Gram-scale synthesis of multipod Pd nanocrystals by a simple solid-liquid phase reaction and their remarkable electrocatalytic properties

Article abstract of DOI:10.1002/ejic.201200236

In this paper, we describe how multipod Pd nanocrystals (NCs) have been synthesized on the gram scale by means of a simple solid-liquid phase reaction route (i.e., thermal reduction of solid Pd(CH₃COO)₂ in the liquid mixture of dodecylamine, oleic acid, and 1-octadecene under a temperature-programmed mode). The nanostructure evolves from the initially generated larger polyhedral NCs into smaller ones and then into the final multipods. The dodecylamine acts as both a mild reductant and a promoter, which affects the reduction rate and decreases the size of the initially formed polyhedral PdNCs. The morphology of the final NCs, such as tri- and tetrapods, may be determined by the number of growing points on each polyhedral NC. According to the temperature- and time-dependent experiments, a multistep growth mechanism including digestive ripening, oriented attachment, and fusion process is proposed. This simple solid-liquid phase reaction route can be extended to prepare other multipod metal nanostructures. The multipod PdNCs are found to have a high electrochemically active surface area and possess excellent electrocatalytic performance toward the oxidation of formic acid. Relative to that of polyhedral Pd and commercial Pd black catalysts, the multipod PdNCs exhibit much higher catalytic activity and long-term stability, which may make them a good candidate catalyst for direct formic acid fuel cells. This developed synthetic strategy together with the provided fundamental understanding of heterogeneous nucleation and growth has great potential for contriving a rational route to the preparation of advanced nanomaterials with specific morphology for catalytic and other functional applications. Multipod Pd nanocrystals were synthesized on the gram scale by means of a simple solid-liquid phase reaction route. A multistep formation mechanism that includes digestive ripening, oriented attachment, and a fusion process is proposed. This Pd nanostructure exhibits excellent electrocatalytic activity and long-term stability toward the oxidation of formic acid. Copyright

Full text of DOI:10.1002/ejic.201200236

Job/Unit: I20236
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Date: 26-06-12 17:36:01
Pages: 8
FULL PAPER
DOI: 10.1002/ejic.201200236
Gram-Scale Synthesis of Multipod Pd Nanocrystals by a Simple Solid–Liquid
Phase Reaction and Their Remarkable Electrocatalytic Properties
[
a]
[a]
[a]
[a]
[a]
[a]
Suli Liu, Min Han, Yi Shi, Chengzhi Zhang, Yu Chen, Jianchun Bao,* and
Zhihui Dai*[
a]
Keywords: Palladium / Nanostructures / Electrocatalysis / Reaction mechanisms / Oxidation
In this paper, we describe how multipod Pd nanocrystals
NCs) have been synthesized on the gram scale by means of
a simple solid–liquid phase reaction route (i.e., thermal re-
duction of solid Pd(CH COO) in the liquid mixture of dodec-
ylamine, oleic acid, and 1-octadecene under a temperature-
programmed mode). The nanostructure evolves from the ini-
tially generated larger polyhedral NCs into smaller ones and
then into the final multipods. The dodecylamine acts as both
a mild reductant and a promoter, which affects the reduction
rate and decreases the size of the initially formed polyhedral
PdNCs. The morphology of the final NCs, such as tri- and
tetrapods, may be determined by the number of growing
points on each polyhedral NC. According to the temperature-
ment, and fusion process is proposed. This simple solid–
liquid phase reaction route can be extended to prepare other
multipod metal nanostructures. The multipod PdNCs are
found to have a high electrochemically active surface area
and possess excellent electrocatalytic performance toward
the oxidation of formic acid. Relative to that of polyhedral
Pd and commercial Pd black catalysts, the multipod PdNCs
exhibit much higher catalytic activity and long-term stability,
which may make them a good candidate catalyst for direct
formic acid fuel cells. This developed synthetic strategy to-
gether with the provided fundamental understanding of
heterogeneous nucleation and growth has great potential for
contriving a rational route to the preparation of advanced
nanomaterials with specific morphology for catalytic and
other functional applications.
(
3
2
and time-dependent experiments,
a multistep growth
mechanism including digestive ripening, oriented attach-
nanoscale devices.[13] To date, many strategies have been de-
veloped to fabricate various branched metal nanostructures
Introduction
Over the past decade, nanostructured materials, and es-
pecially metal nanomaterials, have attracted considerable
attention because their fascinating properties can be effec-
[14,15]
[16]
[17,18]
[19,20]
[21]
including Pt,
Pd,
Au,
Ag,
Rh,
and so
on. A few mechanisms, such as burst nucleation and burst
growth, and self-aggregation-based nanocrystal growth,
have been proposed to explain the formation of NCs. De-
spite this success, exploiting a simple approach to the fabri-
cation of branched metal NCs remains a challenging task,
and the formation mechanism needs to be further iden-
tified. Recently, heterogeneous seeded growth has been used
[
1–12]
tively tuned by rationally tailoring their morphologies.
