Synthesis of highly-branched Au@AgPd core/shell nanoflowers for in situ SERS monitoring of catalytic reactions

Article abstract of DOI:10.1016/j.cclet.2020.04.050

Alloy and small size nanostructures are favorable to catalytical performance, but not to surface-enhanced Raman spectroscopy (SERS) applications. Integrating SERS and catalytic activity into the nanocrystals with both alloy and small size structures is of great interest in fabrication of SERS platform to in situ monitor catalytical reaction. Herein, we report a facile method to synthesize Au@AgPd trimetallic nanoflowers (Au@AgPd NFs) with both SERS and catalytic activities, through simultaneous selective growth of Ag and Pd on Au core to form highly-branched alloy shell. These nanocrystals have the properties of small sizes, defects abundance, and highly-dispersed alloy shell which offer superior catalytic activity, while the merits of monodisperse, excellent stability, and highly-branched shell and core/alloy-shell structure promise the enhanced SERS activity. We further studied their growth mechanisms, and found that the ratio of Ag to Pd, sizes of Au core, and surfactant cetyltrimethylammonium bromide together determine this special structure. Using this as-synthesized nanocrystals, a monolayer bifunctional platform with both SERS and catalytical activity was fabricated through self-assembly at air/water interface, and applied to in situ SERS monitoring the reaction process of Pd-catalyzed hydrogenation of 4-nitrothiophenol to 4-aminothiophenol.

Full text of DOI:10.1016/j.cclet.2020.04.050

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CCLET 5629 No. of Pages 6
Chinese Chemical Letters xxx (2019) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Synthesis of highly-branched Au@AgPd core/shell nanoflowers for
in situ SERS monitoring of catalytic reactions
Yujian Laia,b, Lijie Donga,b, Rui Liua, Shaoyu Lua,b, Zuoliang Hea,b, Wanyu Shana,b
,
Fanglan Genga, Yaqi Caia,b, Jingfu Liua,b,c,
*
a
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing
100085, China
b
University of Chinese Academy of Sciences, Beijing 100049, China
School of Environment, Hangzhou Institute for Advanced Study, University of Chinese Academy of Sciences, Hangzhou 310024, China
c
A R T I C L E I N F O
A B S T R A C T
Article history:
Alloy and small size nanostructures are favorable to catalytical performance, but not to surface-enhanced
Raman spectroscopy (SERS) applications. Integrating SERS and catalytic activity into the nanocrystals
with both alloy and small size structures is of great interest in fabrication of SERS platform to in situ
monitor catalytical reaction. Herein, we report a facile method to synthesize Au@AgPd trimetallic
nanoflowers (Au@AgPd NFs) with both SERS and catalytic activities, through simultaneous selective
growth of Ag and Pd on Au core to form highly-branched alloy shell. These nanocrystals have the
properties of small sizes, defects abundance, and highly-dispersed alloy shell which offer superior
catalytic activity, while the merits of monodisperse, excellent stability, and highly-branched shell and
core/alloy-shell structure promise the enhanced SERS activity. We further studied their growth
mechanisms, and found that the ratio of Ag to Pd, sizes of Au core, and surfactant cetyltrimethy-
lammonium bromide together determine this special structure. Using this as-synthesized nanocrystals, a
monolayer bifunctional platform with both SERS and catalytical activity was fabricated through self-
assembly at air/water interface, and applied to in situ SERS monitoring the reaction process of Pd-
catalyzed hydrogenation of 4-nitrothiophenol to 4-aminothiophenol.
