Designed nanostructured Pt film for electrocatalytic activities by underpotential deposition combined chemical replacement techniques

Article abstract of DOI:10.1021/jp051612e

Multiple-deposited Pt overlayer modified Pt nanoparticle (MD-Pt overlayer/PtNPs) films were deliberately constructed on glassy carbon electrodes through alternately multiple underpotential deposition (UPD) of Ag followed redox replacement reaction by Pt (II) cations. The linear and regular growth of the films characterized by cyclic voltammetry was observed. Atomic force spectroscopy (AFM) provides the surface morphology of the nanostructured Pt films. Rotating disk electrode (RDE) voltammetry and rotating ring-disk electrode (RRDE) voltammetry demonstrate that the MD-Pt overlayer/PtNPs films can catalyze an almost four-electron reduction of O₂ to H ₂O in air-saturated 0.1 M H₂SO₄. Thus-prepared Pt films behave as novel nanostructured electrocatalysts for dioxygen reduction and hydrogen evolution reaction (HER) with enhanced electrocatalytic activities, in terms of both reduction peak potential and peak current, when compared to that of the bulk polycrystalline Pt electrode. Additionally, it is noted that after multiple replacement cycles, the electrocatalytic activities improved remarkably, although the increased amount of Pt is very low in comparison to that of pre-modified PtNPs due to the intrinsic feature of the UPD-redox replacement technique. In other words, the electrocatalytic activities could be improved markedly without using very much Pt by the technique of tailoring the catalytic surface. These features may provide an interesting way to produce Pt catalysts with a reliable catalytic performance as well as a reduction in cost. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp051612e

15264
J. Phys. Chem. B 2005, 109, 15264-15271
Designed Nanostructured Pt Film for Electrocatalytic Activities by Underpotential
Deposition Combined Chemical Replacement Techniques
Minghua Huang, Yongdong Jin, Heqing Jiang, Xuping Sun, Hongjun Chen, Baifeng Liu,
Erkang Wang, and Shaojun Dong*
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun, Jilin, 130022, People’s Republic of China, and
Graduate School of Chinese Academy of Sciences, Beijing, 100039, People’s Republic of China
ReceiVed: March 30, 2005; In Final Form: June 27, 2005
Multiple-deposited Pt overlayer modified Pt nanoparticle (MD-Pt overlayer/PtNPs) films were deliberately
constructed on glassy carbon electrodes through alternately multiple underpotential deposition (UPD) of Ag
followed redox replacement reaction by Pt (II) cations. The linear and regular growth of the films characterized
by cyclic voltammetry was observed. Atomic force spectroscopy (AFM) provides the surface morphology of
the nanostructured Pt films. Rotating disk electrode (RDE) voltammetry and rotating ring-disk electrode (RRDE)
voltammetry demonstrate that the MD-Pt overlayer/PtNPs films can catalyze an almost four-electron reduction
of O2 to H2O in air-saturated 0.1 M H2SO4. Thus-prepared Pt films behave as novel nanostructured
electrocatalysts for dioxygen reduction and hydrogen evolution reaction (HER) with enhanced electrocatalytic
activities, in terms of both reduction peak potential and peak current, when compared to that of the bulk
polycrystalline Pt electrode. Additionally, it is noted that after multiple replacement cycles, the electrocatalytic
activities improved remarkably, although the increased amount of Pt is very low in comparison to that of
pre-modified PtNPs due to the intrinsic feature of the UPD-redox replacement technique. In other words, the
electrocatalytic activities could be improved markedly without using very much Pt by the technique of tailoring
the catalytic surface. These features may provide an interesting way to produce Pt catalysts with a reliable
catalytic performance as well as a reduction in cost.
Introduction
costs.7-9 Many efforts have focused on the development of
techniques to produce Pt catalysts with a high surface area to
achieve reliable catalytic performance and to utilize efficiency.10
For example, various nanosized Pt-alloy electrocatalysts with
narrow size distribution11,12 and nanocomposite multilayer films
containing Pt nanoparticles13,14 were exploited to study the
electrocatalytic activities for dixogen reduction. In addition, Pt
hollow nanospheres7 and thin mesoporous Pt films15 were also
deliberately designed recently and exhibited enhanced electro-
catalytic performance with a cost reduction.
