Pt and Pd decorated Au nanowires: Extremely high activity of ethanol oxidation in alkaline media

Article abstract of DOI:10.1016/j.electacta.2011.04.052

Highly ordered Pt and Pd decorated Au nanowire arrays, Pt/Au NWA and Pd/Au NWA, are prepared by anodized aluminum oxide (AAO) template based electrodeposition combined with chemical reduction. The effect of shell material on the ethanol oxidation currents

Full text of DOI:10.1016/j.electacta.2011.04.052

Electrochimica Acta 56 (2011) 5771–5775
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Pt and Pd decorated Au nanowires: Extremely high activity of ethanol oxidation
in alkaline media
Serhiy Cherevko ,1, Xiaoli Xing , Chan-Hwa Chung
∗
1
∗
Advanced Materials and Process Research Center for IT, School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Highly ordered Pt and Pd decorated Au nanowire arrays, Pt/Au NWA and Pd/Au NWA, are prepared by
anodized aluminum oxide (AAO) template based electrodeposition combined with chemical reduction.
The effect of shell material on the ethanol oxidation currents is studied. The maximum current densi-
ties are several times higher on the modified electrodes than on the unmodified Pt (Pd) NWA. The most
highly active electrode shows almost 4-fold increase in the ethanol peak current. Pt/Au and Pd/Au NWA
electrodes show a similar dependence of the ethanol oxidation current density on the Pd or Pt deposi-
tion time, which most likely, reflects the optimal upper layer thickness. The synergistic effect between
substrate and deposit materials seems to be the most important factor explaining such unusually high
activity.
Received 24 March 2011
Received in revised form 15 April 2011
Accepted 16 April 2011
Available online 27 April 2011
Keywords:
Nanowire array
Ethanol oxidation
Ultra-low loading amount
Synergetic effect
© 2011 Elsevier Ltd. All rights reserved.
1
. Introduction
The direct alcohol fuel cell (DAFC) is a promising candidate as a
(NWA) with high surface-to-volume ratios have been shown to be
highly electrocatalyticaly active for alcohol oxidation [21–23]. This
indicates that nanowire or nanotube array architecture can be a
potential substrate to improve noble metal utilization efficiency.
Besides it can act as a template, which means that no stabilizer
is required, it also serves as an excellent current collector. The
synergistic effect of Pd–Au bimetallic surfaces in Au-covered Pd
nanowires for ethanol oxidation in alkaline media was recently
shown [24].
In this work, we report on the fabrication process and electro-
catalytic properties of Pt/Au and Pd/Au NWA electrodes, which
are produced by the electrodeposition and the chemical reduc-
tion process. The inner part of the fabricated electrode, Au has a
larger lattice constant on which the thin upper layer of Pd or Pt
is expanded. It is believed that such structure would lead to d-
band up-shifting [25]. At the same time, the electron transfer may
take place which results in improvement in their catalytic activity
[25]. To the best of our knowledge, this is the first report on such
extremely high activity of ethanol oxidation in an alkaline medium
in Pt–Au and Pd–Au systems.
portable power source for highly efficient, relatively clean electric
power generation [1–7]. Ethanol, as a promising fuel in such fuel
cells, has recently attracted considerable interest. It appears to be a
very attractive fuel in the DAFCs due to its high energy density and
the fact that it is a renewable energy resource, which can be pro-
duced in great quantities from biomass. Ethanol is also considered
to be a “green” chemical as it is a less toxic than methanol. Direct
ethanol fuel cells (DEFCs) can be divided into acid- and alkaline-
type based on the electrolytes used [8]. With the intensive research
in the area anion exchange membranes development the ethanol
oxidation reaction (EOR) in alkaline medium has attracted much
attention [9–11].
Pt-based electrodes are generally considered to be the best
electrodes for alcohol oxidation as well as the Pd catalysts can
be the alternatives in alkaline medium for EOR [9,12]. However,
the exorbitant cost and CO poisoning of platinum limit its com-
mercial applications. Platinum is often alloyed with an oxophilic
metal to improve its performance [13,14]. To reduce the cost and
increase the Pt utilization efficiency, high surface area supports
such as carbon [15,16], foreign metal [17–19], and metal oxides
2. Experimental
[
7,20], are studied. Highly ordered Pt- and Pd-based nanowire array
Au, Pd, and Pt NWAs were deposited within a homemade
AAO template [26]. A Pt foil and either Ag/AgCl (+0.222 vs. NHE)
(
acidic solutions) or Hg/HgO (+0.098 V vs. NHE) (alkaline solu-
∗ Corresponding authors. Tel.: +82 31 290 7260; fax: +82 31 290 7272.
