Facile synthesis of continuous Pt island networks and their electrochemical properties for methanol electrooxidation

Article abstract of DOI:10.1039/b813935k

A two-dimensional continuous Pt island network was successfully synthesized by pulse-potentiostatic electrodeposition on a flat silicon substrate, which showed markedly enhanced catalytic activity toward methanol electrooxidation and high CO tolerance, probably due to the synergistic effect of the Pt island catalyst and surrounding SiO₂ surface layer. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b813935k

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Facile synthesis of continuous Pt island networks and their
electrochemical properties for methanol electrooxidationw
a
a
a
b
Jitendra N. Tiwari,* Fu-Ming Pan,* Rajanish N . Tiwari and S. K. Nandi
Received (in Cambridge, UK) 12th August 2008, Accepted 23rd September 2008
First published as an Advance Article on the web 6th November 2008
DOI: 10.1039/b813935k
A two-dimensional continuous Pt island network was success-
fully synthesized by pulse-potentiostatic electrodeposition on a
flat silicon substrate, which showed markedly enhanced catalytic
activity toward methanol electrooxidation and high CO toler-
ance, probably due to the synergistic effect of the Pt island
catalyst and surrounding SiO surface layer.
2
Over the past few years, synthesis of nanostructured materials has
received great interest in the technology due to its wide range of
1
a
1b
1c
applications in biosensors, energy system, catalysis, and in
1
d
self-assembly of supramolecular structures. Nanostructured pla-
tinum metals materials are very attractive because of their superior
electrocatalytic performance than the blanket Pt electrode. Re-
Scheme 1 Schematic illustration of the continuous Pt island network
on the flat silicon substrate. The right hand side exhibits the bifunc-
tional mechanism of CO electrooxidation. The adsorbed oxygen
2
cently several methods have been reported for the preparation on
2
containing species on the surface of SiO can facilitate the oxidation
of CO-like poisoning species adsorbed on the active Pt sites.
nanostructure but it is difficult and time-consuming to prepare the
nanostructures. Besides, platinum is very expensive, resource lim-
ited and irreversibly inactivated by CO-like poisoning species.
Therefore, it is essential that the utilization of platinum should be
kept as low as possible without sacrificing the catalytic perfor-
mance. The one best way to accomplish this is to create continuous
Pt island networks. The interconnected structure could have addi-
tional advantages in enhancing catalytic activities for reactions that
involve two or more reactants, because such networks supply
by potentiostatic pulse plating in a three electrode cell system with
4
a saturated calomel reference electrode (SCE). The time durations
for the high potential pulse (+0.08 V) and the low potential pulse
(À0.01 V) were 3 ms and 1 ms, respectively. The blanket Pt
catalyst were prepared by potentiostatic pulse plating
2 6 2 4
(1 M K PtCl /1 M H SO ) on the silicon substrate. The time
durations for the high potential pulse (+0.06 V) and the low
potential pulse (–0.04 V) were 5 ms and 2 ms, respectively. The Ru
decorated blanket Pt electrode was obtained by deposition of Ru
on the blanket Pt by potentiostatic pulse plating at À0.07 V and
3
enough absorption sites for reactant molecules over a close range.
Here, we report a new and simple method for fabricating contin-
uous Pt island networks by pulse-potentiostatic electrodeposition
using Si substrates of low resistivity, which act as the current
collector. And thus, the silicon support was very appropriate for
use as the Pt electrocatalytic electrode in respect of electrical
conductivity. As shown in Scheme 1, the presence of the surface
oxide layer on the silicon substrate can greatly enhanced the
oxidation of CO adsorbed on the active Pt sites according to the
bifunctional mechanism. Moreover, the electrocatalytical study of
the continuous Pt island network on the silicon substrate indicates
the potential application for electrodes in direct methanol fuel cells.
The fabrication steps for the continuous island Pt network
electrode are described in the following: a flat Si substrate was
washed with acetone followed by DI water (18 MO), then etched
in 10 wt% HF at room temperature for 5 min to remove the thin
native oxide layer on the silicon surface. Then, the Pt particles
were electrodeposited on the etched Si in the aqueous solution of
+0.02 V in a 1 M RuCl
(200 mL) solution for 5 ms and 1 ms,
3
À2
respectively. The catalyst mass loading (mg cm ) was calculated
by measuring the difference in the mass of the electrodes before
and after the Pt deposition, using a micro-balance (Sartorious, PB-
SAH) with a resolution of 0.001 mg. The mass loading of Pt on
the continuous island Pt network/Si, the Ru decorated Pt film/Si,
À2
and the blanket Pt on silicon electrodes are B0.37 mg cm
,
À2
À2
B0.89 mg cm , and B0.94 mg cm , respectively.