Since surface-catalyzed reactions are extremely sensitive to
the details of the catalytic surface,16 the deliberate tailoring of
the catalyst surface is an interesting target for the improvement
of the catalytic performance. Recently, a new metal deposition
method that could form near-uniform nanostructured Pt-group
overlayers on substrates has been reported, which involves redox
replacement of an underpotential deposition (UPD) monolayer
by more noble metal cations that could be reduced and
simultaneously deposited.17 Thus-tailored nanostructured films
are of importance with regard to catalysis applications, due to
their high surface-to-volume ratio. Weaver’s group perfected
the UPD-redox replacement technique to prepare Au nanopar-
ticle film electrodes modified with uniform Pt-group overlayers
and succeeded in surface-enhanced Raman scattering (SERs)
applications.18 Kwak et al. performed electrochemical measure-
ments on various UPD systems on layered Au nanoparticle thin
films.19 Very recently, Adzic’s group20 in addition to us21 have
exploited the Pt submonolayers on metal nanoparticles such as
Ru and Au nanoparticles as electrocatalysts. However, to the
best of our knowledge, electrochemical modification of the
Electrocatalytic reactions play a vital role in emerging
technologies related to environmental and energy-related ap-
plications, such as fuel cells that are energy sources in an attempt
to relieve the pollution and energy crisis. In many electrochemi-
cal reactions, the hydrogen evolution reaction occurring in acid
media (H+ + e- f 1/2H2) and the four-electron electroreduction
of dioxygen to water (O2 + 4H+ + 4e- f 2H2O) is extensively
investigated. The former is of considerable significance in
hydrogen-oxygen fuel cells, hydrogen embrittlement, elec-
trodeposition of metals in acid media, and so on.1,2 The latter
is the cathode reaction of most fuel cells that utilize dioxygen
as the reducible reactant.3-5 The fact that the electroreduction
of dioxygen requires high current density, low overpotential,
and nearly synchronous delivery of four electrons gave rise to
many innovative electrocatalyst designs.6 Among various al-
ternative candidates, platinum (Pt) and Pt-based materials are
still the indispensable and most effective electrocatalysts
employed. However, the high cost and limited supply of Pt
constitute a barrier of the commercialization of fuel cells.
Therefore, it is necessary to design economical and effective
Pt-based catalysts with low loading from the viewpoint of
practical applications.
At the same time, the rapid development of nanoscience in
recent years opens a way of designing highly efficient catalysts
with large surface-to-volume ratios from the controlled nano-
scale structures, along with huge savings in the fabrication
* To whom correspondence should be addressed. Fax: +86-431-
5689711; e-mail: dongsj@ciac.jl.cn.
10.1021/jp051612e CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/27/2005

Pt Film for Electrocatalytic Activities
J. Phys. Chem. B, Vol. 109, No. 32, 2005 15265
multi-deposited Pt overlayers on metal nanoparticle monolayer
film electrodes has not yet been exploited for catalytic applica-
tion to date.
When the resulting modified electrode alternately undertook
further UPD Ag/Pt-metal redox replacement cycles, the thick-
ness and amount of Pt overlayers of the as-prepared film
electrodes were readily adjusted by choosing different numbers
of redox replacement cycles of UPD Ag.
In this paper, an alternately multiple UPD-redox replacement
reaction technique was adopted to improve catalytic perfor-
mance. A UPD Ag monolayer was first formed on the pre-
modified Pt nanoparticle (PtNPs) monolayer films. Then, UPD
Ag replacement with Pt (II) yielded a uniform Pt overlayer.
When the modified electrode alternately undertook UPD-redox
replacement reactions in a cyclic fashion, multiple-deposited
Pt overlayer modified Pt nanoparticle (MD-Pt overlayer/PtNPs)
monolayer films could be formed on a glassy carbon electrode
(GCE) pretreated with 4-aminobenzoic acid (4-ABA). Such a
tactic is plausible given that UPD Ag is known to form on Pt.22
Interestingly, as compared to that of the pre-modified PtNPs
monolayer films, thus-tailored nanostructured Pt films by
multiple replacement cycles exhibit more positive potential and
a higher current for dioxygen reduction and hydrogen evolution
reactions (HER). The results provide clear evidence that the
multi-deposited Pt overlayers with low loading could remarkably
improve electrocatalytic activities, which aid in the design of
catalysts for practical applications. Additionally, the film
modified electrode also shows higher electrocatalytic activities
than that of the bulk polycrystalline Pt electrode, in terms of
both the reduction peak potential and the current density.
Electrochemical Measurements. Electrochemical experi-
ments were performed with a CHI 660 electrochemical worksta-
tion in a conventional three-electrode electrochemical cell using
GCE (3 mm diameter) as the working electrode, twisted
platinum wire as the auxiliary electrode, and Ag/AgCl as a
reference electrode in aqueous media or Ag/Ag+ (0.01 M
AgNO3) in anhydrous ethanol solutions. The GCEs were
polished with 1.0 and 0.3 µm R-Al2O3 powders successively
and sonicated in water for about 3 min after each polishing step.
Finally, the electrodes were sonicated in ethanol, washed with
ethanol, and dried with a high purity nitrogen stream im-
mediately before use. Solutions were deaerated for at least 20
min with a high purity nitrogen stream and kept under a pressure
of this gas during the experiments.
