One-step synthesis of three-dimensional Pd polyhedron networks with enhanced electrocatalytic performance

Article abstract of DOI:10.1039/c2cc00154c

Three-dimensional Pd polyhedron networks (Pd PNs) have been fabricated for the first time through a one-step, Cu²⁺-assisted, solution-chemical approach. These as-prepared 3D Pd PNs exhibit high stability and remarkably improved electrocatalytic activity toward formic acid oxidation over commercially available Pd black. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc00154c

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COMMUNICATION
One-step synthesis of three-dimensional Pd polyhedron networks with
enhanced electrocatalytic performancew
You Xu, Rui Xu, Jianhua Cui, Yang Liu and Bin Zhang*
Received 9th January 2012, Accepted 22nd February 2012
DOI: 10.1039/c2cc00154c
Three-dimensional Pd polyhedron networks (Pd PNs) have been
fabricated for the first time through a one-step, Cu2+-assisted,
solution-chemical approach. These as-prepared 3D Pd PNs
exhibit high stability and remarkably improved electrocatalytic
activity toward formic acid oxidation over commercially available
Pd black.
and alloy 3D networks by the synergism of interparticle electro-
static repulsion modulation and heat-induced fusion during
nucleation and growth.7 But, units in these 3D networks are
composed of spherical particles. It remains untouched to develop
a facile method to construct networks of Pd polyhedra, especially
with high density of low-coordinated atomic steps.
Herein, three-dimensional (3D) Pd polyhedron networks
(Pd PNs) have been prepared through a simple, one-pot, Cu2+
-
Controlled synthesis of inorganic nanomaterials with well-defined
morphologies and structures is considered to be a significant
subject in the field of nanotechnology because the intrinsic
properties of nanostructures strongly depend on their shape or
surface atomic arrangement.1 Surface defects on nanocrystals,
such as step and kink atoms with low coordination numbers,
usually exhibit very high chemical reactivity and catalytic
activity toward most of the structure-sensitive reactions.2
Constructing nanocrystals consisting of multitudinous surface
defects through the fine-tuning of the exposure of the specific
facets, corners, edges, and kinks is a promising approach to
fabricate nanomaterials with high activity.
assisted, high yield solution-chemical method. These Pd PNs with
3D structure, high porosity, as well as interconnected network
frames are composed of Pd polyhedra as building blocks. It is
found that these 3D Pd PNs exhibit evidently enhanced catalytic
activity and high stability for formic acid electrooxidation. This
catalytic improvement may be associated with their intrinsic
active sites including high density of atomic steps, ledges, and
kinks, and stable network structure.
The typical synthesis procedure of 3D Pd PNs is described
as follows: a mixture of 0.0446 mmol H2PdCl4, 0.0134 mmol
Cu(NO3)2Á2H2O, 20 mg dodecyl trimethylammonia bromide
(DTAB), and 0.1 mL formaldehyde solution in 10 mL ethylene
glycol (EG) was transferred into a 25 mL Teflon-lined stainless
steel autoclave and heated at 150 1C for 8 h to obtain the
products (see ESIw for details). The morphology and the
structure of the as-obtained samples were firstly characterized
by using scanning electron microscopy (SEM) and transmission
electron microscopy (TEM). As shown in Fig. 1a, a representative
low-magnification SEM image of the Pd samples suggests that
Palladium is extensively used in areas ranging from fuel
cells, hydrogen storage, water treatment, hydrogen purification,
as well as organic catalysis. Great efforts have been devoted to
the preparation of various Pd nanocrystals, such as nanobars,
nanorods, nanocubes, octahedra, icosahedra, and nanoplates.3
Despite considerable investigation of morphological control of
Pd nanocrystals, present studies have mainly focused on the
monodisperse nanocrystals or some unstable assemblies. Using
nanostructures as building blocks for the fabrication of multi-
functional mesoscale architectures such as porous or network-like
structure is an attractive approach to manufacture materials with
new physical, chemical, and mechanical performance originated
from their collective properties. However, to the best of our
knowledge, there are limited studies which are aimed at the
synthesis of Pd porous or network-like nanostructures
composed of structural units such as nanowires,4 nano-
particles,5 and nanospheres.6 Recently, we have developed a
facile and robust synthetic strategy to fabricate noble metal
Department of Chemistry, School of Science, Tianjin University,
Tianjin 300072, China. E-mail: bzhang@tju.edu.cn;
Fax: +86 022 27403475; Tel: +86 022 27403475
w Electronic supplementary information (ESI) available: Experimental
details, XRD patterns of the Pd NNs, SEM images of different Pd
samples. See DOI: 10.1039/c2cc00154c
Fig. 1 Low-magnification SEM image (a), high-magnification SEM
images (b and c) and TEM image (d) of the 3D Pd PNs.
c
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Chem. Commun., 2012, 48, 3881–3883 3881

