One pot synthesis of chain-like palladium nanocubes and their enhanced electrocatalytic activity for fuel-cell applications

1. Nano Energy (2013) 2, 677–687
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanoenergy
RAPID COMMUNICATION
One-pot synthesis of chain-like palladium
nanocubes and their enhanced
electrocatalytic activity for fuel-cell
applications
Palanisamy Kannana,n, Thandavarayan Maiyalaganb,nn,
Marcin Opalloa
a
Institute of Physical Chemistry, Polish Academy of Sciences, 44/52 ul. Kasprzaka, 01-224 Warszawa, Poland
Materials Science and Engineering Program, The University of Texas at Austin, Austin, TX, USA
b
Received 7 May 2013; received in revised form 10 August 2013; accepted 11 August 2013
Available online 23 August 2013
KEYWORDS
Abstract
Chain-like
nanoparticles;
Cubic shape;
Formic acid;
Methanol and ethanol
A simple approach has been developed for the synthesis of anisotropic cubic chain-like Pd nanostructures in an aqueous medium. The cubic chain-like Pd nanostructures show better performance
toward oxidation of formic acid and methanol. The cubic chain-like Pd nanostructures show $ 11.5
times more activity on the basis of an equivalent noble metal mass for the formic acid and methanol
than the spherical shaped Pd nanoparticles and commercial Pd/C catalysts. Further, the superior
electrocatalytic performance of the present anisotropic cubic chain-like Pd nanostructures toward
the oxidation of alcohols makes them excellent candidates as high performance multipurpose
catalysts for direct formic acid fuel cells (DFAFC), direct methanol fuel cells (DMFC), direct ethanol
fuel cells (DEFC), respectively.
& 2013 Elsevier Ltd. All rights reserved.
Introduction
Controlling the morphology of noble metal nanocrystals has
been one of the most recent and attractive research topics
n
Corresponding author. Tel.: +48 223 433 375;
fax: +48 223 433 333.
nn
Corresponding author. Tel.: +1 512 772 3084
E-mail addresses: ktpkannan@gmail.com (P. Kannan),
maiyalagan@gmail.com (T. Maiyalagan).
2211-2855/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.nanoen.2013.08.001
for the past few decades, especially in the field of catalysis,
because the catalytic activity and stability of catalysts
are strongly correlated with the size and shape of the nanocrystals in a variety of chemical reactions [1–3]. It has been
demonstrated that the intrinsic properties of nanostructured material can be dramatically enhanced by shape and
structural variation [4–6]. The one dimensional metal nanostructures like nanowires, nanobelts, and nanotubes have
attracted significant interest due to their exotic technological
applications [7–10]. The surface roughness and inherent high

2. 678
index facets of anisotropic metal nanoparticles stimulate
impressive applications in electro-catalysis, sensors, SERS
and many more [11–15]. At present, Pt is still the most
commonly used electrocatalytic material for fuel cells in
terms of both activity and stability. However, the high
cost of Pt is one of the most important barriers that
limits the large-scale commercialization of fuel cells [16].
Therefore, economical and effective alternative catalysts are
required.
Among the metal based catalysts studied to date, Palladium (Pd) is considerably less expensive than Pt, and Pd
has been focusing on recent research as an attractive and
alternative catalysts nanomaterial for a wide range of applications in catalysis, hydrogen storage and biosensing, reduction of
automobile pollutants and so forth [17–20]. It has been found
that upon appropriate modification of their surface atomic
structure, Pd-based nanomaterials can become promising electrocatalysts by simultaneously decreasing the material cost and
enhancing the performance [20,21]. Recently, Pd nanoparticles
have been accepted as a better alternative material for
polymer electrolyte membrane fuel cells towards catalytic
oxidation of formic acid as well as oxygen reduction in a
proton-exchange membrane (PEM) fuel cell [22,23]. Furthermore, Pd nanoparticles have been chosen as the best alternative material for solving many industrial process problems
and demand the exploration of a new synthetic approach for
production of well-defined shapes and structures. Attempts
have been made to develop anisotropic Pd nanostructures
mainly using surfactants and polymers or high temperature
reactions in organic medium [24–31]. Although, these methods
were successful in producing well-defined Pd nanostructures,
the strong adsorption of these protecting agents resulted in a
decrease of catalytic activity [22]. Nevertheless the aqueous
medium based synthesis of metal nanoparticles is more promising from an environmental standpoint, adding an advantage
over the use of toxic organic solvents [32–34]. Moreover these
protocols are limited to producing nanoparticles with a smooth
surface morphology. However, it has been observed that the
interesting nanoarchitectures with high surface roughness and
surface steps can contribute to the increased accessibility of
reactant species and are more attractive for enhancing catalytic application [35]. Therefore, it is highly desirable to explore
a facile approach to produce nanocatalysts with high surface
areas and high degree of structural anisotropy for improved
performance in technological applications.
Herein, we explore a new approach for the synthesis of
chain-like Pd (branches in a tree) nanostructures (PdCNs)
with a cubic morphology in an aqueous medium. Here
we use the neurotransmitter 5-hydroxytryptamine (HT) to
address the environment-friendly and green perspectives. A
notorious problem in the shape-controlled synthesis of
metal nanoparticles is the shape heterogeneity of the
synthesized product. This report clearly demonstrates the
formation of uniform PdCNs with a unique morphology. To
the best of our knowledge, this is the first report that
describes rapid eco-friendly synthesis of cubic chain-like Pd
nanostructures without any template, polymer, surfactant
or without any heat treatment procedures. The cyclic
voltammetry and chronoamperometry are employed to
investigate the morpho-dependent electrocatalytic activity
of these chain-like cubic Pd nanostructures toward the
oxidations of formic acid, methanol and ethanol.