E-mail addresses: s.cherevko@gmail.com (S. Cherevko), chchung@skku.edu
tions) electrodes were used as the counter and reference electrodes,
respectively. The deposition of the Au NWA was carried out from
an IM-Gold series commercial gold electrolyte at −0.9 V for 10 min.
(
C.-H. Chung).
1
These authors contributed equally to this work.
0
013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.04.052

5
772
S. Cherevko et al. / Electrochimica Acta 56 (2011) 5771–5775
Fig. 1. SEM images of (a) Au and (b) Pt30 min/Au NWAs NWAs electrodes.
Table 1
The Pd NWA were deposited from the electrolyte containing 0.658 g
®
Roughness factors of the electrodes.
of Pd(NH ) Cl , 25 mL of Palladure , and 30 mL of deionized water
3
4
2
at −0.8 V for 5 min. Pt was electrodeposited potentiostatically
Electrode
Au, Pt/Au, Pd/Au
21
Pt
Pd
10
(
E = 0.1 V) for 20 min from an aqueous electrolyte containing 1 wt.%
Roughness factor
13
−3
H PtCl6 and 0.6 mol dm HClO . After transferring the obtained
2
4
structure to a rigid glass and removing AAO [27], the decoration of
Pt on the Au NWA electrodes was carried out by immersing the elec-
factor (ratio of the real surface area of the electrode to its geomet-
ric area) was about 21 for the Au NWA. It was difficult to measure
Pt/Au and Pd/Au electrodes surface area by electrochemical meth-
ods. It can be assumed that the mantle side roughness of the Pd and
Pt nanowires is the same with that of Au nanowires, as the same
AAO template was used. Thus, combining data from the SEM (length
of nanowires and roughness of the side wall) and data from elec-
trochemical surface area measurements of Au nanowires we can
conclude that the roughness factor of Pd and Pt NWAs are approx-
imately 10 and 13, respectively. For Pt/Au and Pd/Au electrodes
the current was normalized by the real surface area of Au NWA.
The roughness factors of all electrodes used in the current work
are summarized in Table 1. Below, the current densities obtained
from the electrodes and divided by roughness factor are stated as
nanowires area normalized current.
The qualitative results on the relative amount of Pt and Au on
the surface of Pt/Au NWA can be obtained from the CV shown in
Fig. 2. The CV scans were taken in the potential region of −0.2 V to
1.6 V vs. Ag/AgCl. The peaks appearing at +0.46 V and +0.99 V orig-
inate from platinum oxide and gold oxide reduction, respectively.
As can be seen, the Pt surface area increased (Pt oxide reduction
peak) with Pt deposition time increasing meanwhile the Au surface
area decreased (Au oxide reduction peak). It is clear that amount
of deposited Pt is increasing with increase in Pt deposition time.
−
3
trodes in 5 mL deionized water containing 40 ␮L of 50 mmol dm
−3
H PtCl6 and 160 ␮L of 100 mmol dm l-ascorbic acid for several
2
minutes. The deposition of the Pd on Au NWA electrodes was also
−
3
achieved by immersing Au into 5 mL of 0.25 mmol dm PdCl2 and
−3
2
5 ␮L of 100 mmol dm l-ascorbic acid [28].
The elemental compositions of the Pt/Au and Pd/Au NWAs elec-
trodes were measured by an EDAX. The morphology information
was investigated by FE-SEM (JEOL JSM-7000F). The real surface
area of the Au NWA electrode was estimated in the oxygen region
−
3
in 0.5 mol dm H SO . The electrochemical activity for EOR was
2
4
−3
−3
inspected by cyclic voltammetry in [1 mol dm KOH + 1 mol dm
−3
−3
ethanol] and [1 mol dm H2SO4 + 1 mol dm ethanol] solutions.
Similar conditions were used for the investigation of methanol oxi-
dation. The electrochemical stability of the as-prepared electrodes
for ethanol electro-oxidation was studied by chronoamperomet-
ric measurements at 0 V in [1 mol dm KOH + 1 mol dm ethanol]
solution. The electrocatalytic activity of Au, Pt/Au, and Pd/Au NWA
was normalized to the real surface area of Au nanowires. Similarly,
current from the Pt and Pd NWA was normalized to the calcu-
lated (see below in the text) surface area of Pt and Pd electrodes,
respectively.