Fig. 1(A) and (B) illustrates a representative scanning
electron microscopy (SEM) image of the blanket Pt on silicon
substrate and Ru decorated on blanket Pt, respectively. From
the SEM images, the blanket Pt and the Ru decorated on the
blanket Pt were completely covered on the silicon substrate
after the electrochemical deposition. Fig. 1(C) show the sur-
face morphology of the pulse electrodeposited Pt on the
Si substarte. The Pt islands are mutually connected over the
Si substrate. The Pt islands forming the continuous Pt island
film have size distribution from B200 nm to B800 nm. X-Ray
photoelectron spectroscopy (XPS) shown in Fig. 1(D) indi-
cated that Pt and Ru were successfully deposited on the silicon
substrate by the pulse electrodeposition.
1
M K PtCl /1 M H SO (100 mL/100 mL) at room temperature
2 6 2 4
a
b
Department of Materials science and Engineering, National Chiao Tung
University, 1001 Ta Hsueh Road, Hsinchu, Taiwan, 300, R.O.C
Department of Physics, Rishi Bankim Chandra College, Naihati,
743165, West Bengal, India. E-mail: jnt_tiw123@yahoo.co.in;
fmpan@faculty.nctu.edu.tw; Fax: 886-3-5724727; Tel: 886-3-5131322
w Electronic supplementary information (ESI) available: Tafel plots.
See DOI: 10.1039/b813935k
The electroactive surface area (ESA) of the electrodes was
determined by the CO-stripping cyclic voltammetry, which was
6
516 | Chem. Commun., 2008, 6516–6518
This journal is ꢀc The Royal Society of Chemistry 2008

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reaction, and hence the continuous Pt island network on the flat Si
substrate possess higher activity towards CO oxidation compared
to the blanket Pt on silicon and the Ru decorated Pt film.
Fig. 2(B) shows the cyclic voltammograms of the three
electrodes recorded in 1 M CH OH/1 M H SO aqueous
3
2
4
À1
solution at a potential scan rate of 20 mV s . The CV curve
of the continuous Pt island network/Si shows that the methanol
oxidation peak had the maximum around 0.63 V vs. SCE and a
very low onset potential of B0.38 V. Also shown in Fig. 2(B) is
the CV curves of the blanket Pt film and the Ru decorated Pt
film, which show a much smaller current density with a higher
onset potential. The negative onset potential shift indicated that
the continuous Pt island network/Si can effectively reduce
6
overpotentials in the methanol electrooxidation reaction.
Noted that the methanol oxidation peak in the forward scan
for the continuous Pt island network/Si electrode was much
larger than the peak in the region of 0.3–0.5 V in the reverse
scan. In the cyclic voltammetric scan, the anodic peaks in the
forward scan and in the reverse scan are associated with
electrooxidation of methanol and removal of incompletely
oxidized carbonaceous species (CO-like poisoning species)
on the electrode, respectively. The catalyst tolerance against
CO adsorption may be evaluated by the ratio of the current
Fig. 1 SEM images: (A) blanket Pt on flat Si substrate; (B) Ru on
blanket Pt; and (C) continuous Pt island network on the flat silicon
substrate; (D) X-ray photoelectron spectrum (XPS) of (a) blanket
Pt/Si; (b) Ru decorated blanket Pt; and (c) continuous Pt island network.
2
performed by flowing a 10% CO/N gas mixture in the 1 M
H SO aqueous solution at +100 mV for 35 min, using a Pt wire
2 4
as the counter electrode and a saturated calomel reference
electrode (SCE). Before scanning, the solution was purged with
density of the forward anodic peak (I
7
f
) to that of the reverse
anodic peak (I ), (I /I ). For the continuous Pt island/Si
b
f b
N gas for 30 min to removed CO remained in the solution.
2
electrode, the (I /I ) ratio was calculated to be B19. This ratio
f b
Representative CO-stripping voltammograms for the conti-
nuous Pt island network/Si, the Ru decorated Pt film and the
blanket Pt/Si electrodes are illustrated in Fig. 2(A).
was more than 9 times and 20 times larger than that of the Ru
decorated Pt film and the blanked Pt film, respectively.
Chroamperometry technique was employed to further test the
activity of theses three electrodes. Fig. 2(C) shows the chrono-
amperogram of electroactivity of the three electrodes at the
2
À1
A high ESA of 67 m g was obtained for the continuous Pt
island network/Si electrode by integrating the CO-electrooxidation
peak of first CO stripping cycle, assuming an oxidation charge
3 2 4
oxidation potential of B0.4 V in the 1 M CH OH/1 M H SO
À2
value of 420 mC cm for a monolayer of CO adsorbed on a
smooth platinum surface. The ESA of the continuous Pt island
aqueous solutions at 25 1C. Steady-state currents for methanol
electrooxidation were measured for more than 800 s. At the
oxidation potential of 0.4 V, the steady-state currents at 800 s for
the continuous Pt island network/Si, Ru decorated Pt film
5
network/Si electrode is much higher than that of the Ru decorated
À1
2
Pt film electrode (21 m g ) and that of the blanket Pt electrode
2
16 m g ). This shows that the continuous Pt island network/Si
À1
À2
À2
(
and blanket Pt/Si are B10 mA cm , B4 mA cm , and
À2
electrode has a relatively high ESA, most likely due to the
interconnect structure of the Pt islands. Such well-defined contin-
uous Pt island network structure provides abundant active sites
for the electrooxidation reaction of methanol.