For example, the position of the transverse resonance peak
of Ag nanobars (from light polarized along the short axis)
stays in the blue region of the visible spectrum. In contrast,
their longitudinal resonance peak shifts from the visible to
[
2]
the near-infrared as the nanobar aspect ratio increases.
as a powerful tool for controlling the morphology of bime-
Also, multioctahedral Pt nanocrystals exhibit improved spe-
cific activity and durability relative to a commercial Pt/C
catalyst for the oxygen reduction reaction.[
[22–24]
tallic nanostructures;
however, reports on the synthe-
sis of monometallic NCs by means of the heterogeneous
6]
[25–27]
reaction route are rare.
Furthermore, it is noted that,
Among the various possible morphologies that can be
taken by metal nanocrystals (NCs) with a face-centered cu-
bic (fcc) phase, branched morphologies are an exciting new
class of nanostructures on account of their unique struc-
tures, physicochemical properties, and their great potential
as catalysts, sensing materials, and building blocks for
in most of the syntheses that have been reported so far, only
sub-gram quantities of metal NCs have been synthesized.
For metal NCs to be used in a wide range of practical appli-
cations, it is necessary to develop a facile and economical
mass production method with controllable morphology.
In this work, multipod PdNCs have been synthesized on
the gram scale by means of a simple solid–liquid phase re-
action route. By designing the heterogeneous-phase reac-
tion model and adjusting the reaction temperature, we have
been able to control the growth and assembly to form
multipod PdNCs. Based on the temperature- and time-de-
pendent experimental results, a multistep formation mecha-
[
a] Jiangsu Key Laboratory of Biofunctional Materials, School of
Chemistry and Materials Science, Nanjing Normal University,
Nanjing 210097, P. R. China
Fax: +86-25-85891767
E-mail: baojianchun@njnu.edu.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200236.
Eur. J. Inorg. Chem. 0000, 0–0
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S. Liu, M. Han, Y. Shi, C. Zhang, Y. Chen, J. Bao, Z. Dai
FULL PAPER
nism (i.e., “digestive ripening,” the oriented attachment and pod length is mainly in the range of 8–10 nm. Further infor-
fusion process) is proposed. Additionally, the multipod mation on the microstructure comes from the HRTEM
PdNCs are used as electrocatalysts for formic acid oxi- analysis (Figure 2, B). It shows that the multipod Pd struc-
dation. In comparison to the polyhedral and commercial ture is composed of associated quasi-spherical particles
Pd black catalysts, the multipod PdNCs possess a high elec- with lattice planes in between [marked with arrows (a) in
trochemically active surface area and excellent electrocata- Figure 2, B]. The lattice spacing was measured to be 2.30 Å,
lytic activity toward formic acid oxidation, which shows which corresponds to the interplanar separation between
their great potential for direct formic acid fuel cells (111) lattice planes of the fcc-phase Pd. In addition, some
(DFAFCs).
defects exposed on the particle surface were also observed
marked with white circles in Figure 2, B; the magnified pic-
(
ture is shown in Figure S1 in the Supporting Information),
which might be one of the important factors in their cata-
lytic properties. The FTIR spectrum (Figure S2 in the Sup-
porting Information) results reveal that the surfaces of the
multipod PdNCs are covered with some organic ligands.
Corresponding thermogravimetric (TG) analysis (Figure S3
in the Supporting Information) demonstrates that the
weight percentage of the adsorbed surface ligands on
multipod PdNCs is about 6%.
Results and Discussion
Characterization of the Multipod PdNCs
Figure 1 (A) shows the XRD pattern of the Pd prepared
under the typical reaction conditions. The diffraction peaks
in the range of 10 Ͻ 2θ Ͻ 90° can be indexed as (111),
(200), (220), (311), and (222) planes of fcc-phase Pd, and
the lattice parameter is a = 3.89 Å, which are all in good
accordance with the American Society for Testing and Ma-
terials (ASTM) standard 65-2867. Energy-dispersive X-ray
spectroscopy (EDS) analysis (Figure 1, B) shows that the
sample is composed of Pd, C, and Cu. The observed C ele-
ment might come from the adsorbed organic ligands or the
carbon film on the TEM grid, and the Cu element origi-
nates from the TEM grid. TEM and HRTEM images of
the Pd are shown in parts A and B of Figure 2. Part A of
this figure shows that the Pd particles are multipod-shaped.
The average diameter of the pod is about 4.6 nm and the
Figure 2. (A) TEM and (B) HRTEM images of multipod PdNCs.
Growth Process and Formation Mechanism of the Multipod
PdNCs
Different reaction routes such as homogeneous/hetero-
geneous, or aqueous/hydrophobic media, and even the reac-
[
28]
tion sequence,
will lead to different reduction kinetics
and formation mechanism, thus obtaining the nanostruc-
tures with various morphologies. In most methods for metal
NCs prepared in organic-medium-based media, metal pre-
cursors are often completely dissolved and the reaction sys-
tems are homogeneous. The majority of growth mecha-
nisms abide by the classical mode (i.e., burst nucleation and
burst growth). Li and co-workers unveiled a three-step “in-
termediate-formation/nucleation/growth” mechanism to ex-
plain the oleylamine-mediated shape evolution of PdNCs in
[
29]
toluene.
agent, dodecylamine (DDA), and a large amount of solid
Pd(CH COO) were used. Due to its poor solubility in the
In the present case, a relatively weak reducing
Figure 1. (A) XRD and (B) EDS patterns of the PdNCs synthe-
sized under the typical reaction conditions.
