Electrodeposition of arrays of Ru, Pt, and PtRu alloy 1D metallic nanostructures

Article abstract of DOI:10.1149/1.3276500

Arrays of Ru, Pt, and PtRu one dimensional (1D) nanowires (NWs) and nanotubes (NTs) were prepared by electrodeposition through the porous structure of an anodic aluminum oxide (AAO) membrane. In each case, micrometer-long NW and NT were formed with an out

Full text of DOI:10.1149/1.3276500

Journal of The Electrochemical Society, 157 ͑3͒ K59-K65 ͑2010͒
K59
0
013-4651/2010/157͑3͒/K59/7/$28.00 © The Electrochemical Society
Electrodeposition of Arrays of Ru, Pt, and PtRu Alloy 1D
Metallic Nanostructures
a,
a,
a
Alexandre Ponrouch, * Sébastien Garbarino, * Stéphanie Pronovost,
b
b,
a,
,z
Pierre-Louis Taberna, Patrice Simon, ** and Daniel Guay **
a
Institut National de la Recherche Scientifique-Énergie, Matériaux et Télécommunications, Varennes,
Québec J3X 1S2, Canada
b
Centre Interuniversitaire de Recherche et d’Ingénierie des Matériaux, UMR CNRS 5085, Université Paul
Sabatier, Toulouse 31062, France
Arrays of Ru, Pt, and PtRu one dimensional ͑1D͒ nanowires ͑NWs͒ and nanotubes ͑NTs͒ were prepared by electrodeposition
through the porous structure of an anodic aluminum oxide ͑AAO͒ membrane. In each case, micrometer-long NW and NT were
formed with an outer diameter of ca. 200 nm, close to the interior diameter of the porous AAO membrane. Arrays of NW and NT
can be formed by varying the metallic salt concentration, the applied potential, and the conductivity of the electrolyte. The Ru and
Pt deposition rates were measured in the various deposition conditions, using an electrochemical quartz crystal microbalance. The
mechanisms responsible for the formation of Ru and Pt NW and NT are discussed based on the observed deposition rates and
models found in the literature. Finally, it is shown that arrays of PtRu alloy NT and NW can be readily prepared and their
compositions can be varied over the whole compositional range by changing the metallic salt concentration of the electrodeposi-
tion bath.
©
2010 The Electrochemical Society. ͓DOI: 10.1149/1.3276500͔ All rights reserved.
Manuscript submitted October 16, 2009; revised manuscript received November 25, 2009. Published January 21, 2010. This was
Paper 2415 presented at the Vienna, Austria, Meeting of the Society, October 4–9, 2009.
Nanotubes ͑NTs͒ and nanowires ͑NWs͒ have attracted significant
able to study how the electrocatalytic activity of these anisotropic
structures is modified with respect to nanoparticles.
interest as a result of their peculiar properties, imposed by the an-
1-4
isotropy of their one-dimensional ͑1D͒ geometry. These 1D mo-
tifs ͑wherein two of the three structural dimensions are smaller than
a few hundreds of nanometers͒ are of interest for several applica-
tions such as biotechnology, catalysis, drug delivery, electronics,
energy storage, magnetic recording device, nanodevices, nanotem-
plating, optoelectronics, photovoltaics, photoelectrochemistry, and
In this paper, we report on the preparation of an array of PtRu
alloy with micrometer-long 1D nanostructures. These arrays are pre-
pared by coelectrodeposition through an AAO membrane, using an
4
+
3+
electroplating bath that contains Pt and Ru metallic ions. We
show that the composition of these 1D nanostructures can be varied
by controlling the composition of the electroplating bath. The Ru
and Pt deposition rate is measured, and we show that the composi-
tion of the PtRu alloy can be assessed from the deposition rate of
both metals. Finally, the NW and NT growth mechanisms are dis-
cussed in the context of the models proposed by Fukunaka et al.
5
-17
sensing.
They have been prepared by various methods including
18,19
vapor-phase synthesis,
ping reagents,
solution-phase methods based on cap-
2
0,21
22-24
and template-assisted method.
Since it was
22
first reported by Frazier et al., the template-assisted method has
attracted much attention due to its simplicity and versatility.
Metallic 1D nanostructures have intrinsically interesting optical
properties and several studies in the literature have made use of
them, including sensing and imaging. In comparison, there are fewer
papers dealing with the electrocatalytic properties of the metallic 1D
31
32
33
͑
coevolution of hydrogen͒, Xiao et al., and Cho and Lee ͑lim-
ited mass transport͒.