An EG and PARC model 636 rotating ring-disk electrode
system and an EG and PARC model 366 bipotentiostat were
used for RDE and RRDE voltammetry experiments. A rotating
GC disk-platinum ring electrode (5 mm diameter) was used as
a working electrode. The collection efficiency (N) of the ring
electrode obtained by reducing ferricyanide at a disk electrode
was 0.23. Before the experiments, the bulk Pt electrode (4 mm
diameter) was mechanically polished and scanned in 0.1 M
H2SO4 at the range of -0.2 to 1.5 V until CV characteristics
for a clean Pt electrode were obtained.
Instrumentation. Samples for TEM were prepared by placing
a drop of solution onto a carbon-coated copper grid and
examined using a JEOL 2010 transmission electron microscope
operated at 200 kV. X-ray photoelectron spectroscopy (XPS)
measurement was performed on an ESCALAB-MKII spectrom-
eter (VG Co.) with AlK_ (X-ray radiation as the X-ray source
for excitation and analyzer pass energy of 50 eV). Typically,
the operating pressure in the analysis chamber was below 10-9
Torr. The resolution of the spectrometer was 0.2 eV. Atomic
force microscopy (AFM) measurements were performed on the
GCE with a SPI3800N microscope (Seiko Instruments, Inc.).
The images were acquired in the tapping mode. Measurements
were made with the Si cantilever in air at room temperature.
The force constant of the cantilever was 0.1-0.6 N/m with a
scan rate of 1-2 Hz.
Experimental Procedures
Materials. 4-Aminobenzoic acid (4-ABA), K2PtCl4, and poly
(diallyldimethylammonium chloride) (PDDA) were purchased
from Aldrich. The absolute ethanol was dried over 3 Å
molecular sieves, and lithium perchlorate was dried at about
90 ×bcC in a vacuum oven before use. H2PtCl6‚6H2O, Ag2-
SO4, H2SO4, and other chemicals were of analytical reagent
grade and used as received. Water used for preparation of
aqueous solution was purified using a Millipore-Q water
purification system.
Synthesis of Citrate-Stabilized Pt Colloids. One milliliter
of 1% H2PtCl6 aqueous solution was added into 100 mL of water
and then heated to boiling. Aging of the H2PtCl6 solution was
not necessary in this synthetic procedure. Then, 3 mL of 1%
sodium citrate aqueous solution was added rapidly, and the
mixture was kept at a boiling temperature for ca. 30 min.
Transmission electron microscopy (TEM) was used to examine
particle size distribution, and an average diameter of 6 nm of
Pt colloids was confirmed.
Results and Discussion
Preparation Procedure of the MD-Pt Overlayer/PtNPs
Films. A GCE was treated with 4-ABA according to the
published procedures.23 The 4-ABA/GCE surface with the
COOH group was first placed in an aqueous solution containing
8.0 wt % PDDA, providing a stable, positively charged surface
on which the negatively charged citrate-stabilized PtNPs can
be bound electrostatically. Then, the PtNPs were immobilized
onto the PDDA/4-ABA/GCE by immersion of the latter into
the sols for more than 12 h. A UPD Ag monolayer was formed
on the PtNPs monolayer films by cycling the potential between
0 and +0.7 V at a scan rate of 20 mV/s for several cycles in
0.1 M H2SO4 containing 1 mM Ag2SO4. After thoroughly
rinsing with water and drying with a nitrogen stream, the
modified electrode was transferred to a solution of 5 mM K2-
PtCl4 in deaerated 0.1 M HClO4 for 30 min to ensure the
complete redox replacement of UPD Ag, which results in a Pt
overlayer on the pre-modified PtNPs monolayer films. The
modified electrode was then thoroughly rinsed and scanned in
0.1 M H2SO4 at the range of -0.2 to 1.5 V for several cycles.
Fabrication of the MD-Pt Overlayer/PtNPs Films. The
deposition protocol employed here is based on a new metal
deposition method recently reported.17 It involved a redox
replacement of a UPD metal monolayer by more noble metal
cations (such as late transition metal cations Pt and Pd) that are
reduced and simultaneously deposited. The spontaneous redox
replacement process, yielding a late transition-metal overlayer,
is driven by the difference at equilibrium potentials between
the transition metal and the UPD metal redox couples. For the
system employed, PtCl42-/Pt, the standard potentials (E°) of the
corresponding half-reaction are more positive than those for the
Ag+/AgUPD couple.24 So, the spontaneous redox replacement
of a previously prepared UPD Ag monolayer by Pt (II) cations
can be used to form Pt metal deposits, consistent with the
anticipated balanced Ag f Ag+/Pt (II) f Pt electron charge
flow.
Through the attachment of 4-ABA containing the COOH
group to GCE, the 4-ABA modified GCE was successively

15266 J. Phys. Chem. B, Vol. 109, No. 32, 2005
Huang et al.