3
2
mixed solvent, it is a solid–liquid phase reaction system. In
2
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Gram-Scale Synthesis of Multipod Pd Nanocrystals
such a heterogeneous-phase system, because the undis- ported in some previous literature.[30,31] For example, Chen
solved solid precursor provides nucleation centers or sites, et al. reported that the size of the Ag particles prepared
the nucleation barrier can be reduced relative to that of a using AgNO as a silver source in the oleylamine-liquid
3
homogeneous-phase system, thus leading to the relatively paraffin system increased as the temperature was increased
[
31]
easy nucleation. The reaction phenomena elucidate this hy- from 100 to 140 °C. The reason why the size of the par-
pothesis. At room temperature, the solid precursor is de- ticles decreases with the increase in the reaction tempera-
posited at the bottom of the reactor and the liquid layer is ture in the present case might be due to digestive ripening/
–1
colorless. When the reactor is heated to 60 °C (6 °Cmin ), oxidative etching. In our experiment, the surfactant is
the color of the surface of the solid gradually changes from DDA, which is in an excess amount and is beneficial to the
reddish brown to black gradually and emits a few bubbles, “digestive ripening” process.[
32–34]
This also indicates that
thereby indicating that the solid Pd(CH COO) is reduced the present solid–liquid reaction and cool injection have a
3
2
by DDA to form Pd particles. Upon further increasing the clear effect on the size and morphology of the nanostruc-
reaction temperature, the solid Pd(CH COO) is continu- tures. (ii) These results imply that the multipod structure is
3
2
ously reduced by DDA. To understand the formation pro- formed, which results from the following ripening process.
cess of the branched Pd nanostructure better, the intermedi- This is different from the case of Pt multipods prepared
ates produced at different reaction temperatures were inves- from 2,4-pentanedionate in organic solvent. The higher the
tigated by TEM. Figure 3 shows the morphological and size reaction temperatures are, the earlier Pt multipods form and
evolution of Pd particles with the reaction temperature. As the sooner they evolve into spheres.[35]
the reaction temperature was raised to 70 °C, polyhedral Pd
To gain further insight into the formation process of the
particles with the size mainly in the range of 5–12 nm were multipod PdNCs, the other two samples were examined af-
formed (Figure 3, A). When the temperature reached ter ripening at 150 °C for 2 and 6 min, respectively. The
120 °C, those large nanoparticles evolved into small, irregu- corresponding TEM images are shown in Figure 4. Com-
lar PdNCs 5–8 nm in size (Figure 3, B). Upon further in- pared with the case in Figure 3 (C), after ripening for about
creasing the temperature to 150 °C, the size of the Pd nano- 2 min, some branched embryonic particles began to appear
particles became close to 5 nm with a relatively narrow size (Figure 4, A). Upon further ripening for about 6 min, the
distribution, and the most regular polyhedral particles population of the branched particles increased. When rip-
(tetrahedral) were formed (Figure 3, C). The average size is ening for 20 min, the branched particles (tri- and tetrapods)
about 5.2 nm. Figure 3 (D) shows the HRTEM analysis of shown in Figure 2 (A) were obtained. It was also found that
the polyhedral Pd particles shown in Figure 3 (C). The after ripening for 1 or 2 h, the size and morphology of the
measured d spacing corresponds to 0.23 nm, which can be Pd did not change further. In addition, although the poly-
assigned to the fcc Pd (111) plane. There are two points hedral particle evolved into the branched nanostructure, the
that should be noted for this part of the reaction. (i) The diameter of branches was similar to that of the polyhedral
size change with temperature is different from the cases re-
Figure 3. TEM images of PdNCs when the reaction temperature is
A) 70, (B) 120, and (C) 150 °C. (D) HRTEM image of the PdNCs
in (C).
Figure 4. TEM images of the Pd particles after ripening at 150 °C
for (A) 2 and (B) 6 min, respectively; (C) HRTEM image of the
particles after ripening for 2 min at 150 °C.
(
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S. Liu, M. Han, Y. Shi, C. Zhang, Y. Chen, J. Bao, Z. Dai
FULL PAPER
particles, thereby implying that the formation of the tion. In the present strategy, DDA is used as a reducing
multipod might be the oriented-attachment process (see dis- reagent. When DDA is substituted with oleylamine, the re-
cussion below).
action also leads to the formation of a multipod Pd nano-
Crystal structure anisotropy is considered to be the main structure, thus indicating that such long-chain amines are
driving force for the anisotropic growth of nanostructures. beneficial to the formation of the branched structure. When
For fcc metals, polyhedra are widely accepted as thermody- using terpineol instead of DDA, flocculated Pd particle ag-
namically stable morphologies, so this multipod Pd struc- gregates composed of small-sized particles of about 6 nm
ture is relatively unexpected. The formation of anisotropic are obtained (Figure 5, A). The diameter of Pd aggregates
morphologies from fcc metals generally requires (i) kinet- is mainly in the range of 100–130 nm. This might result
ically controlled growth conditions, (ii) twinning in the from the relatively fast reaction rate due to the stronger
nanoparticle nuclei, and (iii) the selective binding or non- reducibility of terpineol than that of DDA and the structure
binding of surfactant molecules to different facets of the of terpineol. In addition, we also find that the use of oleic
[36–41]
growing nanocrystal to lead to the anisotropy.
