Simple preparation method of Pd nanoparticles on an Au electrode and its catalysis for dioxygen reduction

Article abstract of DOI:10.1039/b300566f

A simple method for the fabrication of Pd nanoparticles is described. The three-dimensional Pd nanoparticle films are directly formed on a gold electrode surface by simple electrodeposition at -200 mV from a solution of 1 M H₂SO₄ + 0.01 mM K₂PdCl₄. X-Ray photoelectron spectroscopy verifies the constant composition of the Pd nanoparticle films. Atomic force microscopy proves that the as-prepared Pd nanoparticles are uniformly distributed with an average particle diameter of 45-60 nm. It is confirmed that the morphology of the Pd nanoparticle films are correlated with the electrodeposition time and the state of the Au substrate. The resulting Pd-nanoparticle-film-modified electrode possesses high catalytic activity for the reduction of dissolved oxygen in 0.1 M KCl solution. Freshly prepared Pd nanoparticles can catalyze the reduction of O₂ by a 4-electron process at -200 mV in 0.1 M KCl, but this system is not very stable. The cathodic peaks corresponding to the reduction of O₂ gradually decrease with potential cycling and at last reach a steady state. Then two well-defined reduction peaks are observed at -390 and -600 mV vs. Ag/AgCl/KCl (sat.). Those two peaks correspond to a 2-step process for the 4-electron reduction pathway of O₂ in this neutral medium. The former peak is ascribable to the 2-electron reduction of O₂ to H₂O₂, while the latter is assigned to the reduction of H₂O₂ to H₂O. The observed electrocatalysis for the reduction of O₂ is attributable to the extraordinary catalytic activity of the Pd nanoparticles over the bulk gold electrode.

Full text of DOI:10.1039/b300566f

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Simple preparation method of Pd nanoparticles on an Au electrode
and its catalysis for dioxygen reductiony
Yan Shen, Lihua Bi, Baifeng Liu and Shaojun Dong*
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, China.
E-mail: dongsj@ns.ciac.jl.cn; Fax: +86-431-5689711
Received (in Montpellier, France) 12th January 2003, Accepted 27th January 2003
First published as an Advance Article on the web 9th May 2003
A simple method for the fabrication of Pd nanoparticles is described. The three-dimensional Pd nanoparticle
films are directly formed on a gold electrode surface by simple electrodeposition at ꢀ200 mV from a solution of
1 M H2SO4 + 0.01 mM K2PdCl4 . X-Ray photoelectron spectroscopy verifies the constant composition of the
Pd nanoparticle films. Atomic force microscopy proves that the as-prepared Pd nanoparticles are uniformly
distributed with an average particle diameter of 45–60 nm. It is confirmed that the morphology of the Pd
nanoparticle films are correlated with the electrodeposition time and the state of the Au substrate. The
resulting Pd-nanoparticle-film-modified electrode possesses high catalytic activity for the reduction of dissolved
oxygen in 0.1 M KCl solution. Freshly prepared Pd nanoparticles can catalyze the reduction of O2 by a 4-
electron process at ꢀ200 mV in 0.1 M KCl, but this system is not very stable. The cathodic peaks corresponding
to the reduction of O2 gradually decrease with potential cycling and at last reach a steady state. Then two well-
defined reduction peaks are observed at ꢀ390 and ꢀ600 mV vs. Ag/AgCl/KCl (sat.). Those two peaks
correspond to a 2-step process for the 4-electron reduction pathway of O2 in this neutral medium. The former
peak is ascribable to the 2-electron reduction of O2 to H2O2 , while the latter is assigned to the reduction of
H2O2 to H2O. The observed electrocatalysis for the reduction of O2 is attributable to the extraordinary catalytic
activity of the Pd nanoparticles over the bulk gold electrode.
ligands. For example, Crooks et al.28,29 have prepared
Introduction
PAMAM dendrimer-encapsulated Pd and other metal nano-
particles that employ dendrimers as both template and stabili-
zer. Mayer et al.30 have reported that Pd nanoparticles were
prepared by reducing the metal precursors in the presence of
the protective amphiphilic block copolymer polystyrene-b-
poly(methacrylic acid). Bronstein et al.23 have also reported
that Pd colloids were prepared in polystyrene-poly-4-vinylpyr-
idine (PS-b-P4VP) block copolymer micelles and deposited on
Al2O3 . However, a poor stability of the Pd particles under these
conditions has been reported. In addition, this type of stabiliza-
tion leads to the loss of catalytic activity to a great extent.31 In
this work, a direct synthetic method of three-dimensional Pd
nanoparticles on a gold electrode was developed.
