2013 Nano Energy

See discussions, stats, and author profiles for this publication at: http://www.researchgate.net/publication/256605413 

One-pot synthesis of chain-like palladium 

nanocubes and their enhanced electrocatalytic 

activity for fuel-cell applications 

ARTICLE in NANO ENERGY · JULY 2013 

Impact Factor: 10.21 · DOI: 10.1016/j.nanoen.2013.08.001 

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Palanisamy Kannan 

Nanyang Technological University 

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T Maiyalagan 

Nanyang Technological University 

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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 Kannan a,n , Thandavarayan Maiyalagan b,nn , 

Marcin Opallo a 

a Institute of Physical Chemistry, Polish Academy of Sciences, 44/52 ul. Kasprzaka, 01-224 Warszawa, Poland 

b Materials Science and Engineering Program, The University of Texas at Austin, Austin, TX, USA 

Received 7 May 2013; received in revised form 10 August 2013; accepted 11 August 2013 

Available online 23 August 2013 

KEYWORDS 

Chain-like 

nanoparticles; 

Cubic shape; 

Formic acid; 

Methanol and ethanol 

Abstract 

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). 

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]. Ithasbeen 

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 

2211-2855/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. 

http://dx.doi.org/10.1016/j.nanoen.2013.08.001

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. 

Experimental section 

Materials 

PdCl 2 and 5-hydroxytryptamine were obtained from Sigma- 

Aldrich Chemical Company. The phosphate buffer solution 

(PBS; pH=7.2) was prepared using Na 2 HPO 4 and NaH 2 PO 4 . 

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 HNO 3 ), 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 PdCl 2 (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 JSM- 

6301F). 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 cm 2 ), 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 

P. Kannan et al. 

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 140 160 nm and 140 210 nm. Furthermore, we 

examine the stability of cubic chain like-structures PdCNs

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 

almostsameoverthegrowthperiodupto5h(Figure 2aandb). 

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 

observedfeatureisnotduetotheaggregationofnanoparticles. 

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 

stablebyvaryingthetimeintervals(both30min,1and5h).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, Pd 2+ 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, Pd 2+ (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 

Pd 2+ (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 

thegrowthofsphericalshapenanoparticles,andtheydonot 

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 PdCl 2 .AsshowninScheme 2, reduction of 

PdCl 2 occurs due to the transfer of electrons from HT to the 

metal ion, resulting in the formation of Pd 0 . 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

680 


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. 

5-HT 

Pd cubes 

Nucleation 

Primary cubic Pd chain-like nanostructure 

Reconstruction 

Self-assembly 

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) arethemajorproductsin 

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 (–NH 2 ), 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 Pd 2 + concentration can 

be speculated as at high concentration, Pd 2 + 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 Pd 2 + 

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 Pd 2 + to


681 

HT (0.1 mM) 

Pd (0.1 mM) in 10 mL 

+ 

5-Hydroxytryptamine (HT) 

HT (0.3 mM) 

HT (0.5 mM) 

HT (0.75 mM) 

Figure 3 

Pd(II). 

FE-SEM measurements of different morphologies of Pd nanoparticles induced at different concentrations of 5-HT and 

Figure 4 

FE-SEM images of 5-HT-induced (0.3 mM) formation of cubic chain-like small dentries of Pd nanostructures.

682 


Figure 5 

FE-SEM images of 5-HT-induced (0.5 mM) formation of cubic chain-like big-dendrites of Pd nanostructures. 

Pd 0 , 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.


683 

OH 

Pd 2+ 

HN 

5-hydroxytryptamine 

O* 

NH 2 

-H + 

HN 

Pd 0 

NH 2 

HN 

OH 

N 

HO 

O 

a 

N 

N 

NH2 

NH 2 

HO 

OH 

2e - 

2e - 

2H + b 2H + 

O 

H 

N 

O 

NH 

NH 2 

NH 2 

PdNPs 

HN 

NH 

N 

H 

N 

N 

OH 

O 

OH 

OH 

OH 

OH 

O 

N 

NH NH 

HN PdNPs 

NH 

HO 

HO 

NH2 

NH 2 

N 

5,5'-Dihydroxy-4,4'-bitryptamine (DHB) 

N 

H 

tryptamine-4,5-dione (TAD) 

‘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) 

(111) 

Intensity (a.u.) 

(200) 

(220) (311) 

Intensity (a.u.) 

(200) 

(220) 

(311) 

20 40 60 80 

2 Theta (Degree) 

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 HClO 4 at a scan rate 

of 50 mVs 1 . The cyclic voltammogram (CVs) obtained for 

cubic chain-like Pd nanostructures in 0.5 M HClO 4 shows 

sharp distinguished anodic peaks in between 0.25 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

684 


Current Density (mA/cm 2 ) 

0.8 

0.4 

0 

0.4 

0.8 

d 

a 


0 

0.05 

a 

d 

–0.5 0 0.5 1 

Potential (V) vs. Ag/AgCl 

0.5 


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 N 2 -saturated 0.5 M HClO 4 solution at a scan rate of 50 mV s 1 . 

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 0.23 V corresponds to the oxidation 

of the adsorbed hydrogen (H ad ), while the subsequent 

peak at 0.18 V is due to the oxidation of the absorbed 

hydrogen (H ab ) 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 cm 2 

using the reported value of 424 μC cm 2 . 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 cm 2 , 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 cm 2 . 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 CO 2 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 0.15 and 0.13 V, 

respectively (Figure 8A, curve b). The first small anodic 

peak is attributed to the direct oxidation of formic acid to 

CO 2 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


685 


10.0 

8.0 

6.0 

4.0 

2.0 

0.0 

a 

b 

c 

d 

e 

f 


6.0 

5.0 

4.0 

3.0 

2.0 

1.0 

0.0 

-0.1 

a 

b 

c 

d 

e 

f 

-0.3 -0.2 -0.1 0 0.1 0.2 0.3 


-0.3 -0.2 -0.1 0 0.1 0.2 0.3 


Figure 8 Cyclic voltammograms show the oxidation of (A) formic acid (0.25 M) in 0.5 M HClO 4 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 s 1 . 

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 ( 0.10 

and 0.16 V), cubic big-dendrites Pd nanostructures ( 0.10 and 

0.16 V) and fractal Pd nanostructures ( 0.07 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/cm 2 ,whichis 

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 Am 2 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 


8.0 

7.0 

6.0 

5.0 

4.0 

3.0 

2.0 

1.0 

0 

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 HClO 4 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 

a 

b 

c 

d

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 currentforformicacidoxidation 

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. 

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 


[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 


[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.K. Shen, Chemical Reviews 109 (2009) 4183–4206. 

[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.


687 

[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. 

[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 Post- 

Doctoral 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.

2013 Nano Energy

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