For ex- acid (OA) is important in obtaining branched Pd nanos-
ample, Pt tetrapods and octapods were obtained in the po- tructures with good dispersivity but it is not essential for
lyol process through the manipulation of reduction kinet- the formation of the multipod particles. The reaction in the
ics.[
40]
Icosahedral nuclei were used to prepare podlike absence of OA also gives multipod particles, but they are a
[42]
structures in Pt and Pd. Recently, wormlike PdNCs were little aggregated (Figure 5, B), thereby suggesting that the
prepared by reverse micelles or phase-transfer reactions.
[
43]
main role of OA is to protect and disperse the Pd particles.
It is considered that, when using DDA as the stabilizing/ This is different from the case reported by Tilley and co-
coating agent, the formation of wormlike particles is not workers, in which a podlike Pd nanostructure was obtained
related to the synthesis modes but mainly to the weak at- by using only a surfactant mixture of 1:1 OLA and OA at
[
50]
tachment of the coating agent, it being a diffusion-limited room temperature. Such differences might result from the
aggregation. This is an indication that the factor that affects variation of reaction conditions such as temperature and
the anisotropic growth is not simply related to the three reaction mode (solid–liquid phase reaction), thereby lead-
types of conditions mentioned above. The primary nano- ing to the different reaction kinetics.
particle-to-wire or -nanoring transformation has been ob-
served for the oriented growth of some semiconductor NCs,
[
44–48]
in which the dipole moment is the driving force.
Though metals are not expected to form long-lived dipoles
on their surface, the formation of metal nanowires based on
the oriented-attachment mechanism is also observed. For
instance, Au nanowires were obtained through oriented at-
tachment by reacting chloroauric acid in oleylamine (OLA)
under reflux conditions.[ In the present case, the forma-
tion of the multipod PdNCs from a polyhedral Pd nano-
structure should be interpreted as an oriented-attachment
process, which is further confirmed by the HRTEM analy-
sis. Figure 4 (C) shows the HRTEM image of the particles
after ripening for 2 min at 150 °C. The growth is observed
at multiple points on each polyhedral NC, most at the cor-
ners (marked with letters a, b, and c in Figure 4, C), thereby
resulting in the podlike structure. Such assertive evidence
for growth of multipod PdNCs has rarely been reported.
This is rather similar to the Pt growth reported in the work
of Sun and co-workers, in which secondary Pt grew at the Figure 5. TEM images of Pd prepared (A) using terpineol instead
49]
[9]
cube vertices to form the nanopods. It also implies that of DDA and keeping other typical synthesis conditions constant,
and (B) in the absence of OA and keeping other typical synthesis
conditions constant.
the morphology of the final NCs, such as tri- and tetrapods,
might be determined by the number of growing points on
each polyhedral intermediate.
On the basis of the experimental results and analysis
It is worth noting that this simple solid–liquid phase re-
mentioned above, we can be sure that the Pd nanostructure action route can be extended to prepare other branched
evolves from the initially generated larger polyhedral NCs metal nanostructures. For example, when using Ni-
into smaller ones and then into the final multipods. A (HCOO) instead of Pd(CH COO) , a temperature around
2
3
2
multistep growth mechanism that includes digestive ripen- 250 °C, and keeping other reaction conditions constant as
ing, an oriented attachment, and a fusion process can be in the case for multipod PdNCs, the complicated hierarchi-
proposed to explain the formation of multipod PdNCs.
cal Ni nanostructures assembled by small-sized multipod
In addition, several control experiments were carried out NiNCs (Figure 6) were obtained at a yield of 90%. To see
to learn additional information about the reduction reac- clearly, the corresponding field-emission scanning electron
4
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Gram-Scale Synthesis of Multipod Pd Nanocrystals
microscopy (FESEM) images of the hierarchical Ni nano-
structures are shown in Figure S4 in the Supporting Infor-
mation.
Figure 7. CVs of (a) multipod Pd, (b) polyhedral Pd, and (c) com-
4
mercial Pd black catalysts in 0.1 m HClO solution at the rate of
Figure 6. TEM image of the hierarchical Ni nanostructures stacked
by small-sized multipod NiNCs.
–1
50 mVs .
nomenon has been observed by Lee et al. in the electrooxid-
ation of formic acid when using carbon-supported Pd nano-
particles as catalysts.
