Ultralong Pt-on-Pd bimetallic nanowires with nanoporous surface: Nanodendritic structure for enhanced electrocatalytic activity

Article abstract of DOI:10.1039/b922148d

We for the first time report a facile, wet-chemical strategy for the high-yield (-100%) synthesis of ultralong Pt-on-Pd bimetallic nanowires (NWs) with the cores being Pd NWs and the shells being made of dendritic Pt, which exhibit high surface area and enhanced electrocatalytic activity towards methanol oxidation reaction. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b922148d

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Ultralong Pt-on-Pd bimetallic nanowires with nanoporous surface:
nanodendritic structure for enhanced electrocatalytic activityw
Shaojun Guo,ab Shaojun Dong*ab and Erkang Wang*ab
Received (in Cambridge, UK) 22nd October 2009, Accepted 25th November 2009
First published as an Advance Article on the web 13th January 2010
DOI: 10.1039/b922148d
We for the first time report a facile, wet-chemical strategy for the
high-yield (B100%) synthesis of ultralong Pt-on-Pd bimetallic
nanowires (NWs) with the cores being Pd NWs and the shells being
made of dendritic Pt, which exhibit high surface area and enhanced
electrocatalytic activity towards methanol oxidation reaction.
been shown that the performance of catalysts for fuel-cell
reactions could also be improved by changing the morphology
of Pt from NPs to multi-dimensional nanostructures such as
NWs, whose large side surface could provide additional
catalytic active facets.4c–e,7c The typical example was from
Dodelet’s work,7c which demonstrated that Pt NWs/C nano-
composites show enhanced catalytic activity for the oxygen
reduction reaction compared with a state-of-the-art Pt/C catalyst
made of Pt NPs. (3) Moreover, the increase in the inter-
connectivity of Pt NPs can also lead to significantly enhanced
electrocatalytic activities of Pt nanoelectrocatalysts.9
However, with respect to the above designed procedures and
materials reported, high-yield production of binary metal NWs
with a nanoporous surface (dendritic structure) is far from
being trivial despite the fact that the binary complex nano-
architectures will be very promising for the development of
highly active, cost-effective nanocatalysts. In this communica-
tion, we for the first time report a facile, wet-chemical strategy
for the high-yield (B100%) synthesis of ultralong Pd–Pt
bimetallic NWs with the cores being Pd NWs and the shells
being made of dendritic Pt. Specifically, many small single-
crystal Pt nanobranches interweave with each other and thus
form many nanopores on the surface of Pd NWs, which exhibit
a high surface area (90.7 m2 gÀ1) and enhanced electrocatalytic
activity towards methanol oxidation reaction.
One-dimensional metal nanostructures such as nanowires (NWs),
nanotubes and nanobelts have received considerable interest on
account of their various potential applications deriving from their
unique optical, electronic and catalytic properties.1–7 In particular,
Pt and Pd NWs have been more extensively investigated because
of their important technological applications, including as
catalysts in many chemical reactions, as gas sensors, and as
electrode materials in fuel cells.6 To date, various chemical
protocols have been developed for preparing Pt and Pd NWs.4–7
Prominent examples include the synthesis of single-crystal Pt NWs
by a polyol process,4 a phase-transfer method for Pt and Pd
NWs,5 making Pt NWs from electrospinning6 and wet-chemical
approach to the preparation of Pt and Pd NWs.7 Despite these
successful demonstrations, all of these studies have been limited
to NWs with smooth surfaces, which are disadvantageous for
supplying high catalytic activity from the viewpoint of design of
nanocatalysis. Therefore, developing an effective approach to the
synthesis of catalysts with high surface areas for better catalytic
performance and utilization efficiency is highly desirable and
technologically important.
The whole preparation strategy for constructing the Pt-on-Pd
bimetallic NWs with nanodendritic morphology (PBNNM) is
shown in Scheme 1. First, a large-scale and reproducible
hydrothermal route was employed for preparing well-defined
Te NWs according to ref. 7d with a slight modification (for
detailed process see ESIw). Second, the as-prepared small-size
Te NWs were used as both reducing agent and sacrificial
template for obtaining Pd NWs in aqueous solution at room
temperature in less than 5 min, which is different from Yu’s
process7d (reaction time: 13 h, temperature: 50 1C, solvent:
ethylene glycol). Third, the as-obtained Pd NWs were used as
seeds to direct the dendritic growth of Pt upon the reduction of
K2PtCl4 by ascorbic acid (AA) in aqueous solution.
Hybridization (such as designing core/shell nanostructure)
provides an effective strategy for enhancing the functionality of
materials. Particularly, effectively controlling the morphology
of the exterior metal (e.g. Pt) can provide a great opportunity to
improve its surface area and increase its activity on a mass or
specific basis.8 Recently, three areas have attracted particular
attentions in the development of Pt-based nanoelectrocatalysts
with controllable morphology. (1) 3D dendritic nanoparticles
(NPs) have shown the good potential for reducing the Pt
consumption, providing high surface area, and facilitating
enhanced performance in catalytic applications.8 For instance,
Yamauchi et al.8a reported dendritic NPs could be synthesized
in high yield via a block copolymer mediated reduction process.