Experimental
Ru, Pt, and PtRu were electroplated on Ti substrates prepared as
26
previously described. Three different types of deposit were ob-
3,25
25
25
nanostructures. Recently, supportless Pt,
Ag, and Au
have
tained, namely, films, NWs, and NTs. The films were obtained by
deposition on a bare Ti substrate. By contrast, NWs and NTs were
prepared by electrochemical deposition through the porous structure
of an AAO membrane ͑Anodisc 25, Whatman International Ltd.͒.
The experimental setup used was already described in detail
been investigated for the oxygen reduction reaction. Also, in recent
26-29
papers from our group,
we showed that arrays of Ru, Pt, and
PtRu NTs and NWs prepared by electrochemical deposition via the
template-assisted method with a porous anodic aluminum oxide
AAO͒ membrane have interesting electrocatalytic properties for re-
actions that are of interest for the development of effective fuel cell
catalysts.
2
6
͑
elsewhere. No pretreatment ͑metallization or functionalization͒
was performed on the AAO membrane, and the NWs and NTs were
grown on the metallic film that was first deposited in the gap be-
tween the Ti substrate and the porous membrane. The electroplating
solution consisted of HCl ͑10 mM͒, and the metallic salts were
Na PtCl ·xH O and RuCl ·6H O ͑Alfa Aesar͒. In some cases, KCl
Platinum is considered one of the best ͑if not the best͒ electro-
catalysts for polymer electrolyte fuel cell applications. It shows high
electrocatalytic activity for the hydrogen oxidation and the oxygen
reduction reactions. Recently, a few reports have appeared describ-
ing how a micrometer-long metallic Pt 1D nanostructure can be
2
6
2
3
2
͑0.1 M͒ was added to the plating bath. All depositions were per-
formed in potentiostatic mode using Solartron SI 1287
a
2
4,25,29
prepared.
However, it is well known that the electrocatalytic
potentiostat–galvanostat, with all deposition potentials ͑Vdep͒ re-
ferred to the saturated calomel electrode ͑SCE͒ scale. After elec-
trodeposition, the working electrode was removed from the electro-
lyte and, in the case of NW and NT deposits, the AAO membrane
was dissolved by immersion in 1 M NaOH for 2 h at room tempera-
ture.
properties and the long-term resistance of Pt can be further im-
proved by alloying with another element. For example, it is known
that PtRu exhibits improved tolerance to CO poisoning and that
3
0
alloying of Pt can also improve the oxygen reduction kinetics. For
these reasons, it is of interest to develop methods of preparing an
alloyed, Pt-based, micrometer-long, metallic 1D nanostructure to be
Gold-plated crystals ͑9 MHz AT-cut͒ were used for the electro-
chemical quartz crystal microbalance ͑EQCM͒ experiments. The
calibration constant determined by Pb deposition was
−
1
1
.15 ng · Hz , close to the theoretical value calculated from the
34
*
Electrochemical Society Student Member.
Sauerbrey equation. The Ru and Pt deposition rates were deter-
*
* Electrochemical Society Active Member.
z
E-mail: guay@emt.inrs.ca
mined by first electroplating these metals ͑−0.4 V for 300 s͒ on the
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K60
Journal of The Electrochemical Society, 157 ͑3͒ K59-K65 ͑2010͒
1
1
1
1
1
8
6
4
2
0
8
6
4
2
0
Pt (25 ºC)
Ru (25 ºC)
Ru (50 ºC)
Figure 1. SEM micrographs for ͓͑A͒-͑D͔͒ Ru and ͓͑E͒-͑H͔͒ Pt electrochemi-
cally deposited through an AAO membrane. ͑A͒-͑D͒ plating bath: 5 mM
RuCl ·6H O, 10 mM HCl, 0.1 M KCl, and T = 50°C; ͑E͒-͑H͒ plating bath:
3
2
5
mM Na PtCl ·xH O, 10 mM HCl, and T = 25°C.
2 6 2
-0,6
-0,5
-0,4
-0,3
-0,2
-0,1
0,0
bare Au-coated crystals and then adjusting the electrode potential to
the desired value. This was done to prevent the deposition of Ru and
Pt on a foreign metal ͑Au in this case͒ as well as to ensure a constant
initial roughness factor of the substrate. The deposition rate was
calculated by ignoring the initial mass variation, and stationary
deposition rates were obtained in all cases. In all cases, the surface
Vdep (V vs SCE)
Figure 2. Deposition rate of Ru and Pt as a function of Vdep in similar plating
bath conditions as in Fig. 1. The temperature of the solutions is indicated.