P. Kannan et al.
Experimental section
Materials
PdCl2 and 5-hydroxytryptamine were obtained from SigmaAldrich Chemical Company. The phosphate buffer solution
(PBS; pH= 7.2) was prepared using Na2HPO4 and NaH2PO4.
Double distilled water was used to prepare the solutions in
this investigation. All other chemicals used in this investigation
were of analytical grade. All the solutions were prepared with
deionised water (18 mΩ) obtained from Millipore system.
Synthesis of cubic chain-like Pd nanostructures
Glasswares used for synthesis were well cleaned with freshly
prepared aqua regia (3:1 HCl and HNO3), then rinsed
thoroughly with water and dried prior to use (Caution! aqua
regia is a powerful oxidizing agent and it should be handled
with extreme care). In a typical synthesis, 10 ml aqueous
solution of PdCl2 (0.1 mM) was taken in a beaker and then
0.1 ml of 5-hydroxytryptamine (5-HT) (10 mM) was rapidly
injected into the solution, allowed for 30 min in static state.
The resulting nanocolloid was stored at 4 1C for further use.
Characterization
The morphologies of the cubic chain-like, dendrites and
fractal Pd nanostructures were characterized by a field
emission-scanning electron microscopy (FE-SEM JEOL JSM6301F). The specimens were prepared by dropping 3 μl of
colloidal solution onto silicon wafer substrate. The crystallographic information of the prepared Pd chain-like nanostructures were studied by the powder X-ray diffraction
technique (XRD, Shimadzu XRD-6000, Ni filtered CuKα
(λ = 1.54 Å) radiation operating at 30 kV/40 mA).
Electrochemical measurements
Electrochemical measurements were performed using two
compartment three-electrode cell with a glassy carbon
working electrode (geometric area = 0.07 cm2), a Pt wire
auxiliary electrode and Ag/AgCl (3 M KCl) as reference
electrode. Cyclic voltammograms were recorded using a
computer controlled CHI643B electrochemical analyzer. All
the electrochemical experiments are carried out in an argon
atmosphere. The 3 μl of as-synthesized chain-like nanostructures are dispersed over glassy carbon electrode with
nafion and dried prior to electrochemical experiments.
Results and discussion
The cubic chain-like Pd nanostructures have been characterized to establish their nanostructure and mechanism of
shape evolution. Figure 1 shows the FE-SEM images obtained
for the nanoparticle synthesized using 5-HT. The FE-SEM
image shows that Pd nanoparticles were cubic morphology
with chain like-structures (branches in a tree; Figure 1a–d).
The cubic chain-like Pd nanocubes have an average size
between 140 Â 160 nm and 140 Â 210 nm. Furthermore, we
examine the stability of cubic chain like-structures PdCNs

3. One-pot synthesis and enhanced electrocatalytic activity of chain-like palladium nanocubes
679
Figure 1 FE-SEM images of (a) simple branched chain-like cubic Pd nanostructures using 5-HT (0.1 mM), (b) a portion of image “a”,
(c) a portion of image “b” and (d) a portion of image “c”.
prepared at a conc. of 0.1 mM 5-HT, the FE-SEM images were
taken after 1 and 5 h time interval and presented in
Figure 2a–c. From the FE-SEM measurements the density,
length and number of cubic branches per nanochains were
almost same over the growth period up to 5 h (Figure 2a and b).
These time-dependent features can be ascribed either to the
aggregation of nanoparticles or the growth of anisotropic
nanostructures. The morphology of cubic chain-like Pd nanostructures remained the same even after 5 h, implying that the
observed feature is not due to the aggregation of nanoparticles.
The chain-like Pd nanocubes grow up to the size of $80 mm
with more than 25 branches in 5 h (Figure 2d). The observed
results suggest that the cubic chain-like morphology is highly
stable by varying the time intervals (both 30 min, 1 and 5 h). To
the best of our knowledge, this is the first report that describes
rapid eco-friendly synthesis of cubic chain-like Pd nanostructures (branches in a tree) at room temperature without any
heat treatment, template, polymer, or surfactant. The overview of the mechanism for formation of such a unique shape
can be outlined in Scheme 1 as: (i) in the initial stage of
reaction, Pd2 + ions are reduced by 5-HT to form nano-sized
small Pd cubic shaped nanoparticles (around 5–10 min), (ii) a
homogeneous nucleation of these cubic nanoparticles forms
small, primary Pd cubic nano-chains by gradual assembly of
these particles that grows into a one-dimensional interconnected nanostructure and a complete growth of cubic chainlike nanostructures was observed after 30 min of reaction
(iii) these primary particles undergo nucleation/assemble due
course over time into a 1-D nanostructure and reconstruction of
these small nanocubes into cubic dendritic nanowires [36,37].
We propose that, 5-HT molecule (stabilising/reducing agent)
plays a vital role on selective binding on the surface of
1-D nanostructure and reconstruct the tiny nanocubes into
dendritic-like nanochains. The growth of nanostructured particles is highly perceptive to the concentration of 5-HT and
precursor, Pd2 + (Figure 3). Here we demonstrate that cubic
chain-like Pd nanostructures, cubic small dendrites of Pd
nanostructures, cubic big-dendrites of Pd nanostructures and
fractal Pd nanostructures could be selectively prepared by
manipulating and controlling the concentration of 5-HT and
Pd2 + (Figures 4, 5 and Figure S1). It should be noted here that
this procedure establishes a universal protocol for synthesis of
monodisperse Pd nanostructures with a variety of shapes
without adopting different strategies, reaction conditions, or
injection of foreign reagents. Minor change in the concentrations of stabilizer and a fixed concentration of precursor or
vice versa results in the formation of different morphologies of
the nanoparticles assembly (Figure 3).