Catalytic Properties
[57]
Determination of the Electrochemically Active Surface Area
of Pd Catalysts
Electrocatalytic Activity of Catalysts for HCOOH
Oxidation
The multipod PdNCs were tested as electrocatalysts for
formic acid oxidation. For comparison, the electrocatalytic
activities of polyhedral Pd nanostructures (approx. 5.2 nm)
and commercial Pd black catalyst (Ͼ20 nm, Figure S5 in
the Supporting Information) were also measured. Firstly,
the electrochemically active surface areas (ECASA) of those
three kinds of Pd catalysts were calculated from the hydro-
Figure 8 (A) displays the CV plots of those three kinds
of Pd catalysts in 0.5 m HCOOH + 0.1 m HClO solution
4
–1
at a rate of 50 mVs . With its remarkably high ECASA,
the multipod PdNC-modified electrode exhibits a high mass
activity in the electrocatalytic oxidation of formic acid. In
the positive scan direction, the main oxidation peaks of for-
mic acid by multipod PdNCs, polyhedral Pd nanostruc-
tures, and commercial Pd catalysts are located at 0.17, 0.19,
and 0.23 V, respectively, which correspond to the direct oxi-
dation of formic acid. The corresponding main oxidation
gen-adsorption/-desorption cyclic voltammograms (CV) on
Pd-catalyst surfaces.[
51–54]
Figure 7 shows their CV curves
recorded in 0.1 m HClO solution without formic acid. The
4
peak between –0.25 and 0 V can be ascribed to the adsorp-
tion/desorption of hydrogen onto the Pd surface, as well
as adsorption of a small fraction of hydrogen into the Pd
peak currents on those three kinds of Pd catalysts are 170,
–1
5
0, and 90 mAmg Pd, respectively. Meanwhile, their onset
oxidation potential for formic acid is located near –0.15,
0.13, and –0.10 V, respectively. The largest oxidation peak
[
28,55,56]
lattice.
A well-defined cathodic peak at about 0.35 V
–
is attributed to the reduction of Pd oxide formed in the
positive potential scan. According to the Coulombic
amount (Q) associated with the reduction peak area of
current and the lowest onset oxidation potential suggest
that the multipod Pd exhibits significantly better electrocat-
alytic performance than those of polyhedral Pd nanostruc-
tures and commercial Pd catalysts. To evaluate the long-
term stability of catalysts, chronoamperometry tests were
conducted. Figure 8 (B) shows their chronoamperometric
Pd(OH) , the ECASA values of those three kinds of Pd
2
catalysts are estimated from Equation (1).
ECASA = Q/(m·C)
(1)
curves in 0.5 m HCOOH + 0.1 m HClO solution at a po-
4
C is the reduction charge for a unit area of monolayer tential of 0.15 V. After 3000 s measurements, their oxidation
–
2
–1
Pd nanocrystal on the Pd surface (420 μCcm ), and m is peak currents were 31.68, 5.05, and 6.72 mAmg Pd,
the mass of Pd on the catalyst surface. By calculation, the respectively, thereby revealing that the durability of
ECASA of multipod PdNCs, polyhedral Pd nanostructures, multipod PdNC catalyst is higher than those of the other
2
–1
and commercial Pd catalysts are 21.05, 2.90, 1.67 m g , two Pd catalysts. The remarkable enhancement of electroca-
respectively. The largest electrochemical specific surface for talytic performance in a multipod PdNC catalyst toward
multipod PdNCs catalyst might be attributed to its special formic acid oxidation might be attributed to the following
structure (i.e., the way the pods are spatially separated from two reasons. (i) The ECASA of the multipod Pd catalyst is
each other) and small particle size (4.6 nm) of each pod. much higher than that of the polyhedral Pd and commercial
Because the multipod PdNCs possess the largest ECASA, Pd catalysts due to the way the multipods are spatially sepa-
they have the strongest affinity for –OHads species generated rated from each other, thereby leading to the highly favor-
in the positive potential scan, thereby letting their reduction able conditions for maximizing Pd surface area and surface
peak shift to a more negative potential than that of polyhe- permeability,[
29,57,58]
whereas polyhedral Pd and commer-
dral PdNCs and commercial Pd catalyst. A similar phe- cial Pd are easily found in a highly agglomerated form. (ii)
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The particular morphology of multipod Pd composed of Experimental Section
small Pd nanoparticles results in the presence of a relatively
Synthesis of Multipod PdNCs: All chemicals used in this work were
used as received. In a typical synthesis, a solid sample of
Pd(CH COO) (2.5 g), 1-octadecene (ODE) (10 mL), dode-
high density of defects, which provides more active sites for
formic acid oxidation and enhances the rate of the formic
3
2
[
7,59,60]
acid oxidation.
cylamine (DDA) (8 mL), and oleic acid (OA) (0.5 mL) were added
to a 250 mL three-necked flask at room temperature. Subsequently,
–
1
the reactor was heated to 150 °C at a rate of 6 °Cmin and kept
at that temperature for 30 min. Finally, the reactor was naturally
cooled to 30 °C. The crude product was separated and redispersed
in heptane or hexane. Upon adding absolute ethanol (50 mL) into
the dispersed solution, a large amount of precipitates was pro-
duced, which was separated by centrifugation and washed with
heptane and absolute ethanol 2 to 3 times to remove byproducts.
Finally, the precipitate was dried in vacuo for 4 h and used for
further characterization and analysis. The yield of the multipod
PdNCs was about 95% (the adsorbed surface ligands are not de-
ducted). Thus, the detailed amount of multipod PdNCs that can
be synthesized per batch is about 1.13 g.