Oxygen reduction on an electrode surface is one of the most
fundamental electrochemical reaction. This reaction is relevant
to the processes occurring not only in fuel cells but also to those
related to corrosion and metal-air batteries. Consequently
dioxygen reduction has been well-studied. Several electrode
materials with or without modification have been suggested
as cathodes for the reduction of O2 in different media.32,33 How-
ever, it is undisputed that Pt is still the indispensable and most
effective catalyst.34 Recently, surface-modified palladium elec-
trodes have been suggested for many electrocatalytic and elec-
troanalytical applications in view of their superior activity.35,36
On the other hand, the electrodeposition of highly dispersed Pd
nanoparticles on an inert substrate attracts growing interest
because of their extraordinarily high catalytic activity in many
reactions involving small organic molecules.16,37 But electrodes
modified with Pd have been far less intensively investigated
than the Pt electrode for the electrocatalysis of O2 reduction.
In this communication, a simple preparation method of
a three-dimensional Pd nanoparticle film is reported. Ideally,
In recent years, the fabrication of nanostructured materials
and the exploration of their properties has attracted much
attention from scientists in various fields.1–3 The continuing
interest in nanosized noble metal particles has been driven by
their unusual physical and chemical properties, which are quite
different from those of the bulk materials.4–6 Intensive studies
have been devoted to their use as catalysts and sensors, as well
as their potential applications in a new generation of optical,
electronic and magnetic devices.7–11 These applications are clo-
sely related to their size- or shape-dependent properties, such
as size quantization, optical and electronic properties, surface
effects, chemical reactivity, and self-assembly ability. Among
these studies, some have been directed toward the synthesis
of palladium nanoclusters,12–16 partly because of the important
role played by palladium in catalysis where the catalytic activ-
ity has been found to be very sensitive to the particle size,
shape, surrounding media, etc.17–19
Pd colloidal particles have been prepared by a variety of tech-
niques, including chemical liquid deposition,20 refluxing alco-
hol reduction,21–23 hydrogen reduction24 and electrochemical
deposition,25 where, in general, the particles were formed by
the reduction of metal ions in the presence of stabilizers. More-
over, most of the nanoparticles must be pre-synthesized
through reduction of a metal salt by reducing agents in solu-
tion.26,27 Often the catalyst particles are stabilized in a suitable
solid matrix or by surface modification using polymers or
y Electronic supplementary information (ESI) available: XRD pattern
of an evaporated Au electrode and CVs for the reduction of O2 on a
bare Au(111) electrode or a Pd-nanoparticle-film-modified electrode.
See: http://www.rsc.org/suppdata/nj/b3/b300566f/
938
New J. Chem., 2003, 27, 938–941
DOI: 10.1039/b300566f
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2003

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well-dispersed palladium nanocrystals were deposited on a
gold electrode from the electrolyte. AFM and XPS were used
to characterize the Pd nanoparticles. The resulting Pd-nano-
particle-film-modified electrode possesses high catalytic activ-
ity for the reduction of dissolved oxygen.
Experimental
General procedures
Milli-Q water (18 MO resistivity; Millipore systems) was used
for all experiments. Potassium tetrachloropalladate (K2PdCl4),
KCl, H2O2 and H2SO4 were of analytic grade and used as
received.
Analysis of the X-ray photoelectron spectra (XPS) was per-
formed on a ESCLAB MK II using Mg as the exciting source.
AFM measurements were performed with a Digital Nano-
scope IIIa multimode system (DI, Santa Barbara, CA,
USA). The images were acquired in the tapping mode. Mea-
surements were made with the Si cantilever in air at room
temperature. The force constant of the cantilever was 0.1–0.6
N mꢀ1 with a scan rate of 1–2 Hz.
Fig. 1 X-Ray photoelectron spectra of the Pd 3d energy levels for
a gold plate modified with Pd nanoparticle films by electrodeposition
for 10 min and 20 min.
eV, which are the characteristic 3d5/2 and 3d3/2 peaks for Pd0,
indicate the existence of Pd0 on the Au electrode. We can see
that as the electrodeposition time is prolonged, the intensity
of the Pd0peaks is increased.