The morphologies of the resulting products were
investigated by scanning electron microscopy (SEM) and
The as-prepared dendritic NPs gave a surface area of 56 m2 gÀ1
,
which is the highest for unsupported Pt materials. (2) It has
a State Key Laboratory of Electroanalytical Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, Jilin, 130022, China. E-mail: dongsj@ciac.jl.cn,
ekwang@ciac.jl.cn; Fax: +86-431-85689711
b Graduate School of the Chinese Academy of Sciences, Beijing,
100039, China
w Electronic supplementary information (ESI) available: Experimental
section and Fig. S1–S9. See DOI: 10.1039/b922148d
Scheme
1
Procedure to design Pt-on-Pd bimetallic NWs with
dendritic morphology.
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Fig. 1 TEM images of Te NWs (A, B) and Pd NWs (C, D).
Fig. 3 HAADF-STEM-EDS mapping images (A–C) of PBNNM.
The cross-sectional compositional line profiles of individual PBNNM.
(D) Pd-L; (E) Pt-M.
plane of face centered-cubic (fcc) Pt while the fast Fourier
transform (FFT) pattern (Fig. 2G) of the HRTEM image
(Fig. 2F, circled part) further indicates that the Pt nanobranch
is a single crystal. In addition, elemental mapping of Pd and Pt
(Fig. 3A–C) and the line profiles of the corresponding composi-
tions (Fig. 3D and E) on individual PBNNM reveal the
core–shell nanostructure of the bimetallic materials, consisting
of the Pd as a core and a complete shell of Pt.
The formation of PBNNM was further characterized by X-ray
photoelectron spectroscopy (XPS), energy dispersive X-ray
(EDX) spectroscopy and N2 adsorption–desorption. The XPS
pattern (Fig. S4A, ESIw) of the resulting Pd NWs shows
significant Pd 3d signals corresponding to the binding energy of
Pd (low Te XPS signals, data not shown), suggesting that Te
NWs have reacted with the Pd precursor through galvanic
reaction. Fig. S4B, C (ESIw) shows XPS patterns of the PBNNM.
Strong Pt 4f signals corresponding to the binding energy of Pt
(Fig. S4B, ESIw) and Pd 3d signals corresponding to the binding
energy of Pd (Fig. S4C, ESIw) are observed, indicating that Pd/Pt
hybrid NWs have been readily obtained. The EDX spectrum
(Fig. S5, ESIw) of PBNNM shows the peaks corresponding to Pt,
Pd and Si (from Si substrate), further confirming that bimetallic
Pd/Pt hybrid NWs are obtained. The N2 adsorption–desorption
isotherm (A) and corresponding pore-size distribution (B) of the
PBNNM are shown in Fig. S6 (ESIw), which gave a distribution
of pore sizes with a size range of 1.8–2.0 nm [in accordance with
the result from TEM (1.6–2.4 nm)].
Fig.
2 TEM (A–B, D–E), HAADF-STEM (C) and HRTEM
(F) images of PBNNM. FFT pattern (G) of the HRTEM image
shown in Fig. 2F (circled part).
transmission electron microscopy (TEM). Fig. 1A and B show
the typical TEM images of the as-prepared Te NWs. From the
magnified image (Fig. 1B), it is observed that these Te NWs
have the diameter of about 11 nm.
Fig. 1C and D show the TEM images of Pd NWs derived
from the Te NWs. A great number of Pd NWs with diameter
(Fig. 1D) similar to that of pristine Te NWs are obtained.
When the Pd precursor was added into the Te NW solution,
the solution color immediately changes from blue-black to
black (the color of Pd NWs), indicating that the redox reaction
between Te NWs with the Pd precursor is a fast process.
Fig. 2A–E show the typical TEM (A–B, D–E) and high-angle
annular dark-field scanning TEM (HAADF-STEM, C)
images of PBNNM at different magnifications. From the
magnified image (Fig. 2D), it is observed that the as-obtained
bimetallic NWs show a much rougher surface than the Pd
NWs, indicating Pt has grown onto the surface of Pd NWs
(Pd and Pt have a lattice mismatch of only 0.77%).8b Particularly,
many Pt nanobranches had grown from a Pd NW core,
coalesced together and interconnected, forming dendritic tendrils
with nanoporous structures (Fig. 2E and Fig. S1, ESIw).
Further, the SEM image of PBNNM (Fig. S2, ESIw) suggests
the as-prepared PBNNM with lengths 410 mm possess high
purity. In order to further reveal the detailed structure of
PBNNM, the HRTEM image of one part from a typical NW
is shown in Fig. S3, ESIw which clearly shows overgrowth of
Pt branches at multiple sites on the Pd NW. An HRTEM
image of an individual Pt nanobranch (Fig. 2F) shows its
single crystalline structure with highly ordered, continuous
fringe patterns. The measured interplanar spacing for the
lattice fringes is 0.23 nm, which corresponds to the (111) lattice
Interestingly, the density of Pt nanobranches on Pd NWs
could be controlled via simply changing the molar ratio of Pd
NWs to Pt precursor. For instance, Pt-on-Pd bimetallic NWs
with fewer Pt branches could be obtained when 1 mL of
Pt precursor was added into the mixture instead of 2 mL
(Fig. S7A, B, ESIw). Continuously decreasing the amount of
Pt precursor to 0.5 mL, the density of Pt nanobranches could
further decrease (Fig. S7C, D, ESIw).