Figure 3. CV of PtNPs/PDDA/4-ABA/GCE in 0.1 M H2SO4 solution
containing 1 mM Ag2SO4. The dashed line shows the control CV
obtained on PDDA/4-ABA/GCE (PtNPs-free) after the same prepara-
tions. Scan rate: 20 mV s-1
.
is a familiar feature of large surface area material, which might
reflect the nanoparticle nature of the electrode surface.18,21
Then, a solution of 1 mM Ag2SO4 and 0.1 M H2SO4 was
used to deposit the Ag monolayer by the UPD method. Figure
3 displays CVs of UPD Ag on the PtNPs monolayer-modified
electrode surface in a 1 mM Ag2SO4 + 0.1 M H2SO4 solution
over a period of five successive scans. As seen in Figure 3, the
feature is consistent with the formation of a UPD Ag mono-
layer.26 On the negative-going sweep, one cathodic peak (A1)
is observed at ca. 0.369 V, corresponding to the formation of
UPD Ag. One anodic peak at 0.476 V (A1′) is obtained on the
positive-going sweep, consistent with the dissolution of UPD
Ag. Thus, the modified electrode was then transferred to a
deaerated solution containing 5 mM K2PtCl4 + 0.1 M HClO4
at the open circuit for 30 min, resulting in a Pt overlayer on the
pre-modified PtNPs monolayer films. It is noteworthy to
mention that the Pt overlayer was selectively deposited on the
PtNPs monolayer film, not on the GCE surface. In the control
experiment, the PDDA/4-ABA/GCE after UPD Ag exhibits
featureless responses of Ag as shown in Figure 3 (dashed line),
indicating that the Ag monolayer cannot deposit on these sites
without PtNPs.
Figure 1. TEM image of PtNPs as templates for catalyst design (scale
bar ) 20 nm).
The site-selective depositing of the Ag monolayer to PtNPs
determines the selectivity of the Pt overlayer formed by redox
replacement to particle sites, which can be reproducibly prepared
by alternately undertaking further UPD Ag/Pt-metal redox
replacement. With Pt as the redox-active species, the generation
of the MD-Pt overlayers on particle surfaces results in reproduc-
ible electrochemical responses. Figure 2A shows representative
CVs obtained from -0.2 to 1.5 V at the MD-Pt overlayer/PtNPs
monolayer film electrode with different numbers of redox
replacement cycles of UPD Ag from the K2PtCl4 solution. It
can be seen that the CVs are similar to that of a polycrystalline
Pt electrode,25 displaying the characteristic anodic-cathodic
current profile in which near-reversible peaks below 0 V are
associated with hydrogen desorption-adsorption. Also, the
irreversible feature corresponds to the formation and removal
of a Pt surface oxide. In the control experiments (the Pt
overlayers deposit in the absence of PtNPs), no distinctive Pt
feature appears (dashed line in Figure 2B), validating that the
Pt overlayers selectively deposited on PtNPs. Furthermore, we
can see that when the first Pt overlayer is formed on the PtNPs
monolayer films, the onset of the oxide formation and the peak
potential of the oxide reduction are shifted more positively,
indicating that the chemisorptions of OH on the Pt sites may
be inhibited when the Pt overlayer is formed on the PtNPs
films.11 This may have a beneficial effect on the oxygen
adsorption at a low overpotential and thus may lead to
improvement in dioxygen reduction kinetics.11,27 It also should
Figure 2. (A) CVs of the MD-Pt overlayer/PtNPs film electrodes with
different redox replacement cycles of UPD Ag from K2PtCl4 solu-
tions: n ) 1-5 and 7 (from inside to outside), respectively. The dashed
line represents the CV of the PtNPs monolayer film modified electrode.
The inset shows a linear dependence of the platinum oxide reduction
peak currents with the number of layers. (B) A comparison between
the PtNPs films with one replacement cycle (solid line) and the control
CV obtained on PDDA/4-ABA/GCE (PtNPs-free) after the same
preparations (dashed line). Supporting electrolyte: 0.1 M H2SO4
solution and scan rate: 50 mV s-1
.
immersed in positively charged PDDA solutions and negatively
charged citrate-stabilized PtNPs. Here, the aim of the effectively
self-assembled PtNPs with relatively high loading as a template
for the design of Pt overlayers catalyst was mainly to indicate
that after replacement, the MD-Pt overlayers with very low
loading could improve electrocatalytic activities remarkably. The
average particle diameter of 6 nm was characterized by TEM
as shown in Figure 1. Figure 2A (dashed line) shows a
representative cyclic voltammogram (CV) obtained at the
resulted PtNPs monolayer-modified electrode in 0.1 M H2SO4.