−3
−3
3
. Results and discussion
The SEM image of the Au NWA is presented in Fig. 1(a). The elec-
trode consists of closely packed nanowires with a uniform diameter
ca. 55 nm) and length (ca. 600 nm). Similarly, Pd and Pt nanowires
were also investigated and their length was found to be around
00 nm and 400 nm, respectively. After the upper layer material
(
3
deposition, EDAX analysis was performed to confirm the modifi-
cation. In the case of the Pt/Au system, the Pt loading was too low
to be detected by EDAX, unless the Pt deposition time was longer
than 30 min. After 30 min of modification, the amount of Pt was
less than 1 at% and Pt particles could be observed on the SEM image
(
Fig. 1(b)).
The real surface areas of the Au electrodes were investigated
by cyclic voltammetry. The measurements were performed in
−3
−1
H SO at a potential scan rate of 50 mV s . The
2 4
0.5 mol dm
charge obtained by the integration of the cathodic peak for the
−2
reduction of gold oxide was 8410 ␮C cm . Considering the charge
associated with the reduction of the gold oxide monolayer to be
−3
Fig. 2. Cathodic part of CV curves taken in 0.5 mol dm H2SO4 solution on Au, Pt,
−
2
4
00 ␮C cm [29] for a polycrystalline Au electrode, the roughness
and Pt/Au NWA electrodes with Pt deposition time indicated.

S. Cherevko et al. / Electrochimica Acta 56 (2011) 5771–5775
5773
−3
−3
Fig. 3. CVs in 1 mol dm KOH +1 mol dm C2H5OH for Au NWA decorated by (a) Pt and (b) Pd with Pt (Pd) deposition time indicated. The corresponding dependence of the
ethanol oxidation peak currents on the (c) Pt and (d) Pd decoration time. The dashed and dotted lines show the current densities from the Au and Pt (Pd) NWA. The current
−
1
densities are normalized to the real surface area of the Au nanowire array. The scan rate was 50 mV s
.
At the same moment the amount of available Au sites is decreas-
ing, though, even after 20 min. of Pt deposition the Au nanowires
surface is not fully covered by platinum. During the chemical reduc-
tion of platinum ions on the gold by the reducing agent, randomly
distributed Pt nanoparticles are formed, as was shown in Fig. 1(b).
Thus, even after 30 min of Pt deposition there are still available
Au sites as Pt does not form solid film but rather Pt nanoparti-
cles randomly distribute on the surface of Au. For some samples,
the Pt oxide reduction peak potential shift to more positive values
is seen. This result, in principle, may indicate on the d-band cen-
ter shifting of Pt particles [30]. Though, the detail analysis of the
peaks reveals that peak of oxygenated surface species reduction
on the samples with short Pt deposition time (1 min and 3 min)
have the same position as that of pure Pt NWA. Only samples with
longer Pt deposition time exhibit the shift. Nonetheless, as was
pointed out by Tegou et al. [31], cyclic voltammetry is probably
not sensitive enough to reveal the difference in the Pt–O bonds
strength or shift in d-band center. More recent results by the same
group revealed that even the interpretation of Pt–O surface species
striping peaks is problematic. Indeed, the conflicting results were
found by different groups as discussed by Tegou et al. in Ref. [32].
Application of X-ray photoelectron spectroscopy combined with an
electrochemical cell (EC-XPS) in the investigation of CO tolerance
at pure Pt and Pt alloys showed the existence of core level shifts
lower than that for a bulk gold electrode [34]. The oxidation cur-
rent abruptly decreases at around 0.2 V, due to the formation of Pt
oxide which blocks the adsorption of the active species to the elec-
trode surface. The EOR peak current variations were accompanied
by peak potential shifts depending on the Pt modification time. For
short modification times, the peak current rises with increasing
the time (1 min and 3 min). When the Pt deposition time exceeds
3 min, the peak current decreases and reaches values close to those
for the pure Pt NWA electrode when the time is over 30 min. We can
conclude that there is an optimal amount of Pt that leads to the max-
imum activity of the electrode. The peak current on Pt min/Au NWA
3
−
2
was 266 mA cm (normalized to the electrode geometric area) or
−
2
12.7 mA cm (normalized to the nanowires surface area) and the
peak position was 0.19 V for this sample. It is worth mentioning that
the peak position on the CV curves shifts to more positive potentials
for the electrodes with a higher peak current. Thus, the potential
of ethanol oxidation, as well as its current, depends on the amount
of shell material. As the origin of the peak decrease is the electrode
passivation by oxides formation rather than ethanol mass-transfer
limitation, it is most likely that ethanol oxidation diminishes the
surface oxidation. The corresponding graph of the peak current
versus the deposition time is shown in Fig. 3(c). The extremely high
activity of the Pt/Au NWA electrodes (the maximum peak current
was 3.7 times higher than on the pure Pt NWA electrode) cannot be
explained by the increase in the surface area due to the deposition
of Pt particles, because this mechanism cannot explain the maxi-
mum in the electrode activity. The Pt surface area was much lower
on Pt3 min/Au than on Pt180min/Au, but the activity was much higher.