Recently, Willner and co-workers have reported the cataecholamine neurotransmitters mediated growth of Au nanoparticles and their quantification by optical method [38]. The
catacholamine neurotransmitters and their metabolites induce
the growth of spherical shape nanoparticles, and they do not
induce the formation of any anisotropic nanoparticles [38]. In
the present investigation, we observed cubic Pd nanochains,
cubic nanostructures of big-dendrites, and fractal nanostructures by the indoleamine neurotransmitter. The formation of
chain-like nanostructured particles is attributed to the oxidation of 5-HT by PdCl2. As shown in Scheme 2, reduction of
PdCl2 occurs due to the transfer of electrons from HT to the
metal ion, resulting in the formation of Pd0. The metallic Pd
then undergoes nucleation and growth with time to form cubic
chain-like Pd nanostructures, cubic nanostructures of dendrites and fractal nanostructures. It is well documented in the

4. 680
P. Kannan et al.
Figure 2 FE-SEM images of simple branched chain-like cubic Pd nanostructures using 5-HT (0.1 mM) after 1 h (a), 5 h (b) and close
view of image “b”. The image “c” represents the length of the branches in a sample. The image “d” represents maximum length of
chain-like nanocubic particles.
Nucleation
5-HT
Pd cubes
Self-assembly
Primary cubic Pd chain-like nanostructure
Reconstruction
Scheme 1 Schematic presentation showing the formation/
growth process of cubic chain-like Pd nanostructures and
dendritic nanochains.
literature that the oxidation of 5-hydroxyindole generates a
free radical [39–43]. The radical generated by the oxidation of
HT can undergo a chemical reaction in aqueous solution to
yield different products. The reaction of radical strongly
depends on the experimental condition. The hydroxylated
dimers (5,5'-Dihydroxy-4,4'-bitryptamine (DHB)) and tryptamine-4,5-dione (TAD) (Scheme 2) are the major products in
neutral and acidic pH. These oxidation products are expected
to adsorb on the surface of the nanoparticles (vide infra) by
different orientations, resulted that different cubic chain-like
Pd nanostructures. In Scheme 2, “a” and “b” representing the
reversible redox reaction, these oxidized products were
expected to adsorb on the surface of the nanoparticles
through amino groups (–NH2), resulted the Pd nanoparticles.
It is expected that the hydrogen bonding between above two
compounds with PdNPs (cellulose type hydrogen bonding),
resulting that chain-like nanostructures.
We suggest that the balance act between the concentration of precursor and stabilizer determine their shape
evolution process. The effect of Pd2 + concentration can
be speculated as at high concentration, Pd2 + ions are
floating around to seed the growth of the Pd nanocubes
and speed up the nucleation to form compact large nanocubes. On the other hand at lower concentration the Pd2 +
ions are diluted enough to decrease the seeding and small
cubic chain-like nanostructures were formed. Those small
nanocubes undergo controlled nucleation to form the
dendritic morphology. When the concentration of 5-HT
is increased, Pd nanostructures with a big dendritic and
fractal shaped morphology were obtained. This may be due
to the high concentration of 5-HT (reducing/stabilising
agents) which not only expedites the reduction of Pd2 + to

5. One-pot synthesis and enhanced electrocatalytic activity of chain-like palladium nanocubes
681
HT (0.1 mM)
Pd (0.1 mM) in 10 mL
HT (0.3 mM)
+
5-Hydroxytryptamine (HT)
HT (0.5 mM)
HT (0.75 mM)
Figure 3 FE-SEM measurements of different morphologies of Pd nanoparticles induced at different concentrations of 5-HT and
Pd(II).
Figure 4 FE-SEM images of 5-HT-induced (0.3 mM) formation of cubic chain-like small dentries of Pd nanostructures.

6. 682
P. Kannan et al.
Figure 5 FE-SEM images of 5-HT-induced (0.5 mM) formation of cubic chain-like big-dendrites of Pd nanostructures.
Pd0, but also controls the nucleation process. Our examination concludes that the optimal metal precursor concentration for the preparation of Pd nanoparticles with uniform
distribution of nanocubes was of 0.5 mM. When the concentration goes up to 0.75 mM, the chain-like cubic nanoparticles were aggregated. The sufficient numbers of 5-HT
molecules present around the reaction direct the nucleation/assembly of the reduced primary Pd cubic nanoparticles in different arrays or unique direction to produce cubic
chain-like Pd dendritic nanostructures, nanostructures of
small, big-dendrites, and multiple chain-like fractal shapes.
So it can be speculated that the concentration of 5-HT not
only stabilizes the nano-sized particles but its structure
plays a vital role in controlling their nucleation or growth
process in a universal pattern to produce cubic chain-like Pd
nanostructures, cubic chain-like dendritic or fractal shapes.
It is well established that using surfactants or polymers with
different functional groups having different binding strategies such as acids and amines regulate the morphology of
nanopararticles [44]. The formation of different shapes and
morphology is controlled by the faceting tendency of the
stabilizer and the growth kinetics to crystallographic planes
[6,29,30]. We anticipate here that the structure and functional groups of reducing/stabilizing agents play a vital role
in shape and morphology evolution of nanoparticles.
Our efforts are yet underway to establish the structure–
function relationship of the stabilizer for shape evolution of
Pd nanoparticles using the bi-products of 5-HT (DHB and
TAD). We propose a reasonable explanation towards the
balancing act between the concentration of precursor and
stabilizer for shape evolution of Pd nanostructures in
Schemes 1 and 2.