The presence of the hydrogen adsorption/desorption peaks and
preoxidation/reduction peaks shows familiar features of platinum
electrochemistry.25 The large current in the double-layer region

Pt Film for Electrocatalytic Activities
J. Phys. Chem. B, Vol. 109, No. 32, 2005 15267
Figure 4. XPS spectra of PtNPs/PDDA/4-ABA/GCE show (A) Ag
3d: after UPD Ag (curve a) and Ag/Pt redox replacement (curve b)
and (B) Pt 4f: before (curve a) and after one Pt overlayer deposition
by one replacement cycle (curve b).
be mentioned here that no observable changes in the shape and
height of the CVs are found during the whole electrochemical
measurements and that the real surface area of the Pt catalyst
keeps an almost constant value even when there is continuous
cycling between -0.2 and 1.5 V, which excludes the possibility
of severe agglomeration and sintering and also indicates that
the catalyst is very stable. In addition, with increasing the
number of redox replacement cycles, the redox peak currents
increase gradually. Taking the platinum oxide reduction peak
as an example, a good linear relationship between the cycles
and the cathodic peak currents of the Pt overlayers (the inset of
Figure 2A) demonstrates that equal amounts of Pt overlayers
are deposited in each UPD-replacement cycle and also that the
film grows uniformly.
XPS Measurement. XPS measurements were also performed
to identify and characterize the as-prepared PtNPs film as shown
in Figure 4. The XPS spectrum of the Ag 3d3/2 and Ag 3d5/2
doublet in the zero oxidation state before a redox replacement
reaction demonstrates the successful UPD Ag on the PtNPs
monolayer films (curve a in Figure 4A). After a redox
replacement reaction with K2PtCl4, there is little residual silver
left as shown in curve b of Figure 4A. It is possible that a
bimetallic interaction or possible alloying between silver and
platinum cannot be totally excluded. In addition, as shown in
CV of UPD Ag (Figure 3), the broadness and asymmetry of
the peak may also reflect the process of the Ag-Pt surface
alloying to a certain extent, probably arising from the influence
of the grain defect feature (such as steps, kinks, and edges) of
the nanofilms on the electrochemical behaviors and also from
the overlap of another couple of peaks (labeled A2 and A2′).26
Also, citrate-stabilized PtNPs before the redox replacement
reaction were not in the completely zero state as shown in curve
a of Figure 4B. After a replacement reaction, the Pt 4f7/2 and Pt
4f5/2 doublet in the zero oxidation state appear, indicating the
complete formation of Pt (0) (curve b in Figure 4B).
Figure 5. AFM images of the only PtNPs monolayer films (A) and
the MD-Pt overlayer/PtNPs films by three replacement cycles (B).
cycle modified PtNPs. Although there are some agglomerated
blocks also existing in the films, it can be clearly seen that after
multiple replacement cycles, the diameter of the PtNPs has no
remarkable change as compared to that of the PtNPs before
replacement. In addition, the surface roughness of the film
increases from 8.2 to 14.4 nm after replacement cycles.
Electrocatalytic Reduction of Dioxygen on the MD-Pt
Overlayer/PtNPs Films. Our interest in the MD-Pt overlayer/
PtNPs films is primarily related to electrocatalytic applications.
The electrocatalytic activity of the system employed here for
dioxygen reduction was investigated in detail. Figure 6A
presents CVs of the MD-Pt overlayer/PtNPs film electrodes with
different numbers of replacement cycles in a 0.1 M H2SO4
solution in the presence and absence of dioxygen. The dotted
line corresponds to CV of the MD-Pt overlayer/PtNPs films
formed by seven redox replacement cycles of UPD Ag from
K2PtCl4 in a N2-saturated 0.1 M H2SO4 solution. In the presence
of dioxygen, a remarkable catalytic reduction peak current
occurs at 0.57 V, which shifts ca. 210 mV more positive than
that of a typical Pt catalyst in 0.5 M H2SO4.28 In addition, the
reduction potential obtained is more positive than those recently
reported by our group14,29 and another group13 for PtNPs in the
multilayered film modified GCE in 0.5 M H2SO4 and also
Martin’s group30 for PtNPs-containing carbon nanotubule
membranes. Here, it is noted that with the increase of the H2-
SO4 concentration, the reduction potential shifts more positively.
Interestingly, when the first Pt overlayer is formed on the PtNPs
films by the UPD-redox replacement technique, its peak
potential for dioxygen reduction shifts positively in comparison
to that of the PtNPs monolayer films (dashed line in Figure
6A), which indicates that the electrocatalytic activity of the films
is enhanced with the Pt overlayers forming on the PtNPs
AFM Measurement. The surface morphology of the films
was examined by AFM. Figure 5A,B presents the morphological
image of the PtNPs monolayer films and the MD-Pt overlayer/
PtNPs films by three replacement cycles, respectively. Lateral
dimensions are distorted due to the convolution with the AFM
tip. The cross-sectional contour plots of surface morphology
give a particle height of ca. 6-15 nm for the immobilized PtNPs
and 8-16 nm for the MD-Pt overlayers by three replacement

15268 J. Phys. Chem. B, Vol. 109, No. 32, 2005
Huang et al.