−
2
area of the crystal was 0.2 cm .
and then decreases as Vdep is made more negative ͑from −0.30 to
The conductivity of the plating bath was measured with an Oak-
ton CON 11 standard conductivity meter. The morphology of the
deposits was observed using a scanning electron microscope ͑SEM,
JSM-6300F, JEOL͒. The structure and crystallite size of the deposits
were obtained by X-ray diffraction ͑XRD͒ using a Bruker D8 Ad-
vance with Cu K␣ at 1.5418 Å. The diffraction patterns were
achieved in the Bragg–Brentano ␪/2␪ configuration, and structure
−
0.45 V͒. For Vdep ഛ ca. −0.45 V, the Pt deposition rate reaches a
−10
−1
constant value of ca. 0.75 ϫ 10 mol s .
The evolution of the Ru deposition rate with Vdep is not surpris-
ing because it was already reported that the coevolution of hydrogen
38
affects the deposition of Ru, probably due to the formation of
ruthenium hydroxide complexes, which play a key role as an inter-
mediate species during the deposition process. Nevertheless, the
deposition of Ru is not limited by diffusion in the deposition poten-
tial range applied in this study. This is proven to be important in the
following.
In contrast, the evolution of the Pt deposition rate with Vdep
follows more closely the expected trend, namely, an increase in the
deposition rate as Vdep is cathodically shifted from the equilibrium
deposition potential of platinum ͑kinetically controlled deposition͒.
For large cathodic overpotentials ͑ca. Vdep Ͻ −0.30 V͒, the Pt depo-
sition rate decreases and deviates from the “ideal” behavior ͑con-
stant deposition rate͒ expected as Vdep reaches the potential region
3
5
refinement was performed according to the Rietveld method using
36,37
GSAS and EXPGUI software.
Results and Discussion
Pt and Ru nanostructures.— Effect of deposition potential.— Pt
and Ru were electrodeposited through an AAO membrane under the
following electroplating conditions: ͑i͒ For Ru, the plating bath con-
sisted of 5 mM RuCl ·6H O, 10 mM HCl, 0.1 M KCl, and T
3
2
=
50°C. ͑ii͒ For Pt, the plating bath was 5 mM Na PtCl ·xH O,
2 6 2
10 mM HCl, and T = 25°C. For Ru, the deposition potential, Vdep,
4
+
was varied from −0.30 to − 0.45 V, while for Pt it was changed
from 0.00 to − 0.45 V. The effect of Vdep on the morphology of Ru
and Pt deposited through an AAO membrane is depicted in Fig. 1
through a series of scanning electron microscopy micrographs. In
both cases, NWs are achieved for Vdep ജ −0.3 V, while NTs are
obtained for Vdep ഛ −0.3 V. Moreover, the characteristic dimen-
where the growth of the film is limited by the diffusion of Pt
species to the electrode surface. This is because, in our experimental
conditions, hydrogen evolution starts at ca. −0.3 V and may cause a
chemical precipitation of Pt in the vicinity of the electrode, thereby
4
+
39
causing a local reduction of the Pt concentration. Also, for
Vdep ഛ −0.30 V, hydrogen evolution may further limit the accessi-
4
+
sions of the Ru and Pt NTs, namely, the wall thickness ͑W ͒ and the
bility of the electrode surface to Pt species by reducing the effec-
tive surface area between the electrode and the electrolyte. Never-
theless, in this potential range ͑Vdep Ͻ −0.30 V͒, the growth of the
t
internal pore diameter ͑D ͒, vary with Vdep. For example, for Ru and
i
Pt NT, D increases from 65 to 210 nm and from 140 to 235 nm,
i
4
+
Pt film is limited by the diffusion of Pt species to the electrode
surface.
respectively, while, Wt decreases from 85 to 40 nm and from
95 to 35 nm, respectively, as Vdep varies from −0.37 to − 0.45 V.
The deposition rates ͑defined as moles of metal deposited per
Influence of metallic salt concentration.— The equilibrium hydro-
gen evolution potential is ca. −0.3 V in our experimental conditions,
and metal deposition at a very cathodic potential occurs simulta-
neously with hydrogen evolution. This was already shown to dra-
unit of time͒ for Ru and Pt, in similar plating bath conditions as that
shown in Fig. 1, were extracted from EQCM analyses of films de-
posited at different Vdep values, and the results are displayed in Fig.
