Sophisticated construction of au Islands on Pt-Ni: An ideal trimetallic nanoframe catalyst

Article abstract of DOI:10.1021/ja5058532

We have developed a priority-related chemical etching method to transfer the starting Pt-Ni polyhedron to a nanoframe. Utilizing the lower electronegativity of Ni in comparison to Au atoms, in conjunction with the galvanic replacement of catalytically active Au to Ni tops, a unique Au island on a Pt-Ni trimetallic nanoframe is achieved. The design strategy is based on the structural priority mechanism of multimetallic nanocrystals during the synthesis and thus can be generalized to other analogous metal-bimetallic nanocrystal combinations (such as Pd and Cu islands on Pt-Ni nanoframes), which is expected to pave the way for the future development of efficient catalysts.

Full text of DOI:10.1021/ja5058532

Communication
pubs.acs.org/JACS
Sophisticated Construction of Au Islands on Pt−Ni: An Ideal
Trimetallic Nanoframe Catalyst
Yuen Wu, Dingsheng Wang, Gang Zhou, Rong Yu, Chen Chen,* and Yadong Li*
†
†
‡
§
,†
,†
†
Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China
‡
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s
Republic of China
§
Beijing National Center for Electron Microscopy, School of Materials Science and Engineering, Tsinghua University, Beijing 100084,
People’s Republic of China
*
S Supporting Information
catedly anchored with Au islands by galvanic replacement,
masterly utilizing the highly active top Ni atoms. Afterward,
structural priority related chemical etching is used to carefully
regulate the corrosion of Ni and rearrangement of Pt atoms to
achieve the nanosegregated Pt-skin edges. This ingenious
design not only guarantees the major proportion of versatile
active sites but also directly supports synergistic effects between
Au islands and Pt−Ni edges, which will extend our ability to
improve activity, selectivity, and durability. We believe such
previously unreported integration of selective galvanic replace-
ment and priority-related chemical etching gives us the key to
open up a new trimetallic family of nanoframes with surface-
decorating atoms for the selection of ideal NP catalysts.
ABSTRACT: We have developed a priority-related
chemical etching method to transfer the starting Pt−Ni
polyhedron to a nanoframe. Utilizing the lower electro-
negativity of Ni in comparison to Au atoms, in conjunction
with the galvanic replacement of catalytically active Au to
Ni tops, a unique Au island on a Pt−Ni trimetallic
nanoframe is achieved. The design strategy is based on the
structural priority mechanism of multimetallic nanocrystals
during the synthesis and thus can be generalized to other
analogous metal−bimetallic nanocrystal combinations
(
such as Pd and Cu islands on Pt−Ni nanoframes),
which is expected to pave the way for the future
development of efficient catalysts.
We first present an efficient method for synthesizing novel
bimetallic Pt−Ni nanoframes by transforming truncated
octahedral Pt−Ni alloy particles through a priority-related
aving a lower coordination number, corner and edge
12
chemical etching process. Figure 1a shows both transmission
H
atoms, rather than those at the flat surface, are generally
the preferred sites to activate most important catalytic reactions
due to the lower activation energy and altered electronic
electron microscope (TEM) and high-resolution TEM
(
HRTEM) (inset) images of the truncated octahedral PtNi3
NPs. After dimethylglyoxime was applied to a suspension of the
PtNi NPs, the Ni could be gradually etched from the PtNi
3
1
,2
structure. Nanostructures with open surface features such as
3
4
5,6
3
high-index facet, concave surface, and porous structure have
provoked ever-increasing attention in the field of catalysis due
to their exceptional catalytic properties. Metallic nanoparticles
alloy by oxidative etching. Figure 1b reveals that most of the
obtained NPs retained their original symmetry and showed a
narrow size distribution (average size of 12.5 ± 1.5 nm).
Examination of the HRTEM image shown in Figure 1c
indicates that each particle possesses an eroded hollow frame.