Inspired by their attractive dendritic structure and special
property (good solubility in water, Fig. S8, ESIw), the
PBNNM were tested as nanoelectrocatalysts for studying their
catalytic activity. By using hydrogen adsorption–desorption
methods in conjunction with cyclic voltammetry (CV), the
electrochemical surface area (ECSA) of PBNNM and E-TEK
Pt/C catalyst (20 wt%) were measured (see Fig. S9, ESIw). It is
found that the ECSA value of PBNNM (90.7 m2 gÀ1) is higher
than those of E-TEK catalyst (53.5 m2
g
À1) and recent
state-of-art nanomaterials such as Pd–Pt bimetallic nanodendrite
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significant increase in the grain boundaries between Pt NPs, thus
generating discontinuities in the crystal planes of interconnected
Pt NPs and providing a large number of defect sites.8b,9
In summary, we have developed a facile wet-chemical
procedure to synthesize ultralong Pt-on-Pd bimetallic NWs
with nanoporous surface using thin Pd NWs as a support.
Many small single crystalline Pt nanobranches interweave with
each other and thus form many nanoporous structures, which
exhibit an enlarged ECSA and enhanced electrocatalytic
performance. The new hybrid nanostructure is scientifically
interesting and may also find use as catalysts beyond fuel cell
applications.
Fig. 4 (A) CVs (A) of E-TEK catalyst (trace a) and PBNNM (trace b)
modified GC electrodes at a scan rate of 50 mV sÀ1. (B) Transient
current of E-TEK catalyst (trace a) and PBNNM (trace b) for methanol
electrooxidation at 0.50 V in 0.5 M H2SO4–0.5 M CH3OH aqueous
solution. (C) Potential-dependent steady-state current (recorded at
120 s) of methanol electrooxidation on the E-TEK catalyst (trace a)
and PBNNM (trace b). The loading amounts of Pt are 55.5 and
70.7 mg cmÀ2 for PBNNM and E-TEK catalyst, respectively.
This work was supported by the National Natural Science
Foundation of China (No. 20820102037) and the 973 Project
(2009CB930100 and 2010CB933600).
(57.1 m2 gÀ1),8b carbon nanotube (CNT)/ionic liquid/Pt NPs
hybrids (71.4 m2
g
À1),10a CNT/Pt hybrids (44 m2
g
À1),10b
mesoporous Pt with giant mesocages (74 m2
g
À1),10c and
dendritic Pt NPs (56 m2 gÀ1),8a etc., most likely a result of the
particular structure of the bimetallic Pd–Pt NWs. Furthermore,
the surface roughness factors (50.3) on bimetallic Pd–Pt NWs
modified glassy carbon (GC, 3 mm) electrode is higher than that
of E-TEK catalyst (29.7) (indexed to the same amount of Pt).
Therefore, for the present PBNNM, the combination of their
high surface area and surface roughness with the complex
nanoarchitectures is advantageous for catalytic applications.
Methanol was selected as a model molecule for studying the
electrocatalytic performance of PBNNM. Fig. 4A shows the CVs
of E-TEK catalyst (trace a) and PBNNM (trace b) modified GC
electrodes in 0.5 M H2SO4 solution containing 0.5 M methanol.
Relative to the commercial E-TEK catalyst, a significant
enhancement of the peak current and a negative shift of the
onset potential of methanol oxidation can be observed on
PBNNM. It should be noted that the mass activity of PBNNM
is also higher than those of recent state-of-art Pt-based
nanomaterials such as carbon nanofibers or CNTs supported
Pt NPs,11 CNTs/ionic liquid/Pt NPs hybrids,10a CNT/Pt
composite catalysts,12 polyaniline/Pt NPs hybrids,13 and
graphene/Pt NPs hybrid,14 etc. (indexing to 0.5 M methanol).
Chronoamperometry, a useful method for the evaluation of
the electrocatalysts in fuel cells,15 was employed to further
investigate the electrochemical performance of PBNNM, and
a typical result is shown in Fig. 4B. We also investigated
several potentials and the dependence between the steady-state
current and the potential is plotted in Fig. 4C. These results
suggest that the PBNNM exhibit better performance for
methanol electrooxidation than the E-TEK catalyst for all
applied potentials.
Notes and references
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The higher activity in electrochemical performance observed
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(i) Bimetallic NWs with dendritic and nanoporous structure
could provide high ECSA and surface roughness, thus leading
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E-TEK electrocatalysts with low Pt NPs interconnectivity, the Pt
NPs exist as isolated particles, which are likely to impose a high
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increased interconnectivity of Pt NPs could also lead to the
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Chem. Commun., 2010, 46, 1869–1871 | 1871