4
+
2
. Clearly, the evolution of the deposition rate vs Vdep is quite dif-
matically modify the concentration of ͓Pt ͔ in the vicinity of the
surface electrode. To assess how mass transport limitations can af-
fect the morphology of the deposit in the absence of hydrogen evo-
ferent for these two metals. The deposition rate for Ru at 50°C is
extremely low ͑ Ͻ 1 ϫ 10− mol s ͒ for V ജ ca. −0.30 V, and
11
−1
dep
4
+
increases drastically as Vdep is made more cathodic ͑kinetic control͒.
The same behavior is observed at lower temperature ͑T = 25°C͒,
although the deposition rate values are lower. For both temperatures,
no limiting value of the deposition rate is observed, which indicates
that deposition is not limited by the diffusion of electroactive spe-
cies toward the electrode surface. By contrast, the deposition rate of
Pt increases as Vdep decreased from 0.0 to − 0.3 V ͑kinetic control͒,
lution, the ͓Pt ͔ concentration was varied and electrodeposition
was performed at −0.15 V. For ͓Na2PtCl6͔ = 0.5 and 1.0 mM, the
−
10
Pt deposition rates are 0.30 Ϯ 0.07 ϫ 10
and 0.40 Ϯ 0.07
ϫ 10− mol s , respectively, and are almost independent of the
deposition potential for −0.3 ഛ Vdep ഛ 0.0 V ͑data not shown͒.
This suggests that at such low Na PtCl concentration, Pt deposition
10
−1
2
6
4
+
is effectively limited by the diffusion of Pt species to the substrate.
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Journal of The Electrochemical Society, 157 ͑3͒ K59-K65 ͑2010͒
K61
walls would lead to competition between wall nucleation and
diffusion-controlled pore filling as a function of the overall reaction
rate. Thus, NT formation would result from a high deposition rate
and a low ionic diffusion inside the porous structure of the mem-
brane.
33
3
2
For Pt deposition, the model of Xiao et al. and Cho and Lee
appeared to be valid in the potential region where hydrogen evolu-
tion does not occur. Taking into account the geometrical dimensions
of the pores of the AAO membrane ͑290 nm diameter and 60 ␮m
Figure 3. SEM micrographs for Pt electrochemically deposited through an
AAO membrane at Vdep = −0.15 V: ͑A͒ 0.5, ͑B͒ 1, and ͑C͒ 5 mM Na PtCl .
2
6
−18
−18
−17
4+
length͒, ca. 2.0 ϫ 10 , 4.0 ϫ 10 , and 2.0 ϫ 10 mol of Pt
are initially present in a single pore for 0.5, 1, and 5 mM Na PtCl
−
2
In all cases, Q = 40 C cm
.
d
2
6
solutions, respectively. This would yield 0.27, 0.54, and 2.70 nm
4
+
long Pt NWs if an inward flux of Pt does not settle in during the
growth process ͑the Pt NWs and NTs are a micrometer long͒. There-
fore, Pt growth through an AAO membrane is rapidly controlled by
SEM micrographs of Pt nanostructures obtained from 0.5, 1.0, and
.0 mM solutions of Na PtCl are shown in Fig. 3A-C, respectively.
5
2
6
Pt NTs are obtained for ͓Na PtCl ͔ = 0.5 and 1.0 mM solution,
4+
2
6
the diffusion of Pt through the pore of the membrane. As shown
while Pt NWs are formed with ͓Na PtCl ͔ = 5 mM solution. There-
4+
2
6
above, for ͓Pt ͔ = 0.5 and 1.0 mM, the deposition rate is a factor of
fore, Pt NTs can be prepared at −0.15 V, in the absence of hydro-
gen, if the Na PtCl concentration is sufficiently reduced.