The X-ray diffraction (XRD) patterns were recorded to
confirm the lattice facets of cubic chain-like Pd nanostructures as shown in Figure 6. Peaks corresponding to Pd (111),
(200), (220) and (311) were obtained. The relative peak
intensities were compared using the peak area of (111) as a
reference (JCPDS card number: 41-1021). The ratio of the
relative peak intensity of (200) with respect to (111) is
found to be 0.52 versus 0.6 of the standard value. However,
the ratio of the relative peak intensities of high index planes
(220) and (311) are higher than the standard values previously reported (0.73 versus 0.42) and (0.71 versus 0.55),
respectively [17,45,46]. This observation reveals that the
cubic chain-like Pd nanostructures were abundant in high
index facets. Further, XRD patterns for cubic small dendrites
Pd nanostructures, cubic chain-like big-dendrites Pd nanostructures and fractal nanostructures of Pd nanostructures
were recorded to confirm their lattice facets (Figure 6 and
Figure S2). For instance, the ratio of the relative peak
intensities of high index planes (220) and (311) is 0.61 versus
0.42 and 0.65 versus 0.55 respectively for cubic small
dendrites Pd nanostructures, which is lower than the values
obtained for cubic chain-like Pd nanostructures (0.73 versus
0.42) and (0.71 versus 0.55). Moreover, the formations of Pd
nanoparticles were also investigated in the presence of
5-hydroxyindoleacetic acid and N-aceltyserotonin, that are
functionally correlated with respect to 5-HT. In similar
conditions, both these structurally different molecules
induce the formation of Pd nanostructures but the morphology differs (Figure S3). We anticipate here that the
structure and functional groups of reducing/stabilizing
agents affect the shape and morphological evolution of
nanoparticles.

7. One-pot synthesis and enhanced electrocatalytic activity of chain-like palladium nanocubes
683
OH
Pd2+
HN
5-hydroxytryptamine
NH2
-H +
O*
Pd0
HN
O
H
N 2
H
HN
N
OH
OH
OH
2e2H+
b
NH
N
O
H
N
HO
HN
2e2H+
PdNPs
O
N
H
OH
N
H
N
a
HN
N
NH2
OH
NH2
NH
N
PdNPs
O
HO
NH
NH
NH2
HO
OH
N
O
NH2
O
HO
N
H
NH2
NH2
tryptamine-4,5-dione (TAD)
N
5,5'-Dihydroxy-4,4'-bitryptamine (DHB)
‘a’ and ‘b’ represent the reversible redox reaction.
Scheme 2
Mechanism for 5-HT induced formation of Pd nanostructures with different branches with cubic and fractal shapes.
(111)
(200)
20
40
(220)
60
(311)
80
2 Theta (Degree)
Intensity (a.u.)
Intensity (a.u.)
(111)
(200)
(220) (311)
20
40
60
80
2 Theta (Degree)
Figure 6 XRD pattern of as synthesized cubic chain-like Pd nanostructures (a) and cubic chain-like small dendrite of Pd
nanostructures (b).
Pd is well-known to absorb massive quantities of hydrogen in bulk to form hydrides and therefore has a great
technological potential as a hydrogen storage material [47].
We have carried out cyclic voltammetric studies in acidic
medium to examine the affinity of hydrogen toward all
cubic chain-like Pd nanostructures. Figure 7 shows the
voltammetric behavior of the cubic chain-like Pd nanostructures modified electrode in 0.5 M HClO4 at a scan rate
of 50 mVsÀ1. The cyclic voltammogram (CVs) obtained for
cubic chain-like Pd nanostructures in 0.5 M HClO4 shows
sharp distinguished anodic peaks in between À0.25 and
–0.15 V. This broad peak is due to the oxidation of both the
absorbed and adsorbed hydrogen on the cubic chain-like Pd
nanostructures and has been earlier reported for bulk Pd
electrodes [48]. However, the CV for the cubic chain-like Pd
nanostructures coated disk electrode shows two distinct

8. 684
P. Kannan et al.
d
Current Density (mA/cm2)
Current Density (mA/cm2)
0.8
0.4
a
0
0.4
0.8
–0.5
0
0.5
Potential (V) vs. Ag/AgCl
1
0
a
0.05
d
0.5
Potential (V) vs. Ag/AgCl
Figure 7 (A) CVs of fractal Pd nanostructures (a), cubic Pd big-dendrites (b), cubic chain-like small dendrites of Pd nanostructures
(c) and cubic chain-like Pd branched nanostructures in N2-saturated 0.5 M HClO4 solution at a scan rate of 50 mV sÀ1.
voltammetric peaks, one of which is a small shoulder peak
in the negative potential region during the anodic sweep
[48–51]. The first peak at À0.23 V corresponds to the oxidation of the adsorbed hydrogen (Had), while the subsequent
peak at À0.18 V is due to the oxidation of the absorbed
hydrogen (Hab) on Pd surface [49]. The cathodic peak during
the reverse scan is due to both absorption and adsorption of
hydrogen. It is known from the literature that the two wellresolved peaks for the oxidation of adsorbed and absorbed
hydrogen in Pd are manifested exclusively in the case of Pd
nanoparticles. This behavior is attributed to the larger
number of surface sites available for adsorption on the
nanoparticle's surface [48–51]. The large anodic peak corresponding to the desorption of hydrogen in the case of cubic
chain-like Pd nanostructures modified electrode shows its
potential as an excellent hydrogen storage material. The
electroactive surface area of Pd has been measured from
the palladium oxide stripping analysis [48]. The charge
consumed during the reduction of Pd oxides has been
estimated by integrating the area under the reduction
wave. The electric charge of the hydrogen desorption of
cubic chain-like Pd nanochains, small cubic nanostructures
of dendrites, big-dendrites and fractal nanostructures were
measured to be 132.4, 366.2, 509.4, and 784.2 μC (a–d)
respectively. The electrochemically accessible area of cubic
chain-like Pd nanostructures was calculated to be 1.476 cm2
using the reported value of 424 μC cmÀ2. This is a very large
real surface area for the other chain-like Pd nanostructures
such as cubic nanostructures of small dendrites, big-dendrites, and fractal nanostructures are 0.875 0.554, and
0.253 cmÀ2, respectively. This large surface area arises due to
cubic chain-like Pd nanostructures in which the Pd nanocubes
were highly dispersed making a branches-on a tree type of
nanostructure. Since the Pd nanoparticles were directly modified on the electrode surface, the available effective active
centers were also higher, which makes them an ideal electrocatalytic nanomaterial.