The high-angle annular dark-field scanning transmission
electron microscope (HAADF-STEM) micrograph (Figure
(
NPs) with a nanoframe structure, in which edges are joined
together at specific corners to create a skeletal frame, can
harvest the costly materials and supply the three-dimensional
molecular accessibility, thus serving as an ideal model catalyst
to satisfy the ultimate goal of NP catalysis. Several pioneering
works have been established to obtain this unique nanostruc-
1
d) indicates that parts of the framework are much brighter
7
,8
9
than the other region of the particles, further demonstrating the
presence of a nanoframe structure. The structural evolution of
the particles from the parent PtNi3 polyhedrons to final
products was accompanied by a variation in elemental
distribution. According to the elemental maps shown in Figure
1d and cross-sectional compositional line profiles of a truncated
ture, such as monometallic Au and bimetallic Au−Ag and
10
Pt−Cu nanoframes, with outstanding physical and chemical
properties. Very recently, Chen et al. observed that a Pt Ni
3
rhombic dodecahedral nanoframe composed of 24 edges and
1
4 corners enables the maximum utilization of Pt by a three-
11
dimensional accessible surface. The fascinating geometric and
electronic effects induced by the nanosegregated Pt-skin surface
confer a substantial improvement in activity toward the oxygen
reduction reaction to this unique nanoframe catalyst. Because
of this progress, designing multicentered and multifunctional
nanostructures with high efficiency is currently becoming a
promising area. In this report, a previously unknown sequential
strategy is exploited to construct a novel trimetallic nanoframe
catalyst. The corners of Pt−Ni nanoframes can be sophisti-
octahedral PtNi particle (Figure S2, Supporting Information),
3
the distribution of Ni in the starting PtNi particles showed a
3
slight enrichment in the central region, whereas a homogeneous
distribution of Pt throughout the entire particle was observed.
After chemical etching, the as-obtained truncated octahedral
Received: June 14, 2014
Published: August 5, 2014
©
2014 American Chemical Society
11594
dx.doi.org/10.1021/ja5058532 | J. Am. Chem. Soc. 2014, 136, 11594−11597

Journal of the American Chemical Society
Communication
Figure 1. (a) TEM and HRTEM images of truncated octahedral
PtNi . The inset shows the ideal model of a truncated octahedron. (b)
3
Figure 2. Schematic illustration of the proposed etching mechanism of
TEM and (c) HRTEM images of the Pt Ni nanoframe (the insets in
3
truncated octahedral PtNi NPs from DFT calculations, combined
3
(
c) are the fast Fourier transform patterns and corresponding ideal
with crystal structure and etching reaction analysis: (a, b) etching
model of the nanoframe). (d) HAADF-STEM image and correspond-
ing elemental maps (Pt (yellow) and Ni (red)).
processes of {111} and {100} surfaces; (c) formation of [110] frame.
2+
Red, blue, and white balls correspond to Pt and Ni atoms and Ni ,
respectively.
nanoframes retained an overall 36 edges of the truncated
octahedron, in which both Pt and Ni exhibited homogeneous
distributions, implying that the final product formed an alloy
phase.
It is undoubtedly very important to understand the etching-
dominated fabrication methods of alloy NPs with well-designed
shapes. For this purpose, we investigated the surface-dependent
decomposed Pt atoms would readily occupy the Ni vacancies
16
which formed earlier at the edges by migration, thereby
exhibiting the radial thickening of the Pt−Ni@Pt frame with
progressing time (Figure 2c). The nonsusceptibility of Pt to
17
dimethylglyoxime solution and strong Pt−Pt bonds allowed
Pt-protected frames to be retained as far as possible after the
etching. These were in agreement with the experimental
observations. Therefore, it is confirmed that the existence of
open {100} surfaces and Kirkendall effects stemming from
distinct diffusion coefficients of Ni and Pt are the major reasons
etching processes and mechanisms of PtNi NPs by density
3
functional theory calculations, crystal structures, and etching
reaction analyses. A thermodynamic viewpoint showed that the
dissolution rate of Ni atoms is faster than that of Pt atoms,
following the order edge > (110) ≈ (100) > (111) surfaces (see
the Supporting Information for details and Figure 2). The
proposed processes involved in the shape evolution of
truncated octahedral PtNi could yield the hollow structure by
3
chemical etching.
To gain comprehensive insights into the structure−activity
relationship of a catalyst, based on an understanding of priority-
related chemical etching, we designed an experiment
simultaneously involving the effects of morphology and
composition within a single sample. We also chose truncated
octahedral PtNi3 particles as the starting polyhedrons for
depositing Au islands on Pt−Ni nanoframe hybrid structures by
a sequential process. The structural evolution from truncated
octahedral PtNi , to Au on PtNi , to Au on PtNi, and finally to
truncated octahedral PtNi NPs are schematically illustrated
3
12
in Figure 2. As for octahedral PtNi NPs, the etching of (111)
3
surfaces proceeded in a layer-by-layer removal manner (Figure
2a), which in principle yielded a Pt-rich concave solid structure.