B0.05 mA cm , respectively. The observation implied that
most CO-like poisoning species could be oxidized and removed
from the Pt catalyst so that the catalytic oxidation of methanol
could be kept proceeding efficiently on the continuous Pt island
network/Si electrode. Because oxygen containing species on the
From the CO-stripping curve, we noticed a lower onset poten-
tial and smaller peak potential for CO oxidation on the contin-
uous Pt island network electrode in comparison to the Ru
decorated Pt film and the blanket Pt film/Si. Examination of the
CO oxidation curves reveals that the onset potential of the
continuous Pt island network/Si electrode (B0.43 V) is lower
than that of the Ru decorated/Pt (B0.48) and the blanket Pt/Si
2
SiO surface layer can promote the CO removal as described
above, the improvement of the electrooxidation activity can be
ascribed to the synergistic effect of the Pt island catalyst and the
2
SiO surface layer. These results are very consistent with the CV
studies shown in Fig. 2(A) and (B).
Tafel plot for electrochemical oxidation of 1 M CH OH/1 M
3
À1
(B0.60 V). The CO oxidation peak potential for the continuous Pt
H SO aqueous solution at a scan rate of 1 mV s is shown in
2
4
island network/Si (B0.57 V) is also lower than that for the Ru
decorated Pt (B0.60 V) and the blanket Pt/Si (B0.64 V), probably
due to an enhanced CO oxidation rate on the Pt islands
Fig. 1S in the ESI.w The blanket Pt/Si and Ru decorated Pt film
have a Tafel slope of B115 mV/dec and B137 mV/dec, respec-
tively. On the other hand, the continuous Pt island network/Si
exhibits a much larger Tafel slope (B245 mV/dec), suggesting a
great difference in the electrooxidation mechanism for the
continuous Pt island network/Si electrode from the other
two electrodes. This might be ascribed to the Pt island network
2
surrounded by the chemical SiO layer, which was formed on
the Si substrate in the electrolyte. The presence of the oxide layer
on the silicon substrate can promote the oxidation of CO
4
adsorbed on the active Pt sites via the bifunctional mechanism.
The oxygen-containing species on SiO
2
(such as hydroxyl surface
structure and the presence of active oxygenated on the SiO
2
8
group) can transform CO-like poisoning species adsorbed on Pt to
CO , releasing the active sites on Pt for further electrochemical
surface layer. The mechanistic difference could result in the
better catalytic activity and CO tolerance of the 2-D
2
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Fig. 2 Electrochemical measurements of: (A) CO stripping cyclic voltammetry curves recorded at room temperature in a CO saturated 1 M
À1
À1
H
2
SO
4
solution at a scan rate of 20 mV s ; (B) cyclic voltammograms in 1 M CH OH+1 M H
3
2
SO
4
at a scan rate of 25 mV s ; (C)
SO at the potential 0.3 V. The inset
chronoamperometry at the potential of 0.4 V; (D) electrochemical impedance spectra in 1 M CH
in Fig. (D) shows the equivalent circuit model.
3
OH+1 M H
2
4
continuous Pt island network/Si electrode compared to the
blanket Pt/Si and the Ru decorated/Pt film electrode.
The electrochemical impedance spectroscopy (EIS) mea-
surements were used to evaluate the charge-transfer resistance
and the capacitance of these three electrodes during methanol
electrooxidation.
This work was supported by the National Science Council
of R.O.C., under Contract No. NSC93-2120-M-009-007.
Technical support from the National Nano Device Labora-
tories (NDL) is gratefully acknowledged.
Notes and references
Fig. 2(D) shows three Nyquist plots recorded in 1 M CH OH/
3
1
M H SO at the oxidation potential of B0.3 V, where Z and Z
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(
1
2
4
r
i
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respectively. The equivalent circuit model shown in the inset of
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Fig. 2(D) was used to fit the experimental data. The R resistor
2
represents the resistance of the electrolyte solution, Rct the charge
3
046; (b) H. Boo, S. Park, B. Ku, Y. Kim, J. H. Park, H. C. Kim and
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model. The Ru decorated Pt film and the blanket Pt/Si elec-
À2
trodes have charge-transfer resistances about B16 O cm and
B68 O cm , which is over 3 times and 11 times larger than that
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À2
À2
3 X. Teng, X. Liang, S. Maksimuk and H. Yang, Small, 2006, 2, 249.
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In summary, two-dimensional continuous island Pt network
electrode have been successfully fabricated by potentiostatic
pulse plating on the flat silicon substrate, and electrochemical
measurements confirm that this catalyst structure on the silicon
substrate has a better electroactivity toward methanol oxidation
than the blanket Pt/Si and the Ru decorated Pt film/Si
electrodes. The good electrooxidation performance can be
ascribed to the synergistic effect of the Pt island catalyst and
2
5
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1
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6
518 | Chem. Commun., 2008, 6516–6518
This journal is ꢀc The Royal Society of Chemistry 2008