SEM analysis also did not show any difference in the roughness of
Au and Pt3 min/Au nanowires. The EOR peak current densities are
also much higher than the summation of the EOR currents on the
pure Au and pure Pt NWA electrodes, so we may conclude that there
exist a synergistic effect resulting from the interaction between the
Au substrate and the Pt outer layer [25,35,36]. As Pt and Au are both
(
shifts in Fermi level and d-band center position) in Pt atoms elec-
tronic structure by the alloying [33]. The similar investigation of the
Pt/Au system should be performed to find (if any) the changes in
Pt atoms electronic structure by Au. The Au oxide reduction peaks
remain at the same position, which supports the conclusions made
by Tegou.
The cyclic voltammograms in [1 mol dm−3 KOH +1 mol dm−3
C2H5OH] solution for the Pt/Au electrodes after Pt deposition over
various times are shown in Fig. 3(a). The onset potential of the EOR
for all of the modified electrodes was about −0.6 V, which is much

5
774
S. Cherevko et al. / Electrochimica Acta 56 (2011) 5771–5775
−3
−3
Fig. 4. (a) CAs in 1 mol dm KOH +1 mol dm C2H5OH taken on Pt/Au NWA electrodes with Pt deposition time indicated. The applied potential was 0 V vs. Hg/HgO. (b) The
effect of positive (0.75 V, 5 s) and negative (−0.65 V, 10 s) voltage pulses on the ethanol oxidation current on Pt3 min/Au NWA.
noble metals the segregation of the surface layer is unlikely. This
is different to the most core-shell systems with non-noble core
material, e.g. Ni, Fe, Co, where such segregation leads to the for-
mation of Pt surface of few monolayer thickness [31]. In this case,
the active material is, undoubtedly, platinum. As Pt and Au both
can catalyze the electrooxidation of ethanol the exact position of
active sites, that is either Pt or Au or, which is most like, Au–Pt
interface, is not clear. However, synergetic phenomena seem to
be crucial. This synergistic effect also affects EOR mechanism on
Pt/Au NWA electrodes as the hysteresis between the anodic and
cathodic scan are different as the Pt deposition time increased. The
size of the hysteresis loop (e.g. below −0.25 V on Pt3 min/Au) is often
related to the poisoning of the surface [37]. The hysteresis loop
width increases with the Pt deposition time, and it is smaller than
that on Pt NWA when Pt deposition time is shorter than 5 min. This
fact may suggest that there are less strongly adsorbed species dur-
ing the reaction. Though, the mechanism should be further studied
by other techniques such as FT-IR or SERS.
We found that such synergistic effect was only observed for EOR
in alkaline media. The electrocatalytic activity toward the EOR in
acidic solution and the methanol oxidation in both alkaline and
acidic solutions was measured on the Pt/Au NWA electrodes. How-
ever, the activity was found to be much lower than that for the EOR
in alkaline medium, and it was simply proportional to the Pt surface
area for these reactions.
The similar dependence of the electrocatalytic activity of
ethanol oxidation in the 1 M KOH solution on the upper layer mod-
ification time was also observed on Pd/Au NWA electrodes as can
be seen from Fig. 3(b). The corresponding curves for the peak cur-
rent densities versus the shell metal deposition time are shown in
Fig. 3(d). The optimum decoration time for Pd on Au was 5 min. By
comparing the two kinds of electrodes, the Pt3 min/Au NWA elec-
trode shows slightly higher electrocatalytic activity. It is worth to
note, that Pd-based electrodes are reported by other researches
It is most likely that the highest electrode activity corresponds
to the optimal Pt or Pd coverage. At the moment, the main reason
of such behavior is not clear. It can be electron donation from Au
d-orbital, or optimal Pt–Pt, Pd–Pd, or even Au–Au interatomic dis-
tances. Although the mechanism of the synergistic effect on the Pd-
or Pt-decorated Au NWA is not clear, much effort has been made to
explain this behavior resulting from geometric effects and electron
transfer mechanisms [25,35,39,40]. In both cases, the d-band center
shift, which is crucial in catalysis, was considered [41]. When the
lattice of metals (such as Pd and Pt) whose d-band is more than half-
filled is expanded parallel to the substrate of metal with large lattice
constant (e.g. Au) the overlap between the d-electron orbitals of the
neighboring Pd or Pt atoms becomes smaller and the bandwidth
decreases. To keep the d-band occupancy fixed, the energy of the
d-states is up-shifted [30]. As a result, the reactivity of the shell
atoms increases. The geometric effect is gradually weakened as Pd
or Pt layers increased. Besides, the electron transition between the
substrate and the shell atoms can take place and, thus, affect the
d-band center position [30]. Both the geometric effects and elec-
tron transfer between the metal layers depend on the thickness
of the upper layer, which can result in the dependence of the cat-
alytic activity on the shell deposition time described above. That is,
smaller Pt or Pd deposition times results in the most intense effect.