Inspired by the attractive cubic chain-like structures and
unique morphology of Pd nanostructures, we further proved
these as nanoelectrocatalyst. Very recently Mohanty et al.
observed a dramatic catalytic performance of dendritic noble
metal nanoparticles towards the Suzuki–Miyaura and Heck
coupling reaction [17]. Here we use methanol, ethanol and
formic acid as model molecules for studying the
electrocatalytic performance of cubic chain-like Pd nanostructures. The electrochemical oxidation of methanol and
formic acid has attracted much attention due to their
potential energy related applications for direct methanol
fuel cells (DMFC) and direct formic acid fuel cells (DFAFC),
respectively. It is generally accepted that noble Pt metal is
the best catalyst for the formic acid oxidation but it suffers
poisoning due to a strong adsorbed CO species generated as
reaction intermediates [52]. A noble Pd-based catalyst was
explored to be efficient and possesses superior performances
for fuel cell applications because it is highly resistant to CO
poisoning [22]. We have examined the electrocatalytic
activity
of
as-synthesized chain-like cubic Pd nanostructures towards
formic acid and methanol oxidation (Figure 8). Well-defined
voltammograms with prominent peaks in the forward and
reverse scans were obtained, respectively. The oxidation
current has been normalized to the electroactive surface
area of the cubic chain-like Pd nanostructures. This surface
area was determined from the coulombic charge reduction of
Pd oxide (vide supra), according to the reported value of
424 mC cmÀ2. As shown, the as-synthesized cubic chain-like
Pd nanostructures of different branched morphologies exhibit
different electrocatalytic activities.
The oxidation of formic acid on Pt involves a dual path
mechanism, a dehydrogenation path to the direct formation
of CO2 and a dissociative adsorption to form poisoning CO
species (a dehydration path) [53–55]. It can be seen that the
formic acid electro-oxidation on these present cubic chainlike Pd nanostructures shows two anodic peaks in the
forward scan. The onset potential and peak potential on
cubic chain-like Pd nanostructures are À0.15 and 0.13 V,
respectively (Figure 8A, curve b). The first small anodic
peak is attributed to the direct oxidation of formic acid to
CO2 on the remaining sites unblocked by intermediate
species. At higher potentials, the current further increases
and reaches another peak. The second peak is related to the
oxidation of adsorbed intermediate species (CO), which
releases the free surface sites for the subsequent direct
oxidation of formic acid. In the reverse scan, the current
reaches a high peak located at around 0.02 V, which is
much higher than those in the positive scan, due to the bulk
HCOOH oxidation and the absence of CO poison. However,
with the potential scanning to more negative values, the

9. a
b
c
d
e
f
10.0
8.0
6.0
Current Density (mA/cm2)
Current Density (mA/cm2)
One-pot synthesis and enhanced electrocatalytic activity of chain-like palladium nanocubes
4.0
2.0
0.0
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
685
a
b
c
d
e
f
6.0
5.0
4.0
3.0
2.0
1.0
0.0
-0.1
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
Potential (V) vs. Ag/AgCl
Potential (V) vs. Ag/AgCl
surface was again poisoned by CO, resulting in the decrease of
the oxidation current [56–59]. The oxidation of formic acid at
cubic chain-like Pd nanostructures is higher than those on
cubic chain-like small dendrites of Pd nanostructures (À0.10
and 0.16 V), cubic big-dendrites Pd nanostructures (À0.10 and
0.16 V) and fractal Pd nanostructures (À0.07 and 0.18 V),
indicating that the formic acid is easier to oxidize on cubic
chain-like Pd nanostructures. The peak current density for
cubic chain-like Pd nanostructures is $ 760 mA/cm2, which is
much higher than that for cubic small dendrites of Pd
nanostructures ($ 480 mA), cubic big-dendrites Pd nanostructures ($460 mA) and fractal Pd nanostructures ($250 mA).
This should be resulted from its highest ECSA. The specific
surface activities of these catalysts could be obtained from
the normalized peak current density with ECSA to further
investigate the possibility of the enhancement effect of cubic
branched nanostructures on the intrinsic electrocatalytic
activity, which is 10.8, 6.8, 6.5 and 3.5 AmÀ2 for cubic
chain-like Pd nanostructures, cubic small dendrites of Pd
nanostructures, cubic big-dendrites Pd nanostructures and
fractal Pd nanostructures, respectively. The low onset and
oxidative peak potential value (less positive potential), much
higher specific surface activities of the cubic chain-like Pd
nanostructures catalysts towards formic acid oxidation may be
attributed to the presence of branched (branches in a tree)
morphology. On the other hand, the present Pd nanostructures
also efficiently catalyze the oxidations of methanol (Figure 8B)
and ethanol (Figure S4). We summarized the activity of Pd
nanostructures in terms of oxidation potential and generated
catalytic current-density and these data are provided in
(Table S1). Further, we compared the electrocatalytic performance of the present branched cubic chain-like nanoparticles to commercial Pd/C and spherical PdNPs toward the
oxidation of formic acid and methanol (Figures S5 and S6).