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at some specific sites, further reoxidize at varying instants
and assist the next growth to polyhedra. The Cu2+-assisted
growth mechanism can be also found to facilitate the
formation of high-index facets in the synthesis of Au–Pd alloy
nanocrystals.9
In our reaction system, formaldehyde is the main reducing
2À
agent, which can reduce PdCl4 to form Pd at 150 1C. The
cationic surfactant, DTAB, can adsorb onto the as-formed
nuclei or seeds when the metal precursors are reduced, and
make these nuclei or seeds positively charged. As demon-
strated in our previous work, the addition of EG is helpful
to decrease the electrostatic repulsion of charged nuclei and
seeds, and make Velec (electrostatic repulsion potential) smaller
than Vvdw (the van der Waals attraction potential) while still
Fig. 2 XRD pattern (a) of the Pd PNs and CV curves (b) of Pd PNs
and Pd black in 0.1 M H2SO4 solution at a scan rate of 0.05 V sÀ1
.
The current densities were normalized to the electrochemically active
surface areas of the Pd catalysts.
three-dimensional porous network-like architectures were
synthesized on a large scale. Upon closer examination through
high-magnification SEM images (Fig. 1b and c), it can be
distinctly seen that the 3D network-like architectures are
constructed by numerous irregular polyhedra. Upon closer
examination by TEM, the network structure was further
confirmed, as evidenced in Fig. 1d. These polyhedra, as the
building blocks of the network-like architectures, possess
different morphology and size in a range from 200–400 nm.
To further examine the microstructure of the 3D Pd PNs,
typical X-ray diffraction (XRD) pattern is recorded and
presented in Fig. 2a. The diffraction peaks can be assigned
to (111), (200), (220), (311), and (222) diffractions, respectively,
which represents the typical character of a crystalline Pd face-
centered cubic (fcc) phase (JCPDS 05-0681). Interestingly, no
diffraction peaks from Cu or Cu oxide can be seen, which indicate
that Pd samples with high crystallinity and purity were obtained.
Cu2+ has been found to play a key role in the formation of
the polyhedral nanocrystal building blocks. In the absence of
Cu2+ and other conditions kept unchanged, porous network
structure consisting of irregular Pd nanoparticles with an average
size of 50 nm, but not polyhedra, was produced (ESIw, Fig. S1).
In fact, the amount of the copper ions introduced in the synthesis
was important for the formation of the Pd polyhedron units.
When the amount of Cu(NO3)2 is 0.0134 mmol, the yield of the
Pd polyhedra reaches nearly 100%. While the amount of
Cu(NO3)2 decreased to 0.009 mmol, although the network
structure could be obtained, the polyhedral shape selectivity
evidently decreased (ESIw, Fig. S2a). Also, when the amount of
Cu(NO3)2 was increased to 0.0178, 0.0268 and 0.0357 mmol, the
yield of the polyhedra decreased along with the poor network
structure generated (ESIw, Fig. S2b–S2d). These results suggest
that the introduction of an appropriate amount of Cu(NO3)2
would not only facilitate the formation of the Pd polyhedra,
but also promote the generation of interconnected network
structure. Copper ions have been used as the foreign ions in
the synthesis of Pd and Pd-based alloy nanocrystals for shape
control.8,9 As reported in the Cu2+-assisted preparation of rod-like
and branched palladium, some copper ions may be reduced and
reoxidized during the second growth stage and serve as deposition
or nucleation sites due to the relatively close reduction
potential of Cu2+/Cu(0.34 V) with PdCl42À/Pd(0.61 V).8 In
our system, the reduction potential of palladium may be
influenced by the surfactant (DTAB) existing in the reaction
system, which makes their potential to be close. At appropriate
concentration of copper ions, copper atoms preferentially deposit
larger than half of Vvdw. This balance between Velec and Vvdw
,
and the heated fusion will together lead to the formation of
network-like structures.7 Under such solvent-thermal conditions,
the as-prepared Pd nanocrystals further form interconnected
network structure instead of existing independently. Although
the detailed mechanism how the Pd nanopolyhedra tend to
connect together does still need further studies, the synergism
of interparticle electrostatic repulsion modulation and heat-
induced fusion during nucleation and growth may be responsible
for this interesting phenomenon. By introducing a thermodynamic
control of interparticle interactions and heat-induced interparticle
fusion into the nucleation and Ostwald ripening process, 3D
porous network structure can be fabricated.
Note that the relatively large size and stable 3D figure of Pd
PNs make it very difficult to characterize the surface structure
of Pd PNs using high-resolution transmission electron micro-
scopy and selected electron diffraction. In order to further
identify the surface structure of the 3D Pd PNs, electrochemical
voltammetric methods were utilized to obtain overall structural
information. The cyclic voltammogram (CV) of Pd PNs
recorded in 0.1 M H2SO4 solution at a scan rate of 0.05 V sÀ1
at room temperature is presented in Fig. 2b, along with that of
commercial Pd black (Aldrich) for comparison. The electro-
chemically active surface area (ECSA) of the Pd electrodes is
calculated from the electric charges of reduction of monolayer
Pd oxide with an assumption of 424 mC cmÀ2 according to the
CV curves.10 The ECSAs were calculated to be 2.2 m2 gÀ1 for
Pd PNs and 14.5 m2 gÀ1 for commercial Pd black. In comparison
with commercial Pd black, the smaller ECSA of Pd PNs may be
attributed to their larger building blocks (about 300 nm) than the
Pd nanoparticles in Pd blacks. It is worth noting that the oxygen
adsorption/desorption current on the Pd PNs is significantly
larger than that observed on the polycrystalline Pd black
catalyst as shown in Fig. 2b. Similar to oxygen adsorption
on low-coordinated atomic steps of Pt,11 more oxygen adsorp-
tion at low potential indicates that Pd PNs have high density
of low-coordinated atomic steps12 including high density of
atomic steps, ledges, and kinks.
To evaluate the effect of the polyhedron-based network
structure of 3D Pd PNs on their electro-catalytic properties,
HCOOH oxidation on Pd PNs is selected as the probe reaction
owing to the fact that HCOOH electrocatalysis on Pd is known to
be structure sensitive.13 For comparison, formic acid oxidation on
commercially available Pd black is also tested. Fig. 3a displays the
CVs of these two catalysts in 0.1 M HCOOH + 0.1 M H2SO4.
c
3882 Chem. Commun., 2012, 48, 3881–3883
This journal is The Royal Society of Chemistry 2012