Atomic force microscopy (AFM) provides further detailed
information involving the surface morphology and homogene-
ity of the films. Fig. 2 presents the typical tapping-mode AFM
images of the palladium nanoparticle films electrodeposited on
gold plates at ꢀ200 mV for 10 and 30 min. After electrodeposi-
tion for 10 min [Fig. 2(B)], the Pd nanoparticles can be seen
clearly. They form a densely packed film, in which each particle
is in contact with adjacent ones. The Pd particles are homoge-
neously distributed on the electrode surface and relatively
uniform with a size of 45 nm. The mean roughness (7.30 nm)
is higher than that of the bare Au substrate [Fig. 2(A), 5.57
nm], but the Au electrode surface is not completely covered
with Pd nanoparticles. When electrodeposition is prolonged
to 30 min [Fig. 2(C)], the nanoparticles become more closely
packed. In this case, the surface mean roughness is higher
(9.87 nm), and the diameter (60 nm) of the Pd nanoparticles
is bigger than that of the Pd nanoparticles obtained after elec-
trodeposition for 10 min. In addition, from Fig. 2(C) we can
see that the nanoparticles are arranged in lines and look to
be in a very ordered arrangement. A tentative explanation
might be that the bare Au substrate plate has some line ter-
races [Fig. 2(A)], because the metal nanoparticles tend to form
at terrace zones of the surface.38 From this point, we can infer
that the morphology of the Pd nanoparticle films directly
depend on the surface state of the substrate and the electro-
deposition time. This interpretation is further supported by
experiments in which the Pd nanoparticles are electrodeposited
on an Au(111) single-crystal electrode. Fig. 2(D) and Fig. 2(E)
show the AFM morphology for the bare Au(111) single-crystal
surface and Pd nanoparticles electrodeposited onto an Au(111)
electrode, respectively. It is clear from Fig. 2(E) that the Pd
nanoparticles nicely dispersed on the surface of the Au(111)
electrode and that they are very different from those shown
in Fig. 2(C). This is because the surface of the bare Au(111)
electrode (surface roughness: 1.63 nm) is smoother than that
of the polycrystalline Au electrode (surface roughness: 5.57 nm).
The resulting Pd nanoparticle films are very stable. When
immersed in 0.1 M KCl solution for one month, the morphol-
ogy shows no change. This is a very valuable property for cat-
alysts and device fabrication. It also allows one to obtain
uniform charge transport throughout the whole films.
Electrochemical measurements
Cyclic voltammetry was performed with a CHI 660 electroche-
mical workstation (USA) in a conventional three-electrode
electrochemical cell. The working electrode was an Au wire
(f ¼ 3.5 mm) sealed in a Teflon jacket. The exposed geometric
surface area of the electrode was 0.096 cm2. Twisted platinum
wire and Ag/AgCl/KCl (sat.) were the counter and the refer-
ence electrode, respectively. Prior to each experiment, if neces-
sary, O2 was bubbled directly into the cell for 30 min to obtain
an O2-saturated 0.1 M KCl solution and during the measure-
ments O2 was flushed over the cell solution. Electrolyte solu-
tions were, if necessary, deaerated by bubbling N2 for at
least 30 min prior to electrochemical measurements. All the
measurements were performed at room temperature.
Electrode modification procedure
The polycrystalline Au electrodes and Au plates were polished
with aqueous slurries of successively finer a-Al2O3 powder
(down to 0.05 mm) and were sonicated in water. After being
washed carefully with distilled water, the Au electrodes were
electrochemically pretreated in a 0.5 M H2SO4 solution by
repeating the potential scan in the range of ꢀ0.2 to 1.5 V vs.
Ag/AgCl/KCl (sat.) at 100 mV sꢀ1 until the cyclic voltammo-
gram (CV) characteristic for a clean Au electrode was
obtained. The glass-covered gold films were purchased from
Beijing (The Institute of Electronics, Chinese Academy of
Sciences, China). The gold films were cleaned in a mixture of
H2SO4–H2O2 (7:3; piranha solution), washed carefully with
distilled water and dried in a stream of nitrogen. Then the
Au films were characterized with XRD and shown to be Au
(111) single crystal. (see Electronic supplementary informa-
tion, ESI). Cleaned gold electrodes were modified by electro-
deposition at ꢀ200 mV from a solution containing 0.01 mM
K2PdCl4 + 1.0 M H2SO4 with magnetic stirring of the electro-
lyte solution. In this paper, all the modified Au electrodes used
to catalyze O2 are prepared by electrodeposition for 30 min.