However, the open (100) surfaces exhibited a distinctly
different etching process and mechanism because of alternate
stacking of full Ni and Pt−Ni alternately in the structure. More
precisely, after Ni dissolution, the surface Pt aggregated, driven
by the near-neighbor bonding to form a “porous Pt shell”. It
inhibited the binding between exposed subsurface Ni and
dimethylglyoxime, which is a crucial stage of oxidative etching
3
3
Au on Pt Ni is illustrated in Figure 3. The first step of this
3
process starts with a mild galvanic replacement between Au(III)
and Ni(0), which mainly results from the greater electro-
1
2
of Pt−Ni alloy. The subsequent etching process was
negativity of Au with respect to Ni. When the PtNi truncated
3
2
+
unilaterally controlled by outward diffusion of Ni from NPs,
accompanying the inward flow of vacancies, through the pores
of te inert Pt shell (Figure 2b). It followed a mechanism
octahedrons with sharp corners react with a trace amount of
HAuCl solution, the replacement initiates local low-coordina-
4
18
tion sites owing to their high surface free energy. It is
reasonable to anticipate that the replacement reaction prevails
at corners rather than edges and faces, which guarantees that
Au(III) ions have a greater probability oxidizing Ni atoms at
corners, thereby forming top Au(0) atoms in situ. As the
reaction proceeds, the Au atoms form isolated clusters or very
small NPs at the top of the original truncated octahedron. The
HRTEM image shown in Figure 3b indicates that the core of
the particles remained a truncated octahedron, and the
protruding parts, which are much darker, suggest that the
1
3,14
analogous to the Kirkendall effect.
As the reaction
proceeded deeper into the Ni center, the etching rate should
increase significantly, which allowed mobility of vacancies and
growth and coalescence of small cavities. When reaching the
Kirkendall hole, the etching of {111} surfaces would open the
NPs (Figure 2c). Meanwhile, growing stress-corrosion cracks
would in time decompose the Pt-segregated layers, which was
14
not present in the hollowing of metals. Following the step-
15
induced/terrace-assisted growth mechanism of alloy NPs, the
1
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Journal of the American Chemical Society
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Figure 3. TEM images of (a) 10% Au on PtNi , (d) 10% Au on PtNi,
3
Figure 4. (a) Pt 4f XPS spectra of Pt Ni frame and 10% Au on Pt Ni
nanoframe (the inset illustrates the charge transfer from Au islands to
and (g) 10%Au on Pt Ni. HRTEM images of (b) 10% Au on PtNi ,
3
3
3
3
(
e) 10% Au on PtNi, and (h) 10% Au on Pt Ni. EDS elemental
3
Pt Ni frames in trimetallic systems; red, blue and yellow balls
mapping images of (c) 10% Au on PtNi , (f) 10% Au on PtNi, and (i)
3
3
correspond to Pt, Ni, and Au, respectively). (b) Selectivity and TOFs
of 4-nitrobenzaldehyde hydrogenation with different catalysts as a
function. (c) Specific ECSAs for different catalysts recorded in 0.1 M
1
0% Au on Pt Ni. (j) Schematic illustration of the structural change
3
from the truncated octahedral PtNi to trimetallic hybrid Au islands on
3
the Pt Ni nanoframe.
3
HClO . (d) Cyclic voltammograms of methanol oxidation on different
4
catalysts in 0.1 M HClO4 containing 1 M MeOH. (e) Cyclic
heavier gold islands clustered around the core. To further
elucidate the nature of this unique hybrid structure, we also
carried out EDS elemental mapping measurements. Unlike the
homogeneous distribution of Pt and Ni throughout each
particle (Figure 3c2 and 3c3), the turquoise dots in Figure 3c4
clearly indicate the existence of isolated Au islands. The second
step after replacement was the chemical etching process. With
an increase in the amount of etchant, the compositions of these
hybrid structures continued to evolve from 10% Au on PtNi to
voltammograms of CO stripping on different catalysts recorded in 0.1
−1
M HClO with a scan rate of 100 mV s . (f) Loss of peak current
4
density in a forward scan as a function of cycling numbers.
reducible groups such as CC and CO, the nitro groups
22
are difficult to reduce in a selective manner. Utilizing the
hydrogenation of 4-nitrobenzaldehyde as a model probe
reaction, we could evaluate the catalytic behavior of truncated
octahedral PtNi NPs and Pt Ni, Pt Ni nanoframes and hybrid
3,
1
0% Au on PtNi, to 10% Au on Pt Ni, which was consistent
3
3
3
3
with the morphological transformation of the Pt−Ni cores from
truncated octahedrons to truncated octahedral nanoframes.