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This work was financially supported by the National Natural
Science Foundation of China (20901057 and 11074185) and the
Tianjin Natural Science Foundation (10JCYBJC01800), Innova-
tion Foundation of Tianjin University, and the Scientific Research
Foundation for the Returned Overseas Chinese Scholars,
State Education Ministry.
Notes and references
Fig. 3 CV curves at a scan rate of 0.05 V sÀ1 (a) and chronoampero-
metric curves at 0.1 V (vs. SCE) (b) of Pd black and Pd PNs in 0.1 M
HCOOH + 0.1 M H2SO4. The currents were normalized to the
ECSAs of Pd catalysts. The arrows in CVs (a) indicate the potential
scan direction.
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The currents are normalized to the ECSAs of the Pd catalysts.
The forward peak current density for formic acid oxidation on
Pd PNs is 1.57 mA cmÀ2, which is about 12.1 times higher than
that on Pd black (0.13 mA cmÀ2). The currents were normalized
to the mass amount of Pd catalysts to obtain the mass activities.
The forward peak current density for formic acid oxidation on
Pd PNs is 27.5 mA mgÀ1, which is about 1.46 times higher than
that on Pd black (18.8 mA mgÀ1), as shown in Fig. S3a (ESIw).
The higher current densities indicate that the Pd PNs exhibit
enhanced electrocatalytic activity toward formic acid oxidation.
The improved electroactivity of Pd PNs may be attributed to
the effective electronic conduction through the high porosity
and highly interconnected networks and the electronic states of
the surface atoms because of the unique morphology of the 3D
Pd networks and the exposure of their intrinsic active sites
including high density of atomic steps, ledges, and kinks, and
stable network structure.
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catalysts, which can be attributed to the intermediate poisoning
species formed from formic acid oxidation. Nevertheless, the
current densities on the Pd PNs are evidently higher than those
on Pd black over the entire time range, significantly confirming
the better catalytic ability and stability of Pd PNs toward
HCOOH oxidation than commercial Pd black.
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In conclusion, 3D Pd PNs have been successfully prepared
by a simple, one-pot, Cu2+-assisted, solution-chemical synthesis
method. The Pd PNs catalyst appears to exhibit high stability
and remarkably improved catalytic activity for formic acid
oxidation over commercially available Pd black, with promising
applications in fuel cells and organic synthesis. Importantly, this
synthesis method provides a new synthetic strategy to construct
other noble metal or alloy polyhedron networks.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3881–3883 3883