Results and discussion
XPS and AFM characterization of Pd nanoparticles
electrodeposited on Au electrode
O2 reduction by Pd nanoparticles electrodeposited on
an Au electrode
In order to confirm the existence of the palladium nanoparti-
cles electrodeposited on the gold electrode surface, XPS experi-
ments were carried out. In Fig. 1, two peaks at 335.0 and 342.5
Fig. 3 shows the cyclic voltamograms (CVs) obtained for the
reduction of O2 with bulk and Pd-nanoparticle-modified Au
New J. Chem., 2003, 27, 938–941
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Fig. 3 CVs obtained for the O2 reduction at: (a) freshly prepared Pd
nanoparticles deposited on an Au electrode; (b) bulk Au electrode; (c)
modified electrode of (a) after potential cycling for hours in air-satu-
rated 0.1 M KCl solution. The dashed and dotted lines correspond
to the bare and modified Au electrode, respectively, in N2-saturated
0.1 M KCl solution. Potential scan rate: 100 mV sꢀ1
.
O2 to H2O2 and the further reduction of H2O2 to H2O, respec-
tively.39 The difference between the two peak potentials is very
large. This means that the reduction of H2O2 on Au is rather
slow. For the Au electrode with electrodeposited nanosized
Pd particles, the single reduction peak at ꢀ200 mV is rationalyl
considered the 4-electron reduction of O2 .
In order to sustain this suggestion, the CVs were measured
with a freshly prepared modified Au electrode. Fig. 4(A)
shows cyclic voltammograms of the nanosized Pd particles
electrodeposited on an Au electrode in the same solution satu-
rated by O2 at different scan rates. The peak currents vary as
a linear function of the square root of the scan rate, indicat-
ing a diffusion-controlled electrochemical process. The num-
ber of electrons (n) for the catalytic reduction of dioxygen
can be calculated from the slope of the plot of ip vs. v1/2 40
.
The experimental results almost match the calculated peak
currents for the reduction of dioxygen to water (n ¼ 4), as
shown in Fig. 4(B). But the situation of curve a in Fig. 3 is
not very stable. The peak corresponding to the reduction of
O2 gradually decreases with potential cycling and reaches a
steady state at last as shown by curve c in Fig. 3. Two
well-defined reduction peaks were observed at ꢀ390 and
ꢀ600 mV vs. Ag/AgCl/KCl (sat.). These two peaks are also
ascribable to the 2-electron reduction of O2 to H2O2 and
the further reduction of H2O2 to H2O. Compared with curve
b in Fig. 3, the peak current is higher and the difference
between the potentials of O2/H2O2 and H2O2/H2O is smaller.
This indicates that the reduction of H2O2 by Pd nanoparticles
deposited on a polycrystalline Au electrode is more rapid than
on a bulk Au electrode.
To further confirm the mechanism of O2 reduction at the
modified Au electrode after potential cycling for hours, the
Fig. 2 AFM images of Pd nanoparticle films prepared on an Au
plate by electrodeposition for (A) 0; (B) 10 and (C) 30 min, respec-
tively; (D) bare Au(111) electrode and (E) Pd nanoparticle film on
an Au(111) electrode (electrodeposition for 30 min).
electrodes in air-saturated 0.1 M KCl solution. This figure
reflects two important catalytic features of the Au electrode
loaded with the Pd nanoparticles: (i) a significant positive shift
and combination of the O2 reduction peak from ꢀ280 and
ꢀ700 mV (in the case of the bulk Au electrode) to ꢀ200 mV
upon loading of the Pd nanoparticles and (ii) an increase in
the peak current (compares curves a and b). At the bulk Au
electrode, the reduction peaks at ꢀ280 mV and ꢀ700 mV vs.
Ag/AgCl/KCl (sat.) are due to the 2-electron reduction of
Fig. 4 (A) Cyclic voltammograms of an O2-saturated solution at 20,
40, 60, 80, 100, 120, and 150 mV sꢀ1. (B) Plot of the electrocatalytic
dioxygen reduction peak current as a function of the square root of
the scan rate (solid circles and line) and the calculated curve for the
reaction with n ¼ 4 (dashed line). Electrolyte: 0.1 M KCl.
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potential cycling for hours in 0.1 M KCl saturated with N2
or air, in the absence and presence of H2O2 . It is clear from
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In this communication, we report a simple method for the pre-
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method, simple and fast, is a general approach to the prepara-
tion of metal nanoparticles on solid surfaces. X-Ray photo-
electron spectroscopy verifies the constant composition of
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Acknowledgements
This work is supported by the National Science Foundation of
China (No. 20275036). We are grateful to Dr. Aiguo Wu for
his help with the atomic force microscopy experiments.
New J. Chem., 2003, 27, 938–941
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