Figure 7. CVs for O2 reduction at the bulk Pt electrode (curve a) and
the MD-Pt overlayer/PtNPs film electrodes by one (curve b) and seven
(curve c) replacement cycles. Supporting electrolyte: air-saturated 0.1
M H2SO4 solution and scan rate: 50 mV s-1
.
electrode by one redox replacement (curve b) exhibits more
positive potential for dioxygen reduction. Increasing the number
of replacement cycles to seven times (curve c), the catalytic
activity is enhanced obviously, in terms of both reduction peak
potential and current density. From curve c, the reduction peak
potential is comparable with that of the most positive achievable
by currently available catalysts in solutions containing 1 M H+
(0.55 V vs the SCE),32 and the current density is slightly higher
than that of the bulk Pt electrode, which saves precious metal
Pt.
Figure 6. (A) CVs of O2 reduction at the MD-Pt overlayer/PtNPs film
electrodes with different redox replacement cycles: n ) 1-4, 6, and
7 (from top to bottom), respectively. The dotted line is the CV at the
MD-Pt overlayer/PtNPs films by seven replacement cycles in N2-
saturated 0.1 M H2SO4 solution. The dashed line is the CV of O2
reduction at the PtNPs monolayer film electrodes. Supporting electro-
Kinetic Analysis of Dioxygen Reduction on the MD-Pt
Overlayer/PtNPs Films. To study the kinetics of the catalytic
reduction of dioxygen, voltammetric measurements at RDE and
RRDE have been performed. Figure 8A reveals current-
potential curves of dioxygen reduction in air-saturated 0.1 M
H2SO4 at various rotation rates using the rotating glassy carbon
(GC) disk electrode assembled with the MD-Pt overlayer/PtNPs
films by seven replacement cycles. It can be seen that potential-
independent plateau currents are present at all rotation rates and
that the catalytic current increases gradually with the increase
of the electrode rotation rate. The Levich and Koutecky-Levich
plots obtained from the plateau currents at 0.30 V (Figure 8A)
are shown in Figure 8B,C, respectively. If the mass-transport
process in the solution solely controls the reduction of dioxygen
at the MD-Pt overlayer/PtNPs film electrodes, the relationship
between limiting current and rotation rate should obey the
Levich equation.33 From Figure 8B, the limiting current
increases linearly with the square root of the rotation rate at
lower rotation rates and then deviates slightly from the Levich
plot, which has been observed in other systems.34 This suggests
that the catalytic reaction is controlled not by diffusion but by
kinetics. The slope of the Levich plot is very close to that of
the calculated line for the four-electron reduction of O2. When
the mass-transport process in the solution and the catalytic
reaction becomes dominant, the Koutecky-Levich equation33
can be used to determine the number of electrons transferred
for the O2 reduction and the rate constant for the catalytic
reaction rate between film electrodes and O2. As seen from
Figure 8C, the Koutecky-Levich plot is linear with a slope
very close to that of the dashed line calculated for the four-
electron reduction of O2 by a kinetics-controlled process. The
calculated number of electrons involved in the reduction of O2
from the slope of the Koutecky-Levich plot is found to be about
3.80 for the MD-Pt overlayer/PtNPs films.33 The kinetic current
value obtained from the intercept of the Koutecky-Levich plot
is founded to be 17.3 mA. The rate constant, calculated from
the kinetic current value, is found to be 0.40 cm s-1, which is
higher than that reported for Pt (110), the most active face at
almost the same potential.35 Such a large rate may be attributed
to the decrease in the coverage of PtOH, which can enhance
O2 reduction kinetics.36
lyte: air-saturated 0.1 M H2SO4 solution and scan rate: 50 mV s-1
.
(B) CVs of O2 reduction at the MD-Pt overlayer/PtNPs film electrodes
by seven replacement cycles with different scan rates: 0.01, 0.03, 0.05,
0.07, 0.09, 0.11, and 0.13 V s-1. The dashed line is the CV at 0.01 V
s-1 in N2-saturated 0.1 M H2SO4 solution. The inset shows a linear
dependence of the electrocatalytic dioxygen reduction peak currents
with the square root of scan rates. Supporting electrolyte: air-saturated
0.1 M H2SO4 solution.
surfaces. It also can be seen that with increasing the number of
replacement cycles, the catalytic current of dioxygen reduction
increases gradually, and interestingly, the reduction peak
potential shifts positively. Therefore, by simply choosing
different numbers of replacement cycles, the amount of Pt
overlayers of the as-prepared films was readily adjusted while
enhancing the catalytic activity toward dioxygen reduction. It
is possible that a lack of a significant inhibition of the PtOH
might be partly responsible for the enhanced dioxygen reduction
kinetics.20,31 In addition, by the multiple UPD-replacement
technique, the surface structure of the PtNPs might be changed
(such as low coordination of a considerable number of surface
atoms and an enhanced surface roughness),31 as compared to
that of the PtNPs monolayer films, thus decreasing the activation
energy of the dioxygen reduction reaction. Therefore, the
reduction peak potential shifts positively. The results provide
clear evidence that after multiple replacement cycles, the multi-
deposited Pt overlayers with very low loading can remarkably
improve the electrocatalytic activities.