The current densities were also normalized to the electroactive surface area, which were measured from the electric
charge of hydrogen adsorption/desorption on Pd surfaces.
Such normalization allowed the current density to apply
directly for comparison of the catalytic activities of the
different catalysts. The cubic chain-like Pd nanostructures
show Z11.5 (comparison for both cases) times more activity
on the basis of an equivalent noble metal mass for the formic
acid and methanol than the spherical shaped PdNPs and
Current Density (mA/cm2)
Figure 8 Cyclic voltammograms show the oxidation of (A) formic acid (0.25 M) in 0.5 M HClO4 and (B) methanol (0.25 M) in
0.1 M KOH at (a) bare GC, (b) cubic chain-like Pd nanochains, (c) cubic chain-like Pd nanochains in the absence of both formic acid
and methanol, (d) cubic small dendrites of Pd nanostructures, (e) cubic big-dendrites Pd nanostructures and (f) fractal Pd
nanostructures modified electrodes. Scan rate: 50 mV sÀ1.
8.0
7.0
6.0
a
5.0
4.0
3.0
b
c
2.0
1.0
0
d
0
2000
4000
6000
Time (Sec)
Figure 9 Polarization current vs. time plots of branched cubic
chain-like Pd nanostructures (a), cubic small dendrites of Pd
nanostructures (b), cubic big-dendrites of Pd nanostructures
(c) and fractal Pd nanostructures (d) modified electrodes measured
in formic acid (0.25 M) in 0.5 M HClO4 solution.
commercial Pd/C catalysts. This may be attributed to the
high degree of structural anisotropy of cubic chain-like Pd
nanostructures and branched tree-like morphology contributing to their superior electrocatalytic activities. The rich
branched sharp edges and corner atoms of tree-like structures were highly preferred for improving the catalytic
activity [60]. The long-term stability of these catalysts was
evaluated by the chronoamperometry method and also the
current decay, which is associated with the poisoning of
intermediate species on the nanoparticles determining the
fate of its practical application. The chronoamperometry
profile is sufficiently stable for cubic chain-like Pd nanostructures and cubic nanostructures of dendrites morphologies. The chronoamperometric stabilities of cubic chain-like
Pd nanostructures, cubic nanostructures of small dendrites,
cubic big-dendrites and fractal nanostructures were investigated using towards formic acid oxidation and as shown in
Figure 9. The polarization current for the formic acid
oxidation reaction on these three supported Pd catalysts
shows a significant decay initially and reaches a stable value
after polarization at 0.20 V for $ 400–600 s. The current decay
for the formic acid oxidation reaction indicates the slow
deactivation of the nanostructued Pd-based electrocatalysts

10. 686
by the slow adsorption of CO or CO-like intermediates [61–63].
This is supported by the detection of the adsorption of CO
species on Pd catalysts during formic acid oxidation by
surface-enhanced infrared absorption spectroscopy [64]. However, the stable mass specific current for formic acid oxidation
reaction on cubic chain-like Pd nanostructures is $540 mA,
significantly higher than cubic nanostructures of small dendrites ($170 mA) and cubic big-dendrites ($90 mA). This
indicates that cubic chain-like Pd nanostructures catalysts
possess much better stability against the poisoning by
adsorbed CO or CO-like intermediate species. The stability
of these catalysts was also studied by the cyclic voltammetric
method. The first peak currents at forward scan are recorded
as a function of the cycle numbers, as shown in Figure S7.
Using the peak current of the 10th cycle as the baseline, the
peak current after 100 cycles on cubic chain-like Pd nanostructures is reduced by 29.6%, which is significantly lower
than the reduction in activity on cubic nanostructures of small
dendrites (38.5%) and cubic big-dendrites (57.2%) nanocatalysts, further indicating that cubic chain-like Pd nanostructures exhibit the highest catalytic stability and is in good
agreement with the results of chronoamperometry curves.
Therefore, the as-synthesized cubic chain-like Pd nanostructures were highly resistant to poisoning due to intermediates
and were favorable for practical uses. We could also evidently
argue that the presence of cubic chain-like Pd nanostructures
significantly enhances the electrocalytic activity and diminishes the poisoning of the Pd catalysts for the highest stability.
We anticipate that the cubic chain-like Pd nanostructures
(Figure 1) expose a larger number of active sites for the
adsorption of active oxygen atoms, which readily oxidizes the
reaction intermediate species, protecting it from surface
poisoning [65,66]. No change in the morphology of branched
cubic chain-like Pd nanostructures was observed after several
voltammetric cycles (Figure S8) and hence acclaims its robust
nature. Our efforts are underway to modify these cubic chainlike Pd nanostructures on the graphene, carbon support and
compare their enhanced electrocatalytic activity with the
commercially available Pd/C.
Conclusions
In summary, we have explored a facile approach to tailor
the shape and tree-like branched morphology of Pd nanostructures in aqueous medium. We propose that, 5-HT
molecule (stabilising/reducing agent) plays a vital role on
selective binding on the surface of 1-D nanostructure and
reconstruct the tiny nanocubes into dendritic-like cubic
nanochains. The as-synthesized chain-like, small and bigdendrites, and fractal Pd nanostructures showed cubic
shape and morphology dependent electrocatalytic activity
towards oxidation of formic acid, methanol and ethanol for
DFAFC, DMFC and DEFC applications.
Acknowledgments
Palanisamy Kannan and Marcin Opallo thank NanOtechnology
Biomaterials and aLternative Energy Source for ERA Integration
[FP7-REGPOT-CT-2011-285949-NOBLESSE] Project from European Union.
