Abdallah, T.; Talaat, M. H.; El-
Sayed, M. A. Appl. Phys. Lett. 2011, 98, 183115. (b) Zhang, S. P.; Xu,
H. X. ACS Nano 2012, 6, 8128.
(18) Chu, M. W.; Myroshnychenko, V.; Chen, C. H.; Deng, J. P.;
Mou, C. Y.; de Abajo, F. J. G. Nano Lett. 2009, 9, 399.
(19) (a) Feng, Y. H.; Wang, Y.; Wang, H.; Chen, T.; Tay, Y. Y.; Yao,
L.; Yan, Q. Y.; Li, S. Z.; Chen, H. Y. Small 2012, 8, 246. (b) Chen, T.;
Wang, H.; Chen, G.; Wang, Y.; Feng, Y. H.; Teo, W. S.; Wu, T.; Chen,
H. Y. ACS Nano 2010, 4, 3087. (c) Chen, G.; Wang, Y.; Yang, M. X.;
Xu, J.; Goh, S. J.; Pan, M.; Chen, H. Y. J. Am. Chem. Soc. 2010, 132,
3644.
(20) Funston, A. M.; Novo, C.; Davis, T. J.; Mulvaney, P. Nano Lett.
2009, 9, 1651.
(21) Huang, Y. F.; Zhu, H. P.; Liu, G. K.; Wu, D. Y.; Ren, B.; Tian, Z.
Q. J. Am. Chem. Soc. 2010, 132, 9244.
(22) Gdowski, G. E.; Felter, T. E.; Stulen, R. H. Surf. Sci. 1987, 181,
L147.
(23) Huang, J. F.; Vongehr, S.; Tang, S. C.; Lu, H. M.; Meng, X. K. J.
Phys. Chem. C 2010, 114, 15005.
(24) (a) Dong, B.; Fang, Y. R.; Chen, X. W.; Xu, H. X.; Sun, M. T.
Langmuir 2011, 27, 10677. (b) Sun, M. T.; Zhang, Z. L.; Zheng, H. R.;
Xu, H. X. Sci. Rep. 2012, 2, 647. (c) Xie, W.; Walkenfort, B.; Schlucker,
̈
S. J. Am. Chem. Soc. 2013, 135, 1657.
(25) Joseph, V.; Engelbrekt, C.; Zhang, J. D.; Gernert, U.; Ulstrup, J.;
Kneipp, J. Angew. Chem., Int. Ed. 2012, 51, 7592.
(26) Nahm, T. U.; Jung, R.; Kim, J. Y.; Park, W. G.; Oh, S. J.; Park, J.
H.; Allen, J.; Chung, S. M.; Lee, Y.; Whang, C. Phys. Rev. B 1998, 58,
9817.
(27) Nutt, M. O.; Hughes, J. B.; Wong, M. S. Environ. Sci. Technol.
2005, 39, 1346.
8561
dx.doi.org/10.1021/ja4004602 | J. Am. Chem. Soc. 2013, 135, 8552−8561