Figure 3 shows TEM images of the samples at three different
stages, which demonstrated a morphological yield of up to 95%.
Very recently, it was demonstrated that a third metal can
indeed affect the adsorption energy of an adsorbed substrate
such as CO on the bimetallic surface by the direct electronic
Au on Pt−Ni with respect to both activity and selectivity. As
shown in Figure 4b, due to their larger surface area, the Pt Ni
3
frames exhibited a higher activity turnover frequency (TOF)
than the truncated octahedral PtNi3 and Pt Ni particles,
3
whereas much lower selectivity was measured (46%). The
Pt−Ni alloys presented a mixture of two reduction products (4-
aminobenzaldehyde and 4-nitrobenzyl alcohol). With small
19−21
coupling between different domains.
As shown in Figure
amounts of Au islands decorated on the Pt Ni frames, the Au
3
4
a, the Pt 4f7/2 and 4f peaks of the Pt Ni frames
on Pt−Ni showed an even better initial activity turnover
5/2
3
0
corresponded to Pt . Similarly, the Ni 2p peak could be
assigned to Ni (Figure S12, Supporting Information).
Comparatively, an apparent decrease in binding energy was
frequency (TOF) than the Pt Ni frames and almost 100%
3
/2
3
0
selectivity toward 4-aminobenzaldehyde. It is not surprising
that the improved selectivity resulted from the selective
23
observed for Au islands on the Pt Ni frame structures. The
adsorption of nitro groups on the surface of Au corners,
3
results strongly indicate that the Au islands on the Pt−Ni frame
do have electron communication with Pt and Ni, which is in
reasonable agreement with our DFT calculations, where
while the enhanced activity TOF might be attributed to
localized electron communication between the Au corners and
Pt Ni edges. However, an increase in the amount of Au did not
3
electrons were transferred from Au islands to Pt Ni frames
cause a further enhancement in activity but instead a significant
decay. It is possible that the overgrowth of Au islands may
occur on the NPs, blocking the reactive sites and thereby
3
(
see inset of Figure 4a). In particular, some exposed Au at the
+
interface was reduced to electron-deficient δ , an indication of
synergistic effects, which might favor the diversity or/and
selectivity of catalytic reactions. The hydrogenation reaction of
aromatic nitro groups is an important industrial process.
However, when the reactant molecules consist of other
hindering the contact between the substrate and inner Pt Ni
3
core.
Furthermore, we used the optimal amount of Au on the
Pt Ni catalysts (10%) to further study the particles’ catalytic
3
1
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Journal of the American Chemical Society
Communication
properties toward the electro-oxidation of methanol. Given the
charge formed during the hydrogen adsorption/desorption
process after the correction for double-layer formation, the
nanoframe catalysts exhibited notably larger electrochemically
ACKNOWLEDGMENTS
This work was supported by the State Key Project of
Fundamental Research for Nanoscience and Nanotechnology
■
(
2011CB932401 and 2011CBA00500), National key Basic
active surface areas than those of the Pt Ni truncated
3
Research Program of China (2012CB224802), and the
National Natural Science Foundation of China (Grant Nos.
21221062, 21171105, 21322107, and 21131004). This work
made use of the resources of the Beijing National Center for
Electron Microscopy.
octahedrons (Figure 4c). Similarly, the nanoframes also
exhibited excellent electrocatalytic activity toward the electro-
oxidation of methanol (Figure 4d). Remarkably, the doping of
Au islands on the Pt Ni nanoframes could significantly increase
3
the I /I ratio (I and I are the forward and backward current
f
b
f
b
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ASSOCIATED CONTENT
■
*
S
Supporting Information
(
́ ́
ez-Arellano, C.; Iglesias, M.; Sanchez, F. Appl.
Text, figures, and tables giving details of trimetallic Pt−Ni
nanoframe control experimental synthesis, characterization
data, and their catalytic reactions and detailed information
about DFT calculations. This material is available free of charge
via the Internet at http://pubs.acs.org.
(
(
(
2
(
AUTHOR INFORMATION
■
Corresponding Authors
6
*
E-mail for C.C.: cchen06@mails.tsinghua.edu.cn.
E-mail for Y.L.: ydli@mail.tsinghua.edu.cn.
*
Notes
The authors declare no competing financial interest.
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