Received 25 February 2020
Received in revised form 24 April 2020
Accepted 27 April 2020
Available online xxx
Keywords:
Nanoflower
SERS
Bifunctional platform
AgPd alloy
Growth mechanism
© 2020 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Tracking reaction processes on the surface of nanocrystals (NCs)
in situ is a key issue for understanding the catalytic mechanisms
and reaction kinetics [1]. As a promising fingerprint technique,
surface-enhanced Raman spectroscopy (SERS) with greater sensi-
tivity down to single molecules allows revealing surface catalytic
processes with fast response and surface selectivity [2,3]. To this
end, great efforts have been devoted to fabrication of desired
bifunctional platform which possess both of the high SERS and
catalytic activity [4,5]. However, integrating both SERS and
catalytic activity into small-sized NCs is still of great challenge,
let alone the small-sized alloy nanostructures. This is because both
of alloy and small size nanostructures are detrimental to SERS
applications [6], but favorable to catalytical performance [7].
From the structural point of view, core/shell NCs with highly-
branched alloy shell have the ideal structure to achieve this goal.
Firstly, the highly-branched shell with rough surface and core/shell
structure can offer high SERS activity [2,8], meanwhile the
abundant atomic steps, edges, and corner atoms in the branches
can boost their catalytical performance [9]. Both of alloy and core/
shell structures can markedly enhance the stability and prevent
agglomeration [10], which is of great importance for in situ
monitoring reaction in aqueous dispersion. Further, the morphol-
ogy and size distribution of core/shell NCs can be easily controlled
through the seed-mediated-growth method. Considerable re-
search efforts have been devoted to synthesis of multi-metallic
branched NCs with alloy or core/shell structures [11,12]. However,
few studies devoted to synthesis of highly branched NCs with both
alloy and core/shell structures and their application in in situ
monitor catalytical reaction.
Herein, we report the facile synthesis of flower-like Au@AgPd
trimetallic nanostructure (Au@AgPd NFs), through simultaneous
selectivegrowthofAgandPdonAu coreforforminghighlybranched
and highly dispersed AgPd alloy shell. The as-synthesized Au@AgPd
* Corresponding author at: State Key Laboratory of Environmental Chemistry and
Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy
of Sciences, Beijing 100085, China.
E-mail address: jfliu@rcees.ac.cn (J. Liu).
https://doi.org/10.1016/j.cclet.2020.04.050
1001-8417/© 2020 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Lai, et al., Synthesis of highly-branched Au@AgPd core/shell nanoflowers for in situ SERS monitoring of
catalytic reactions, Chin. Chem. Lett. (2020), https://doi.org/10.1016/j.cclet.2020.04.050

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NFs were highly monodispersed and superior stability in H2O2
solution. Most importantly, this small-sized Au@AgPd NFs showed
excellent SERS and catalytic activity, and was successfully applied to
in situ monitor catalytic reaction process. The exploration of such
ideal nanostructures and the involved growth mechanisms are of
great fundamental interest. The role of capping agent, Ag/Pd ratio,
and core sizes were systematically studied. This study provides a
vivid example to rational design versatile NCs by means of
heterogeneous construction for fabrication of SERS platform, and
may offer inspiration for tailoring other metal NCs to monitor
different reactions with the understanding of involved growth
mechanisms.
Seed-mediated growth method enables the controlled synthe-
sis of monodispersed core/shell nanostructures owing to their
manageable sizes and morphologies, and was thus employed to
construct the highly-branched nanostructures. Gold nanocrystals
(AuNCs) as the commonly used seeds for epitaxial growth endow
shell with enhanced catalytic activity and boosted stability in
aqueous dispersions. More importantly, the crystallographic
defects in the twinned Au seed particles facilitates the growth
of AgPd branches [13]. Therefore, the fine Au seeds were firstly
synthesized with a narrow size distribution of 13.5 Æ1.0 nm
(Fig. S1 in Supporting information). Utilizing these AuNCs as seeds
for simultaneous overgrowth of palladium and silver enables the
formation of core/shell structured Au@AgPd NFs with highly-
branched AgPd alloy shell, a narrow size distribution, enhanced
stability, and high yield. In brief, AgNO3 and H2PdCl4 were co-
reduced by L-ascorbic acid (AA) and overgrown on Au seeds using
CTAB as the capping agent. Experimental details can be found in
Supporting information. To ensure a highly-dispersed alloy shell,
the kinetics of Pd growth was controlled using a syringe pump, and
the nucleation and growth processes will be discussed vide infra.