Physical Measurements: X-ray diffraction (XRD) patterns were re-
corded on a powder sample with a D/max 2500 VL/PC dif-
fractometer (Japan) equipped with graphite-monochromated Cu-
α
K radiation (λ = 1.54060 Å) with 2θ ranging from 10 to 90°. The
corresponding work voltage and current were 40 kV and 100 mA,
respectively. MDI Jade 5.0 software was used to deal with the ac-
quired diffraction data. The TEM images were taken with a JEM-
200CX instrument (Japan) using an accelerating voltage of 200 kV.
The HRTEM images were recorded with a JEOL-2100 apparatus
at an accelerating voltage of 200 kV. The FTIR spectra were re-
corded with a NEXUS670 infrared spectrometer (USA). The ther-
mogravimetric analysis was performed with a Perkin–Elmer TG/
DTA synchronous thermoanalysis instrument. FESEM images
were recorded with an S-4800 scanning electron microscope that
operated at an accelerating voltage of 5 kV.
Figure 8. (A) CVs and (B) chronoamperometric curves of (a)
multipod Pd, (b) polyhedral Pd, and (c) commercial Pd black cata-
4
lysts in 0.5 m HCOOH + 0.1 m HClO solution.
Electrocatalytic Measurement: Electrochemical measurements were
performed in a conventional three-electrode electrochemical cell
with a CHI 600 electrochemical analyzer (CH Instruments, Shang-
hai Chenghua Co.). A Pt plate auxiliary electrode and a saturated
calomel reference electrode (SCE) were used. All potentials refer to
Conclusion
SCE. Before the electrochemical measurements, N
through the solution for 10 min to remove the dissolved O
the electrochemical experiments, a continuous N flow was main-
tained over the solution. All the electrochemical measurements
2
was bubbled
In summary, we have synthesized multipod PdNCs in
high yield by means of a developed solid–liquid phase reac-
tion route in DDA and 1-octadecene (ODE) mixed solvent.
2
. During
2
Such a simple and effective reaction route, combined with were carried out at (30Ϯ1) °C.
a varying temperature mode, enables the size and the mor-
Before the electrochemical experiments, multipod PdNCs (4 mg)
phology of NCs to be controlled and their economical mass
production to be achieved. The temperature- and time-de-
pendent experimental results show that the nanostructure
evolves from polyhedral particles to multipods with an in-
were dispersed in ethanol (2 mL) by ultrasonication for 30 min to
generate a uniform suspension. The concentration of the multipod
–1
PdNCs suspension was about 2 mgmL . The glassy carbon (GC)
electrodes (3 mm in diameter) were polished to a mirrorlike finish
crease in reaction temperature by growing on the vertices with 1.0, 0.3, and 0.05 μm alumina slurry (Beuhler) followed by
of polyhedral NCs. This can be described as the digestive rinsing thoroughly with doubly distilled water. The electrodes were
ripening, oriented attachment, and fusion process. Relative successively sonicated in 1:1 nitric acid, acetone, and doubly dis-
tilled water, and then allowed to dry at room temperature. The
resulting suspension (5.0 μL) was laid on the surface of the pre-
to the polyhedral and commercial Pd black catalysts, the
multipod PdNCs show a very high electrochemically active
cleaned GC electrode and allowed to dry at 40 °C. Finally, Nafion
surface area and a significant increase in electrocatalytic ac-
solution (3.0 μL, 5 wt.-%) was dispersed on the electrode surface
tivity toward formic acid oxidation, which makes them pref-
and dried again at 40 °C. Thus, the working electrode was fabri-
erable catalysts for DFAFCs. This facile heterogeneous re-
action method as well as the understanding of the growth
process is of great potential toward the rational synthesis of
cated, and the specific loading of metal Pd on the electrode surface
–2
was about 140 μgcm . For comparison, the polyhedral Pd nano-
structures and commercial Pd-catalyst-modified GC electrodes
advanced nanomaterials for catalytic and other functional were also prepared by using the same procedure. All the modified
applications. electrodes were pretreated by cycling the potential between –0.2
6
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Eur. J. Inorg. Chem. 0000, 0–0

Job/Unit: I20236
/KAP1
Date: 26-06-12 17:36:01
Pages: 8
Gram-Scale Synthesis of Multipod Pd Nanocrystals
and 1.2 V for several cycles in a 0.1 m HClO
electrochemical test.
4
solution prior to the
[25] M. Yamamoto, M. Nakamoto, J. Mater. Chem. 2003, 13, 2064–
2065.
[
[
[
[
26] X. Wang, J. Zhuang, Q. Peng, Y. D. Li, Nature 2005, 437, 121–
24.
27] Y.-H. Chen, H.-H. Hung, M. H. Huang, J. Am. Chem. Soc.
009, 131, 9114–9121.
28] Y. W. Lee, M. Kim, S. W. Han, Chem. Commun. 2010, 46,
535–1537.
1
Supporting Information (see footnote on the first page of this arti-
cle): HRTEM image, FT-IR spectrum, and thermogravimetric plot
of multipod Pd NCs; FE-SEM images of hierarchical Ni nano-
structures and TEM image of commercial Pd black catalyst.