P. Kannan et al.
Appendix A.
Supplementary material
Supplementary data associated with this article can be found
in the online version at http://dx.doi.org/10.1016/j.nanoen.
2013.08.001.
References
[1] D. Kim, Y.W. Lee, S.B. Lee, S.W. Han, Angewandte Chemie
International Edition 51 (2012) 159–163.
[2] Y. Xia, Y. Xiong, B. Lim, S.E. Skrabalak, Angewandte Chemie
International Edition 48 (2009) 60–103.
[3] M. Grzelczak, J. Perez-Juste, P. Mulvaney, L.M. Liz-Marzan,
Chemical Society Reviews 37 (2008) 1783–1791.
[4] R. Jin, Y. Charles Cao, E. Hao, G.S. Metraux, G.C. Schatz,
C.A. Mirkin, Nature 425 (2003) 487–490.
[5] C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chemical
Reviews 105 (2005) 1025–1102.
[6] C.J. Murphy, T.K. Sau, A.M. Gole, C.J. Orendorff, J. Gao,
L. Gou, S.E. Hunyadi, T. Li, The Journal of Physical Chemistry B
109 (2005) 13857–13870.
[7] X. Teng, W.-Q. Han, W. Ku, M. Hücker, Angewandte Chemie
International Edition 47 (2008) 2055–2058.
[8] K.K. Caswell, C.M. Bender, C.J. Murphy, Nano Letters 3 (2003)
667–669.
[9] S. Guo, S. Dong, E. Wang, Chemical Communications 46 (2010)
1869–1871.
[10] N. Tian, Z.-Y. Zhou, S.-G. Sun, Chemical Communications
0 (2009) 1502–1504.
[11] S. Cheong, J.D. Watt, R.D. Tilley, Nanoscale 2 (2010) 2045–2053.
[12] Y. Wook Lee, M. Kim, S. Woo Han, Chemical Communications
46 (2010) 1535–1537.
[13] B. Hu, K. Ding, T. Wu, X. Zhou, H. Fan, T. Jiang, Q. Wang,
B. Han, Chemical Communications 46 (2010) 8552–8554.
[14] B. Lim, M. Jiang, P.H.C. Camargo, E.C. Cho, J. Tao, X. Lu,
Y. Zhu, Y. Xia, Science 324 (2009) 1302–1305.
[15] M.J. Mulvihill, X.Y. Ling, J. Henzie, P. Yang, Journal of the
American Chemical Society 132 (2009) 268–274.
[16] R. Bashyam, P. Zelenay, Nature 443 (2006) 63–66.
[17] A. Mohanty, N. Garg, R. Jin, Angewandte Chemie International
Edition 49 (2010) 4962–4966.
[18] M. Cao, J. Lin, H. Yang, R. Cao, Chemical Communications 46
(2010) 5088–5090.
[19] C. Langhammer, I. Zorić, B. Kasemo, B.M. Clemens, Nano
Letters 7 (2007) 3122–3127.
[20] N. Tian, Z.-Y. Zhou, N.-F. Yu, L.-Y. Wang, S.-G. Sun, Journal of
the American Chemical Society 132 (2010) 7580–7581.
[21] L. Xiao, L. Zhuang, Y. Liu, J. Lu, H.C.D. Abruna, Journal of the
American Chemical Society 131 (2008) 602–608.
[22] V. Mazumder, S. Sun, Journal of the American Chemical
Society 131 (2009) 4588–4589.
[23] C. Bianchini, P Shen, Chemical Reviews 109 (2009) 4183–4206.
.K.
[24] X. Huang, N. Zheng, Journal of the American Chemical Society
131 (2009) 4602–4603.
[25] Y.-H. Chen, H.-H. Hung, M.H. Huang, Journal of the American
Chemical Society 131 (2009) 9114–9121.
[26] J. Watt, S. Cheong, M.F. Toney, B. Ingham, J. Cookson,
P.T. Bishop, R.D. Tilley, ACS Nano 4 (2009) 396–402.
[27] V.P. Ananikov, N.V. Orlov, I.P. Beletskaya, V.N. Khrustalev,
M.Y. Antipin, T.V. Timofeeva, Journal of the American Chemical Society 129 (2007) 7252–7253.
[28] Y. Sun, L. Zhang, H. Zhou, Y. Zhu, E. Sutter, Y. Ji,
M.H. Rafailovich, J.C. Sokolov, Chemistry of Materials 19
(2007) 2065–2070.
[29] Y. Xiong, Y. Xia, Advanced Materials 19 (2007) 3385–3391.

11. One-pot synthesis and enhanced electrocatalytic activity of chain-like palladium nanocubes
[30] J. Watt, N. Young, S. Haigh, A. Kirkland, R.D. Tilley, Advanced
Materials 21 (2009) 2288–2293.
[31] S.-W. Kim, J. Park, Y. Jang, Y. Chung, S. Hwang, T. Hyeon,
Y.W. Kim, Nano Letters 3 (2003) 1289–1291.
[32] B.K. Jena, S.C. Sahu, B. Satpati, R.K. Sahu, D. Behera,
S. Mohanty, Chemical Communications 47 (2011) 3796–3798.
[33] Q. Yuan, X. Wang, Nanoscale 2 (2010) 2328–2335.
[34] B. Lim, M. Jiang, J. Tao, P.H.C. Camargo, Y. Zhu, Y. Xia,
Advanced Functional Materials 19 (2009) 189–200.
[35] L. Wang, Y. Yamauchi, Journal of the American Chemical
Society 131 (2009) 9152–9153.