Fig.1 shows transmission electron microscopy (TEM) and scanning
transmission electron microscopy (STEM) of the as-prepared
Au@AgPd NFs, which is highly-branched with distinct core/shell
structure. The Au@AgPd NFs show a narrow size distribution of
diameter (38.4 Æ 2.1 nm), as well as branch length (10.5 Æ1.3 nm)
and width (5.4 Æ 0.7 nm) as shown in Fig. S2a (Supporting
information). The inductively coupled plasma mass spectrometer
(ICP-MS) analysis showed the overall weight percentages of Au, Ag
and Pd in the Au@AgPd NFs were respectively 14.2%, 40.6% and
45.3%, which agreed with the ratio of Au/Ag/Pd precursors. The
high yield of Au@AgPd NFs (90.0%, based on the total amount of Ag
and Pd) indicates most of AgNO3 and H2PdCl4 were reduced and
grew on AuNCs.
To explore the shell structure, the wide-angle X-ray diffraction
(XRD) pattern was collected (Fig. S3 in Supporting information).
Although the peaks of Ag and Au cannot be resolved due to their
small lattice mismatch (0.9%), the presence of two characteristic
diffraction peaks confirmed the alloy composition. Furthermore,
Fig. 1. Morphology characterization of Au@AgPd NCs. (a) Low-magnification TEM image, (b) high-magnification STEM image, (c) HAADF-STEM image, (e-h) EDX elemental
mappings and EDX line profile.
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catalytic reactions, Chin. Chem. Lett. (2020), https://doi.org/10.1016/j.cclet.2020.04.050

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X-ray photoelectron spectroscopy (XPS) was adopted to analyze
the surface composition and state of Au@AgPd NFs within 2 nm,
and results showed that both Ag and Pd existed in the surface of
Au@AgPd NFs (Fig. S4 in Supporting information). The binding
energy of Pd 3d is evidently shifted to 335.7 and 340.9 eV
compared to the standard Pd 3d peaks (335.2 and 340.4 eV), which
suggests that there is electron transfer from Pd to Ag due to the
alloying of Ag and Pd. To directly observe the spatial distributions
of Ag, Pd and Au in Au@AgPd NFs, high-angle annular darkfield
TEM (HAADF-TEM) equipped with energy-dispersive X-ray (EDX)
elemental mapping was employed, and results are shown in Fig. 1.
By overlaying these elemental maps, it can be seen that Au element
clearly separated from Ag and Pd elements, confirming the core/
shell structure; while the well overlapped flower-like Ag and Pd
maps indicated the alloy structure of AgPd branch. The cross-
sectional composition line profiles also suggest the homogeneity of
the AgPd shell. Taken together, the highly-branched shell are
demonstrated alloy structure in full-scale. The stability of this
alloyed shell was demonstrated by unchanged morphology of
Au@AgPd NFs after reaction with H2O2 (2 wt%) for 20 min (Fig. S5
in Supporting information). This result also implied that the alloy
structure can protect Ag from etching. In addition, the Au@AgPd
NFs showed excellent SERS activity in comparison with Au@Pd NCs
(Fig. S6 in Supporting information). It should note that the Au@Pd
NCs tightly coated with Pd shell also showed slight SERS activity,
which suggest the core/shell structure endowed Pd shell with SERS
activity. The result of finite difference time domain (FDTD)
simulation in Fig. S7 (Supporting information) further confirmed
this conclusion. The reason for markedly enhanced SERS activity of
Au@AgPd NFs may be multiple. Highly-branched shell, addition of
Ag, core/alloy-shell structure may endow the small-sized
Au@AgPd NFs with SERS activity together. However, reliable and
accurately quantitative statements on the SERS enhancement
factor of this complex nanostructure are difficult to be achieved
due to the unknown parameters of molar extinction coefficient and
exact surface area of the Au@AgPd NFs [4].
nanotwins (Figs. 2b and c), which could act as highly efficient
catalytic sites.