2
1
29] Z. Q. Niu, Q. Peng, M. Gong, H. P. Rong, Y. D. Li, Angew.
Chem. 2011, 123, 6439; Angew. Chem. Int. Ed. 2011, 50, 6315–
6
319.
30] V. Mazumder, S. H. Sun, J. Am. Chem. Soc. 2009, 131, 4588–
589.
Acknowledgments
[
4
This work was supported by the National Natural Science Founda-
tion of China (NSFC) under the research projects 20901041,
[31] M. Chen, Y. G. Feng, X. Wang, T. C. Li, J. Y. Zhang, D. J.
Qian, Langmuir 2007, 23, 5296–5304.
2
1175069, and 21171096, the program for New Century Excellent
Talents in University of Chinese Ministry of Education (NCET-
9-0159), the Priority Academic Program Development of Jiangsu
[32] A. C. S. Samia, J. A. Schlueter, J. S. Jiang, S. D. Bader, C. J.
Qin, X. M. Lin, Chem. Mater. 2006, 18, 5203–5212.
[
33] A. C. S. Samia, K. Hyzer, J. A. Schlueter, C. L. Qin, J. S. Jiang,
0
S. D. Bader, X. M. Lin, J. Am. Chem. Soc. 2005, 127, 4126–
Higher Education Institutions, and the Program for Outstanding
Innovation Research Team of Universities in Jiangsu Province
4
127.
[
34] B. L. V. Prasad, S. I. Stoeva, C. M. Sorensen, K. J. Klabunde,
Chem. Mater. 2003, 15, 935–942.
(11KJA150003).
[
[
35] X. W. Teng, H. Yang, Nano Lett. 2005, 5, 885–891.
36] J. Watt, N. Young, S. Haigh, A. Kirkland, R. D. Tilley, Adv.
Mater. 2009, 21, 2288–2293.
[
[
1] T. K. Sau, A. L. Rogach, Adv. Mater. 2010, 22, 1781–1804.
2] B. Wiley, Y. G. Sun, Y. N. Xia, Acc. Chem. Res. 2007, 40, 1067–
[37] L. Y. Ruan, C. Y. Chiu, Y. J. Li, Y. Huang, Nano Lett. 2011,
1076.
11, 3040–3046.
[
3] A. Kisner, M. Heggen, E. Fernández, S. Lenk, D. Mayer, U.
[38] S. Maksimuk, X. W. Teng, H. Yang, Phys. Chem. Chem. Phys.
2006, 8, 4660–4663.
[39] X. W. Lou, C. L. Yuan, L. A. Archer, Chem. Mater. 2006, 18,
3921–3923.
Simon, A. Offenhäusser, Y. Mourzina, Chem. Eur. J. 2011, 17,
9503–9507.
[4] S. Shanmugam, U. Simon, Chem. Eur. J. 2008, 14, 8776–8779.
[5] D. S. Wang, Q. Peng, Y. D. Li, Nano Res. 2010, 3, 574–580.
[6] B. W. Lim, X. M. Lu, M. J. Jiang, P. H. C. Camargo, E. C.
Cho, E. P. Lee, Y. N. Xia, Nano Lett. 2008, 8, 4043–4047.
[40] J. Y. Chen, T. Herricks, Y. N. Xia, Angew. Chem. 2005, 117,
2645; Angew. Chem. Int. Ed. 2005, 44, 2589–2592.
[41] S. H. Chen, Z. L. Wang, J. Ballato, S. H. Foulger, D. L. Carroll,
J. Am. Chem. Soc. 2003, 125, 16186–16187.
[
[
[
[
[
7] L. Wang, H. J. Wang, Y. Nemoto, Y. Yamauchi, Chem. Mater.
2010, 22, 2835–2841.
[42] S. Maksimuk, X. Teng, H. Yang, J. Phys. Chem. C 2007, 111,
8] A. X. Yin, X. Q. Min, W. Zhu, H. S. Wu, Y. W. Zhang, C. H.
Yan, Chem. Commun. 2012, 48, 543–545.
14312–14319.
[43] K. Naoe, C. Petit, M. P. Pileni, J. Phys. Chem. C 2007, 111,
16249–16254.
[44] Z. Y. Tang, N. A. Kotov, Adv. Mater. 2005, 17, 951–962.
[45] A. B. Panda, S. Acharya, A. Efrima, Adv. Mater. 2005, 17,
2471–2474.
[46] K.-S. Cho, D. V. Talapin, W. Gaschler, C. B. Murray, J. Am.
Chem. Soc. 2005, 127, 7140–7147.
[47] A. P. Alivisatos, Science 2000, 289, 736–737.
[48] R. L. Penn, J. F. Banfield, Science 1998, 281, 969–971.
[49] A. Halder, N. Ravishankar, Adv. Mater. 2007, 19, 1854–1858.
[50] J. Watt, S. Cheong, M. F. Toney, B. Ingham, J. Cookson, P. T.
Bishop, R. D. Tilley, ACS Nano 2010, 4, 396–402.
[51] L. Xiao, L. Zhuang, Y. Liu, J. T. Lu, H. D. Abruna, J. Am.