4+
4+
1
0 smaller than for ͓Pt ͔ = 5.0 mM, and limiting the Pt mass
2
6
transport is critical to favor the formation of Pt NTs. In the potential
region where hydrogen evolution occurs, i.e., from −0.3 to
Figure 4 shows the corresponding XRD patterns for Pt nano-
4
+
structures depicted in Fig. 3A-C ͑Vdep = −0.15 V, whereas ͓Pt ͔ in
−
0.45 V, the Pt deposition rate decreased, which could suggest that
solution is varied from 0.5 to 5.0 mM͒. The deposition charge den-
4+
the limited diffusion of Pt to the growing Pt surface could be
responsible for the formation of Pt NTs. However, at Vdep
−
2
sity, Q , was kept constant at 40 C ϫ cm and at least 10 ␮m long
d
=
NTs and NWs were grown in each case. All the peaks can be in-
dexed to the hexagonal phase of Ti ͑substrate͒ and face-centered
cubic ͑fcc͒ Pt, indicating that metallic Pt is formed. The crystallite
sizes were extracted from the XRD patterns by performing a Ri-
etveld refinement analysis. The Pt crystallite sizes decrease from
−
=
0.37 V, the Pt deposition rate is almost identical to that at Vdep
−0.15 V ͑see Fig. 2͒, where hydrogen evolution does not occur.
This points to the fact that hydrogen evolution ͑at Vdep ഛ −0.30 V͒
is also important in the mechanisms that yield to the formation of Pt
NT.
1
6 to 7 nm as the concentration of Pt4+ in solution increases from
For Ru, the variation of the deposition rate with Vdep observed in
Fig. 2 is not compatible with the NT growth mechanisms model
0
.5 ͑NTs͒ to 5.0 mM ͑NWs͒. It seems straightforward to conclude
that a low Pt salt concentration limits the number of nucleation sites
for Pt growth inside the confined pore of an AAO membrane and
leads to larger crystallites.
32
33
proposed by Xiao et al. and Cho and Lee because the deposition
rate is not limited by diffusion. Instead, the deposition rate of Ru
increases steadily as Vdep is made more cathodic. In these deposition
conditions, the NT growth mechanism suggested by Fukunaka et
Two hypotheses have been proposed in the literature to explain
the growth of NTs inside a porous AAO membrane: ͑i͒ Fukunaka et
31
3
1
al. might prevail for Ru NT formation.
al. suggested that concurrent gas evolution ͑hydrogen͒ during
metal deposition would limit the accessibility of ionic species to the
inside of the pores of the AAO membrane, and hence limit the metal
deposition to regions that are close to the pore wall. This hypothesis
There are several important differences between this work and
the previous ones. First, in most of the previous studies, the growth
of NTs was performed on a gold-coated AAO membrane, with the
40
gold coating making an annular shape at the pore basis of the
was further investigated by Philippe and Michler and is consistent
with some recent reports suggesting that concurrent gas evolution
31,32,40,43
membrane.
A similar strategy was used elsewhere to grow
44
an array of Pt NT using silver instead of gold. This conductive
͑
hydrogen͒ and metal electrodeposition have an effect on the struc-
41,42 32 33
annular base might initiate the nucleation at the circumference of the
ture of the deposit.
͑ii͒ Xiao et al. and Cho and Lee sug-
45,46
pore thanks to the well-known “tip effect.”
This is not the case
gested that fast, rate-limiting nucleation on the surface of the pore
here because both Pt and Ru NTs were grown on a Pt and a Ru
underlayer that filled the gap between the substrate and the AAO
3
2
template. Also, this study differs from the works of Xiao et al.,
Cho and Lee, and Martin et al.
3
3
23,43
as no pretreatment ͑metalliza-
20
tion or functionalization͒ was necessary to obtain NTs. Furthermore,
in contrast with Ref. 23, 32, and 33, the nanostructures formed with
our setup remain in the form of NTs even for a long deposition time,
allowing us to synthesize NTs of about 60 ␮m length ͑imposed by
the thickness of the AAO membrane͒. This represents an aspect ratio
of about 200, considering the average external diameter of these
structures, at ca. 300 nm.
1
6
12
8
4
0
0
1
2
3
4
5
6
4+
[Pt
] /
mM
Influence of supporting electrolyte.— The concentration of Pt4+ ions
in the precursor solution strongly influences the deposition rate and
the nanostructure of the resulting deposit, but other factors turn out
to be equally important such as the supporting electrolyte concen-
tration. Figure 5 shows the SEM micrographs of Pt nanostructures
C
B
A
Pt
Ti
deposited at −0.15 V in 1 mM Na PtCl plating bath. Pt NTs are
2
6
achieved in a KCl-free solution ͑Fig. 5A͒, while Pt NWs are ob-
tained by the addition of 0.1 M KCl ͑Fig. 5B͒. The conductivity of
2
0
30
40
50
60
70
80
90
100
−
1
the electrolyte linearly increases from 4.3 to 17.0 mS cm as ͓KCl͔
is varied from 0 to 0.1 M. Therefore, it seems obvious that the con-
ductivity of the electrolyte is also an important parameter for the
formation of NTs. A possible explanation is based on previous stud-
2ꢀ
Figure 4. XRD patterns for Pt electrochemically deposited through an AAO
membrane at Vdep = −0.15 V and Q = 40 C cm . The Na PtCl concentra-
tion was ͑A͒ 0.5, ͑B͒ 1.0, and ͑C͒ 5.0 mM. The inset depicts the evolution of
the crystallite size with ͓Na PtCl ͔.