To elucidate the mechanism involved in the formation of
highly-branched Au@AgPd NFs,
a set of experiments were
conducted. The role of CTAB was firstly investigated because it
has been widely used in morphological regulation [14–16]. With no
addition of CTAB, the synthesized Au@AgPd NFs showed rough
surface and wide branches (Fig. S8 in Supporting information). This
result suggests that CTAB may not be the key factor governing the
branch formation, but can assist AgPd bloom on Au seeds forming a
highly-branched shell. The possible explanation is that the Ag and/
or Pd nucleated on AuNPs surface and formed Ag and/or Pd islands,
then CTAB capped on the surface of the islands [17] with lower
density on the island top than that on the sides due to their
difference in curvature [18]. This resulted in the deposition of Ag
and Pd atoms on the island top rather than its sides, therefore, the
formation of narrowed AgPd branches. In the absence of CTAB,
however, the difference of AgPd deposition rate between island top
and sides is not that large, and hence broad branches formed.
The molar ratio of Pd to Ag was found to markedly affect the
shell growth. As illustrated in Fig. 3, core/shell structured Au@Pd or
Au@Ag NCs with smooth and rather highly-branched shell was
formed when H2PdCl4 or AgNO3 was used as precursor alone.
However, distinct branches appeared when co-reducing both Pd
and Ag precursors with various Pd/Ag ratios, which imply the
critical role of the coexistence of Ag and Pd metals in the branch
formation. Specifically, the branches changed from sparse to
compact and then to sparse again with increasing of Pd/Ag ratio,
and the Pd/Ag ratio of 1:1 is the optimal point at which closely
spaced, and uniform branches were obtained. This was attributed
to the large lattice mismatch (6.3%), as metals tend to deposit on
substrate in the manner of island growth (Volmer-Weber mode)
when lattice mismatch larger than 5% [19].
The formation of highly-branched shell is also dependent on the
size of Au seed. Citrate-coated AuNCs with different sizes were at
first prepared (Table S1 and Fig. S1 in Supporting information) to
study the branch shell. As shown in Fig. 4, large sized Au core
inhibited the formation of closely compact branch shell. When the
diameter of Au seeds increased to 60 nm, branches can hardly be
observed. Furthermore, the branches tend to grow at the corner of
Au seeds (Fig. 4c), by which the high surface energies of corner can
To further investigate the structure of AgPd branches down to
the atomic level, aberration-corrected HAADF-STEM imaging was
conducted. The lattice fringes were clearly observed when the
Au@AgPd nanoparticle is viewed along the [110] zone axis,
indicating the good crystallization (Fig. 2a). Although discrimina-
tion of Ag and Pd atoms through Z-contrast difference is impossible
due to their adjacent atomic number, AgPd alloy can be confirmed
in atomic level by measuring their lattice spacings. As shown in
Fig. 2a, the average lattice spacing of AgPd branch is determined as
0.2323 nm, which is between that of the (111) plane of Ag
(0.2370 nm) and Pd (0.2230 nm), confirming the AgPd alloy
structure. It is noteworthy that in the AgPd alloy branch there
are abundant lattice defects including steps, atomic corners and
be released and thus
a more stable structure is adopted.
Consistently, smaller sized Au seeds have more twin boundary
sites facilitating the AgPd nucleation, and thus densely branches
tend to grow on small sized AuNPs. Additionally, it should be noted
that the sodium citrate (SC) concentration in all growth solutions
with different AuNCs sizes were kept closely in the range of 0.13–
0.14 mmol/L (Fig. S9 in Supporting information), which suggested
the impact of SC on branches growth is not remarkable.
Fig. 2. (a) Aberration-corrected HAADF-STEM image of branches in Au@AgPd NCs. Step (b) and nanotwins (c) in branches. The inset image in (a) shows the fast Fourier
transform (FFT) pattern.