Figure 6B shows CVs of dioxygen reduction at the as-
prepared film electrodes in an air-saturated 0.1 M H2SO4
solution at different scan rates. The peak currents of dioxygen
reduction vary as a linear function of the square root of the
scan rates, demonstrating that the kinetics of the overall process
is controlled by mass transport of dioxygen from the bulk
solution to the modified electrode surface.
For the purpose of comparison, voltammograms for O2
reduction at a bulk polycrystalline Pt electrode and the MD-Pt
overlayer/PtNPs film electrodes prepared by one and seven
replacement cycles are shown in Figure 7. Compared with that
of the bulk Pt electrode (curve a), the as-prepared modified

Pt Film for Electrocatalytic Activities
J. Phys. Chem. B, Vol. 109, No. 32, 2005 15269
Figure 9. RRDE voltammograms of the MD-Pt overlayer/PtNPs film
electrodes by seven replacement cycles (disk scan rate: 20 mV s-1
and ω: 100 rpm) in air-saturated 0.1 M H2SO4 solution. The potential
of the platinum ring electrode was set to 1.0 V to oxidize H2O2 to O2
completely.
Figure 10. Polarization curves for the HER on the MD-Pt overlayer/
PtNPs films with different redox replacement cycles of 1, 3, and 7.
The dashed line is the polarization curve on the only PtNPs monolayer
films. The inset represents polarization curves for the HER on the bare
GCE (dashed line), bulk Pt electrode (dotted line), and the MD-Pt
overlayer/PtNPs films by seven replacement cycles (solid line).
Supporting electrolyte: N2-saturated 0.1 M H2SO4 solution, scan rate:
5 mV s-1, and rotation rate: 3500 rpm.
Figure 8. (A) Current-potential curves of O2 reduction at a RDE
assembled with the MD-Pt overlayer/PtNPs films by seven replacement
cycles with different rotating rates in air-saturated 0.1 M H2SO4 solution.
Levich plot (B) and Koutecky-Levich plot (C) of the kinetic limiting
currents of the voltammograms. The solid line is from the experimental
data, and the dashed lines are from the calculated data considering the
reduction of O2 by two and four electrons, respectively.
dioxygen, recorded at RRDE with the MD-Pt overlayer/PtNPs
films by seven replacement cycles immobilized on the GC disk
electrode. The disk potential was scanned from 0.9 to 0.1 V,
while the ring potential was held at 1.0 V to oxidize H2O2
generated by O2 reduction at the disk electrode. Only a very
small ring current was observed as the potential was swept
through the electrocatalytic reduction potential, which indicated
almost no H2O2 production by the catalyst. The ratio of the ring
to the disk current, iR/iD, is 0.002 for the MD-Pt overlayer/PtNPs
films, which is much smaller than the collection efficiency of
0.20 (standard deviation 0.02) determined with the use of the
[Fe(CN)6]3-/4- redox couple under the same solution conditions.
From the value of iR/iD, 0.002, the calculated number of
electrons involved in the reduction of O2 was found to be 3.98
according to the equation n ) 4 - 2(iR/iDN),40 which is almost
identical to that acquired from the Koutecky-Levich plot (vide
supra). Also, from the value of iR/iD, 0.002, the efficiency of
H2O formation of the MD-Pt overlayer/PtNPs films amounts
to 98 ( 0.2%.41 This result indicates that the reduction of
dioxygen at the MD-Pt overlayer/PtNPs film electrodes mainly
supports the four-electron pathway process, realizing nearly
complete electroreduction for dioxygen to water.
Electrocatalytic Activities for HER of the MD-Pt Over-
layer/PtNPs Films. The electrocatalytic activity of thus-
prepared modified electrode for HER was also investigated.
Figure 10 shows polarization curves for the HER on the MD-
Pt overlayer/PtNPs films with different numbers of replacement
cycles in N2-saturated 0.1 M H2SO4 solutions. The dashed line
represents the polarization curve on the only PtNPs film without
a UPD-replacement reaction. With the increase of the replace-
ment steps, the current for the HER increases gradually. We
The Tafel behavior in the mixed kinetic-diffusion controlled
region was also investigated using mass transport-corrected Tafel
plots of E versus log [i/(il - i)] for the MD-Pt overlayer/PtNPs
film electrodes by seven replacement cycles at a rotation rate
of 2500 rpm (figure not shown). It is well-known that the bulk
polycrystalline Pt shows two Tafel slope regions, in which the
small Tafel slope of ca. -60 mV/decade (the low potential
region) corresponds to O2 electroreduction at the oxidized
(PtOH) surface, whereas the large Tafel slope of ca. -120 mV/
decade (the high potential region) corresponds to that at a clean
Pt surface.37,38 The transition in the two Tafel slopes is probably
related to the potential-dependent coverage of surface oxides
like PtOH, which inhibit the adsorption of O2.31,39 For the MD-
Pt overlayer/PtNPs film electrodes, the result shows that the
transition in the Tafel slopes at the entire potential region is
absent. The single Tafel slope -113 mV/decade was obtained,
suggesting that there was a lack of a significant inhibition of
PtOH on the Pt overlayers,20,39 which is in agreement with the
results obtained from the growth process of the MD-Pt over-
layers characterized by cyclic voltammetry. It is close to the
value of -120 mV/decade, indicating that the transfer of the
first electron to O2 was the rate-limiting step in four-electron
reduction of dioxygen.