Chem. Soc. 2009, 131, 602–608.
[52] Z. Y. Bai, L. Yang, L. Li, J. Lv, K. Wang, J. Zhang, J. Phys.
Chem. C 2009, 113, 10568–10573.
[53] C. Batchelor-McAuley, C. E. Banks, A. O. Simm, T. G. J. Jones,
R. G. Compton, ChemPhysChem 2006, 7, 1081–1085.
[54] R. Pattabiraman, Appl. Catal. A 1997, 153, 9–20.
9] M. N. Mankin, V. Mazumder, S. H. Sun, Chem. Mater. 2011,
23, 132–136.
10] S. J. Guo, S. J. Dong, E. K. Wang, ACS Nano 2010, 4, 547–
555.
11] C. J. Murphy, L. B. Thompson, A. M. Alkilany, P. N. Sisco,
S. P. Boulos, S. T. Sivapalan, J. A. Yang, D. J. Chernak, J. Y.
Huang, J. Phys. Chem. Lett. 2010, 1, 2867–2875.
[
[
12] Q. A. Yuan, X. Wang, Nanoscale 2010, 2, 2328–2335.
13] B. Lim, Y. N. Xia, Angew. Chem. 2011, 123, 78; Angew. Chem.
Int. Ed. 2011, 50, 76–85.
[
[
[
[
[
[
[
[
[
[
[
14] X. W. Teng, X. Y. Liang, S. Maksimuk, H. Yang, Small 2006,
2, 249–253.
15] L. Wang, Y. Yamauchi, J. Am. Chem. Soc. 2009, 131, 9152–
9153.
16] J. Watt, N. Young, S. Haigh, A. Kirkland, R. D. Tilley, Adv.
Mater. 2009, 21, 2288–2293.
17] H. G. Liao, Y. X. Jiang, Z. Y. Zhou, S. P. Chen, S. G. Sun, An-
gew. Chem. 2008, 120, 9240–9243.
18] Z. Q. Li, W. Y. Li, P. H. C. Camargo, Y. N. Xia, Angew. Chem. [55] Y. Liang, Y. Zhou, J. Ma, J. Y. Zhao, Y. Chen, Y. W. Tang,
2
008, 120, 9799–9802.
19] J. Y. Chen, B. Lim, E. P. Lee, Y. N. Xia, Nano Today 2009, 4,
1–95.
T. H. Lu, Applied Catalysis B: Environ. 2011, 103, 388–396.
[56] J. L. Haan, K. M. Stafford, R. I. Masel, J. Phys. Chem. C 2010,
114, 11665–11672.
[57] W. J. Zhou, J. Y. Lee, J. Phys. Chem. C 2008, 112, 3789–3793.
[58] L. Wang, Y. Nemoto, Y. Yamauchi, J. Am. Chem. Soc. 2011,
133, 9674–9677.
[59] H. Chen, R. S. Liu, M. Y. Lo, S. C. Chang, L. D. Tsai, Y. M.
Peng, J. F. Lee, J. Phys. Chem. C 2008, 112, 7522–7526.
[60] W. P. Zhou, A. Lewera, R. Larsen, R. I. Masel, P. S. Bagus, A.
Wieckowski, J. Phys. Chem. B 2006, 110, 13393–13398.
Received: March 7, 2012
8
20] M. J. Mulvihill, X. Y. Ling, J. Henzie, P. D. Yang, J. Am. Chem.
Soc. 2010, 132, 268–274.
21] S. M. Humphrey, M. E. Grass, S. E. Habas, K. Niesz, G. A.
Somorjai, T. D. Tilley, Nano Lett. 2007, 7, 785–790.
22] S. E. Habas, H. Lee, V. Radmilovic, G. A. Somorjai, P. D.
Yang, Nat. Mater. 2007, 6, 692–697.
23] P. H. C. Camargo, Y. Xiong, L. Ji, J. M. Zuo, Y. N. Xia, J. Am.
Chem. Soc. 2007, 129, 15452–15453.
24] Z. Peng, H. Yang, J. Am. Chem. Soc. 2009, 131, 7542–7543.
Published Online: ᭿
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7

Job/Unit: I20236
/KAP1
Date: 26-06-12 17:36:01
Pages: 8
S. Liu, M. Han, Y. Shi, C. Zhang, Y. Chen, J. Bao, Z. Dai
FULL PAPER
Palladium Nanocrystals
Multipod Pd nanocrystals were synthesized
on the gram scale by means of a simple so-
lid–liquid phase reaction route. A multistep
formation mechanism that includes diges-
tive ripening, oriented attachment, and a
fusion process is proposed. This Pd nano-
structure exhibits excellent electrocatalytic
activity and long-term stability toward the
oxidation of formic acid.
S. Liu, M. Han, Y. Shi, C. Zhang,
Y. Chen, J. Bao,* Z. Dai* ................ 1–8
Gram-Scale Synthesis of Multipod Pd
Nanocrystals by a Simple Solid–Liquid
Phase Reaction and Their Remarkable
Electrocatalytic Properties
Keywords: Palladium / Nanostructures /
Electrocatalysis / Reaction mechanisms /
Oxidation
8
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