[36] F. Viñes, F. Illas, K.M. Neyman, Angewandte Chemie International Edition 46 (2007) 7094–7097.
[37] H. Kura, T. Ogawa, Journal of Applied Physics 107 (2010)
074310–074315.
[38] R. Baron, M. Zayats, I. Willner, Analytical Chemistry 77 (2005)
1566–1571.
[39] E. Perez-Reyes, R.P. Mason, Journal of Biological Chemistry 256
(1981) 2427–2432.
[40] M.Z. Wrona, G. Dryhurst, Bioorganic Chemistry 18 (1990)
291–317.
[41] C.R. Raj, S. Chakraborty, Biosensors and Bioelectronics 22
(2006) 700–706.
[42] R.N. Goyal, A. Sangal, Journal of Electroanalytical Chemistry
578 (2005) 185–192.
[43] M.Z. Wrona, G. Dryhurst, The Journal of Organic Chemistry 52
(1987) 2817–2825.
[44] E. Ramirez, S. Jansat, K. Philippot, P. Lecante, M. Gomez,
A.M. Masdeu-Bultó, B. Chaudret, Journal of Organometallic
Chemistry 689 (2004) 4601–4610.
[45] Y. Sun, Y. Xia, Science 298 (2002) 2176–2179.
[46] B.D. Cullity, Elements of X-ray Diffraction, 3rd Edition,
Prentice-Hall, Upper Saddle River, NJ, 2001.
[47] J.C. Barton, J.A.S. Green, F.A. Lewis, Nature 197 (1963)
1293–1294.
[48] W. Pan, X. Zhang, H. Ma, J. Zhang, The Journal of Physical
Chemistry C 112 (2008) 2456–2461.
[49] H.-P. Liang, N.S. Lawrence, T.G.J. Jones, C.E. Banks, C. Ducati,
Journal of the American Chemical Society 129 (2007)
6068–6069.
[50] Y. Gimeno, A. Hernández Creus, S. González, R.C. Salvarezza,
A.J. Arvia, Chemistry of Materials 13 (2001) 1857–1864.
[51] C. Batchelor-McAuley, C.E. Banks, A.O. Simm, T.G.J. Jones,
R.G. Compton, ChemPhysChem 7 (2006) 1081–1085.
[52] S. Zhang, Y. Shao, G. Yin, Y. Lin, Angewandte Chemie International Edition 49 (2010) 2211–2214.
[53] A. Capon, R. Parson, Journal of Electroanalytical Chemistry
and Interfacial Electrochemistry 44 (1973) 1–7.
[54] A. Capon, R. Parsons, Journal of Electroanalytical Chemistry
and Interfacial Electrochemistry 45 (1973) 205–231.
[55] M.E. Vela, R.O. Lezna, N.R. De Tacconi, A.J. Arvia, B. Beden,
F. Hahn, C. Lamy, Journal of Electroanalytical Chemistry 323
(1992) 289–302.
[56] Y. Li, Y. Jiang, M. Chen, H. Liao, R. Huang, Z. Zhou, N. Tian,
S. Chen, S. Sun, Chemical Communications 48 (2012) 9531–9533.
687
[57] H. Zhao, C. Yu, H. You, S. Yang, Y. Guo, B. Ding, X. Song,
Journal of Materials Chemistry 22 (2012) 4780–4789.
[58] H. Yang, L. Dai, D. Xu, J. Fang, S. Zou, Electrochimica Acta 55
(2010) 8000–8004.
[59] T. Jin, S. Guo, J.-l. Zuo, S. Sun, Nanoscale 5 (2013) 160–163.
[60] L. Wang, H. Wang, Y. Nemoto, Y. Yamauchi, Chemistry of
Materials 22 (2010) 2835–2841.
[61] Z. Cui, C. Gong, C.X. Guo, C.M. Li, Journal of Materials
Chemistry A 1 (2013) 1179–1184.
[62] X. Yu, P.G. Pickup, Electrochemistry Communications 11 (2009)
2012–2014.
[63] X. Yu, P.G. Pickup, Journal of Power Sources, 187, 493–499.
[64] H. Miyake, T. Okada, G. Samjeske, M. Osawa, Physical
Chemistry Chemical Physics 10 (2008) 3662–3669.
[65] X. Zhang, W. Lu, J. Da, H. Wang, D. Zhao, P.A. Webley, Chemical
Communications 0 (2009) 195–197.
[66] R. Manoharan, J. Prabhuram, Journal of Power Sources 96
(2001) 220–225.
Palanisamy Kannan is currently working
as Research Fellow in Institute of Physical
Chemistry, Polish Academy of Sciences,
Warsaw, Poland. He received his Ph.D. in
2010, Gandhigram Rural University and
completed the Post-Doctoral Program in
School of Chemical and Biomedical Engineering, Nanyang Technological University,
Singapore. His research interests are mainly
on the development of novel nanomaterials
for electrochemical biosensor and fuel cell applications.
Thandavarayan Maiyalagan is currently
working as Research Fellow in Texas Materials Institute, The University of Texas at
Austin, United States. He received his Ph.D.
in 2007, IIT Madras and completed the PostDoctoral Programs at New Castle University,
United Kingdom and Nanyang Technological
University, Singapore. His current research
interest includes development of nanomaterials for energy conversion and storage
devices, electro-catalysis, fuel cells and biosensors.
Marcin Opallo is a Professor of Department
of Electrode Process, Institute of Physical
Chemistry, Polish Academy of Sciences,
Warsaw, Poland. He received his Ph.D. in
1987 completed the Post-Doctoral Programs
at Tohoku University and University of
California, Davis, United States. He has
published more than 100 papers. Currently,
his research focuses on developing new
materials for energy conversion and sensing.