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Fig. 3. Morphologies of Au@AgPd NCs with various Pd/Ag ratio of 0:10 (a), 2:10 (b), 5:10 (c), 10:10 (d), 20:10 (e) and 10:0 (f).
Fig. 4. Morphologies of Au@AgPd NFs (down) with the corresponding core diameter (up) under the proposed synthesis conditions.
To further elucidate the formation mechanism of this highly-
branched shell, the shape-evolution process of the Au@AgPd NFs
was studied. It should be mentioned that the growth of Ag was
slow due to the retarded kinetics in the presence of CTAB [19], and
the Ag reduction relies mostly on galvanic replacement between
Ag and Pd [20], i.e., the Ag growth kinetics is dependent on the
amount of Pd. Thus, the growth kinetics of both Ag and Pd can be
controlled by slowly injecting H2PdCl4 with a syringe pump, which
provides enough time to prepare TEM samples to track the
Au@AgPd NFs evolution. Fig. S10 (Supporting information) shows
the TEM images of aliquots sampled from reaction solution at
different reaction times. After 3 min from injecting AgNO3, only
multiply twinned Au NCs are shown in the TEM images (Fig. S10a).
Then, a smooth and thin shell on Au seed was observed after
reaction for 6 min (Fig. S10b). After 9 min, branches were appeared
at the twin boundary sites which is consistent with above
conclusion (Fig. S10c). Further, branched shell was formed at
12 min (Fig. S10d) and become more densely at 15 min (Fig. S10e).
With the extension of reaction time to 30 min, no apparent
changes in morphology were further detected (Fig. S10f).
Given the highly-branched Au@AgPd NFs possessed abundant
defects and rough surface that are favored to both catalysis and
SERS applications, it was applied to in situ SERS monitor the model
reaction of Pd-catalyzed hydrogenation of 4-nitrothiophenol (4-
NTP) to 4-aminothiophenol (4-ATP). To acquire stable and high-
resolution temporal SERS spectra, Au@AgPd NFs were deposited on
a quartz plate and dried at room temperature. Specifically, a
monolayer of Au@AgPd NFs was assembled at the air/liquid
interface at first. Then, the assembled monolayer was transferred
onto a hydroxylated quartz plate for fabricating a SERS platform
with uniform hotspots. With the prepared platform, the hydro-
genation of 4-NTP was in situ recorded and their reaction kinetics
were achieved as shown in Fig. 6 (for more experimental details
see Supporting information). As shown in the in situ SERS spectra
(Fig. 6b), initially on the AgPd branches were pre-adsorbed with a
monolayer of 4-NTP molecules with four characteristic bands
located at 854, 1076, 1337, and 1572 cmÀ1, which are assigned to
CÀÀH wagging (nCH), CÀÀS stretching (nCS), OÀÀNÀÀO stretching
Collectively, the growth mechanism of high-branched shell can
be concluded as illustrated in Fig. 5. Firstly, a thin film of Ag and/or
Pd was formed on Au seed. Then, Ag and Pd were selectively co-
deposited on the twin boundary sites of Au NCs forming small
islands. After that, Ag and Pd deposit on the top of the islands with
the assistance of CTAB to form branches. Finally, uniform and dense
branches are formed on Au NCs surface with continuing growth.
Fig. 5. Schematic diagram of the branch growth process.
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Fig. 6. In situ SERS monitoring of the catalytic reduction of 4-NTP to 4-ATP. (a) Reaction routes of 4-NTP reduction on the catalytic surface. (b) In situ SERS spectra. (c) Plots of
the logarithm of the corresponding SERS intensity at 1337 cmÀ1 (ln(I1337(t)/I1337(0)) versus the reaction time.