The relatively low activity during very short deposition time is due
to small amount of such active material (active sites). Though, at
the moment, we cannot provide any experimental confirmation of
such d-band modification in Pt/Au and Pd/Au NWAs.
The obtained result can also be explained by the effect of
deposited ad-layers on the overall surface morphology. Borkowska
et al. [42] have shown that methanol oxidation current on activated
rough gold electrode highly exceeds the one on an un-activated
electrode. The authors proposed that the formation of small crys-
−
tallites that can trap OH species is the most important factor for
the unusually high activity. Though, Borkowska studied methanol
oxidation whereas we report on ethanol oxidation, there can be
similarities in the mechanisms. As was proposed by Tremiliosi-
Filho et al. [43] adsorbed hydroxide species also plays crucial role
in the ethanol oxidation on gold. It is reasonable to propose that a
tiny amount of Pt or Pd ad-layer can facilitate such processes. Also,
the platinum ad-layer with larger Pt–Pt distance may facilitate the
[
16,38] to be better than Pt-based for EOR in alkaline media. The
onset of the C H5OH oxidation on Pt modified electrode starts at
2
lower potential than that on Pd/Au NWA. On the other hand, peak
potential of ethanol oxidation is ca. 100 mV more negative for Pd/Au
NWA. The other interesting observation is the current profile during
negatively going sweeps. The peak current ratio of the backward to
forward scans (I /I ) is usually used as a measure of the surface poi-
−
−
OH adsorption strength (opposite to the effect of OH inhibition
for shorter Pt–Pt distances in O2 reduction Pt alloy electrocatalysts
[44,45], though the effect was questioned more recently [46]) and,
thus, enhance the overall ethanol oxidation activity. Nonetheless,
further studies of such structures should be carried out to explain
their unique behavior toward the oxidation of ethanol in alkaline
media.
b
f
soning by CO or CO-like surface species. Comparing the CV profiles
for Pt/Au NWA and Pd/Au NWA it is most likely that the peak on the
backward scan is due to the poisons oxidation rather than ethanol
oxidation (similar diffusion conditions in both case would result
in similar profile). The ratios I /I for the electrodes with optimal
b
f
shell coverage, i.e. Pt3 min/Au and Pd5 min/Au NWAs were 0.31 and
.05, respectively. The results for the Pd5 min/Au NWA is similar to
1
The electrochemical stability of the Pt/Au electrodes for ethanol
electro-oxidation was studied by analyzing chronoamperometric
that obtained on Au/Pd NW and Pd@Au/C electrodes [19,24]. At the
same time Pt3 min/Au shows much better result.
−
3
−3
KOH + 1 mol dm C2H5OH]
(CA) response at 0 V in [1 mol dm

S. Cherevko et al. / Electrochimica Acta 56 (2011) 5771–5775
5775
solution (Fig. 4(a)). The variations of current density at initial period
of CA on different Pt/Au NWA and Pt NWA electrodes were in good
agreement with CV results. Pt3 min/Au NWA which had smaller
hysteresis loop size on CV curve had the highest electrochemical
activity and stability. After few minutes, however, a sharp decay
in polarization current for the ethanol oxidation on all Pt/Au elec-
trodes was observed. The possible explanation of such behavior
can be the poisoning of electrodes by CO and CO-like species, or
oxidation of Pt [9,37].
In order to determine the exact reason of such current decrease,
the CA with additional steps were performed. To oxidize any CO or
CO-like species further, the potential pulse of 0.75 V over 5 s was
applied. Similarly, to reduce any Pt oxide, the potential pulse of
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This research was supported by Basic Science Research Pro-
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