A rotating GC disk-platinum ring electrode was employed
to determine the quantity of H2O2 that was generated by a two-
electron reduction process of O2 at the disk electrode, which
will give direct evidence to detect the effectiveness of this
catalyst through four-electron reduction of dioxygen to water.
Figure 9 shows the voltammetric curves for the reduction of

15270 J. Phys. Chem. B, Vol. 109, No. 32, 2005
Huang et al.
also evaluated the electrocatalytic ability of the MD-Pt overlayer/
PtNPs films with respect to the HER in a 0.1 M H2SO4 solution
as compared with a bulk Pt electrode and bare GCE (the inset
of Figure 10). As seen from the inset, the current density value
of the HER at the MD-Pt overlayer/PtNPs films by seven
replacement cycles is slightly higher than that obtained at the
bulk Pt electrode, and the hydrogen evolution potential is more
positive than that of the latter. Importantly, the current at the
MD-Pt overlayer/PtNPs films does not decay as quickly as that
at the bulk polycrystalline Pt electrode. There is no observable
decrease in the current density at the MD-Pt overlayer/PtNPs
films after 1 h, whereas it is known that activated Pt electrodes
lose most of their electrocatalytic activity during a period of
ca. 1 h (up to 90% in acid solutions).42 Even overnight, the
current density at the MD-Pt overlayer/PtNPs films only
decreased 5%. So, the as-prepared modified electrode is very
durable and efficient in longer time periods.
trocatalytic activities for dioxygen reduction, in terms of both
reduction peak potential and current density, when compared
to that of the bulk polycrystalline Pt electrode. Interestingly,
after multiple replacement cycles, the electrocatalytic activities
were improved remarkably, although the increased amount of
Pt is very low in comparison to that of pre-modified PtNPs due
to the intrinsic feature of a UPD-redox replacement technique.
The results indicate that the multiple replacement cycles are
beneficial for the improvement of the catalytic performance. In
a word, through using the technique of tailoring the catalytic
surface, the electrocatalytic activities could be improved without
using very much Pt. These features may provide an interesting
way to produce Pt catalysts with reliable catalytic performance
and a reduction of costs. Additionally, this approach could be
extended to the fabrication of nanostructured Au and Pd deposits
and will be attractive to the design of nanoparticle-based
materials for catalytic applications.
In addition, in comparison to that of bulk Pt electrodes, the
CVs of PtNPs monolayer film electrodes and the MD-Pt
overlayer/PtNPs film modified electrode show some distortion
of the hydrogen adsorption and desorption peaks (figure not
shown), which may imply a difference in H adsorption behavior
between modified electrode and bulk Pt electrode.43 Also, shifts
in the peak potential of H adsorption can be observed in the
CVs of PtNPs monolayer films electrode and the MD-Pt
overlayers/PtNPs film modified electrode, which indicates that
changes may be made in the binding energy of adsorbed
hydrogen, caused by some change in the electronic nature of
the MD-Pt overlayers/PtNPs film electrode.43 However, now it
is difficult to determine precisely the main factor effecting
chemical properties of the film modified electrode, since the
details about it are probably very complicated and need to be
studied using other techniques. Further investigation into the
details is underway.
Stability of the MD-Pt Overlayer/PtNPs Films. The stabil-
ity of the MD-Pt overlayer/PtNPs film electrodes by comparing
the changes in voltammetric peak currents before and after
potential scanning 4 h between -0.2 and 1.5 V at 50 mV s-1
in 0.1 M H2SO4 was found. There was no remarkable change
in the voltammetric currents of Pt and the catalytic current.
Furthermore, no observable change in the shape and height of
the catalytic current was found, after the as-prepared modified
electrode was exposed to air or soaked in the supporting
electrolyte for 2 months. Chronoamperometry experiments show
that the MD-Pt overlayer/PtNPs film exhibits a better stability
and is able to maintain higher current density for over 1 h, as
compared to that of the bulk Pt electrode. The MD-Pt overlayer/
PtNPs films are very stable and difficult to remove from the
electrode surface. The only way to remove the film is to polish
the electrode. The good stability of the films is very useful in
the preparation of the modified electrode and the catalytic
reaction.
Acknowledgment. This work was supported by the National
Natural Science Foundation of China (20275036 and
20210506).
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