(
nNO2), and CÀÀC stretching (nCC), respectively [5]. The intensities
defect abundance that were favorable to catalytic process but
detrimental to its SERS activity; while the advantages of
monodispersion, highly dense branches, core/alloy-shell struc-
tures settled this dilemma. The growth mechanism of this
fascinating nanostructure was thus explored. The Pd/Ag ratio
and twin boundary sites were shown to be crucial for the formation
of branched shell, while CTAB assists AgPd to bloom on Au seed
with narrowed branches. Owing to their SERS and catalytic activity,
the Au@AgPd NFs were successfully applied to fabricating
bifunctional SERS platform which was demonstrated enable to
in situ monitor the catalytic conversion of 4-NTP to 4-ATP. This
study provides a vivid example to construct bifunctional nano-
crystals with small sizes and alloy structure to in situ monitor
catalytic reaction, and this may offer inspiration for tailoring other
metal NCs to monitor different reactions with the understanding of
involved growth mechanisms.
of R-NO2-related bands decrease as the reaction proceeds, and
concomitantly new bands of 1593 cmÀ1, corresponding to CÀÀC
stretching (nCC), gradually appeared. This change indicates that 4-
NTP was efficiently reduced to 4-ATP on the alloy branches.
The SERS measurement also enabled the elucidation of in
detailed reaction mechanisms. Previous studies [21–23] showed
that two reaction routes schemed in Fig. 6a are involved in the 4-
NTP reduction: (1) The molecules are adsorbed on the catalyst,
then dimerized into 4,4'-dimercaptoazobenzene (DMAB) with
adjacent 4-NTP, and further reduced into 4-ATP (Route 1); (2)
hydrogen dissociated on the catalyst surface and then directly
reduce the 4-NTP molecules into 4-ATP (Route 2). The characteris-
tic bands of the intermediate located at 1140, 1390 and 1435 cmÀ1
were not detected, suggesting Route 2 dominate the reduction
process [22,23]. This is because the Pd NCs are highly active for
hydrogen dissociation and thus facilitate the Route 2 reaction.
The reaction kinetics was further measured by quantitatively
monitoring the intensity decrease of the characteristic bands at
1337 cmÀ1. Because no DMAB intermediates involved in the
reduction reaction as analysed above, and the large excess of
NaBH4, the reduction reaction can be assumed to follow pseudo-
first-order kinetics. The reaction rate constant (k) can be calculated
by Eq. (1):
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgments
kt = –ln (It/I0)
(1)
This work was supported by the National Key R&D Program of
China (No. 2016YFA0203102), and the National Natural Science
Foundation of China (Nos. 21620102008 and 21827815).
where t is the reaction time, and I is the intensity of the band at
1337 cmÀ1. As plotted in Fig. 6c, the reaction rate is slow in the
initial 900 s, suggesting the requirement of a period of induction
time. Although further studies are required to clarify this induction
period, a possible tentative explanation is the fact that both of
Appendix A. Supplementary data
–
Supplementarymaterialrelatedtothisarticlecanbefound, inthe
online version, at doi:https://doi.org/10.1016/j.cclet.2020.04.050.
diffusion process of BH4 to the catalysis center and activation
process are time-consuming. Besides, the duration time of the
induction period is closely related to laser power, but the reaction
rate constant (k) is not influenced based on our previous study [24].
Thus, the first-ordered fit was conducted in the range of 900–
1200s, and a linear correlation between ln(It/I0) and reaction time
(t) was found as shown in Fig. 6c with reaction rate constant of
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Please cite this article in press as: Y. Lai, et al., Synthesis of highly-branched Au@AgPd core/shell nanoflowers for in situ SERS monitoring of
catalytic reactions, Chin. Chem. Lett. (2020), https://doi.org/10.1016/j.cclet.2020.04.050

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Please cite this article in press as: Y. Lai, et al., Synthesis of highly-branched Au@AgPd core/shell nanoflowers for in situ SERS monitoring of
catalytic reactions, Chin. Chem. Lett. (2020), https://doi.org/10.1016/j.cclet.2020.04.050