−
2
d
2
6
4
7
48
ies by Oldham and Amatore et al., which showed that the resis-
tance along the diameter of a disk microelectrode embedded in an
2
6
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K62
Journal of The Electrochemical Society, 157 ͑3͒ K59-K65 ͑2010͒
0,8
0,6
0,4
0,2
0,0
A
-
0,2
0,4
0 V
-0.15 V
-0.3 V
-0.45V
-0.6 V
-
Figure 5. SEM micrographs for Pt electrochemically deposited through an
AAO membrane at Vdep = −0.15 V, with ͓Na PtCl ͔ = 1 mM and ͓HCl͔
2
6
1
2
0
8
6
4
2
0
=
=
10 mM: ͑A͒ without KCl and ͑B͒ with 0.1 M KCl. In all cases, Q
0,6
0,4
d
B
−
2
40 C cm
.
1
0
,2
,0
0
insulator is not uniform over the disk surface. Indeed, the resistance
decreases from the center to the outer diameter of the disk, resulting
in a higher current density at the edge. It is straightforward to com-
pare the surface resistance behavior of a disk microelectrode embed-
ded in an insulator with that of an NW ͑or NT͒ that is growing
inside the pore of an insulating AAO membrane. This inhomoge-
neous current density distribution was dependent on the overall sys-
0
5
10 15
48
0
50 100 150 200 250 300 350 400
Time (s)
tem resistance. Thus, the fact that different morphologies ͑NW or
4
+
NT͒ were grown at fixed Vdep and ͓Pt ͔ depending on the conduc-
tivity of the electrolyte results from an inhomogeneous distribution
of equipotential lines as the KCl concentration is decreased. This
could also explain why the nucleation is initiated at the edges of the
growing disk, resulting in inverse conic top end Pt NWs ͑cf. Fig. 1E
and F͒ for deposition under kinetic control. For low salt concentra-
tion where the growth mechanism is under diffusion-controlled con-
ditions, this effect could further help the growth of NTs ͑cf. Fig. 3A
and B͒.
Figure 7. ͑Color online͒ Time evolution of ͑A͒ the electrode OCP and ͑B͒
the mass of the electrode following potentiostatic deposition of Pt at Vdep
0.00, −0.15, −0.30, −0.45, and −0.60 V on gold-plated quartz crystal. The
=
deposition conditions were ͓Na PtCl ͔ = 5 mM and ͓HCl͔ = 10 mM ͑KCl-
2
6
free plating bath͒.
Influence of the OCP period after deposition.— In Fig. 6, Pt
nanostructures were deposited through an AAO membrane at
deposition at different values of Vdep is displayed, while the simul-
taneous evolution of the deposited mass is shown in Fig. 7B. As
shown in Fig. 7A, the electrode potential increases from the initial
value of Vdep to a limiting value close to 0.6 V. However, the time
needed for the OCP to reach this limiting value increases as applied
Vdep is made more negative. Fast recovery time ͑less than 30 s͒ of
the OCP is reached for Vdep ജ −0.3 V, while it can take as long as
−
0.15 V in a 0.5 mM Na PtCl , KCl-free plating bath. Pt NTs were
2 6
obtained by pulling out the electrode immediately after Pt deposition
Fig. 6A͒, whereas Pt NWs were achieved in the same deposition
conditions with the electrode left for 5 h at open-circuit potential
OCP͒ in the plating bath after deposition ͑Fig. 6B͒. The morphol-
͑
͑
ogy of such Pt NWs differs from that of NWs displayed in Fig. 1
and 3. Indeed, the top end of these former NWs presents a dense and
smooth circumference, while the center is very rough, porous, and
tip shape ͑cf. Fig. 6B͒. As evidenced in Fig. 6C, damaged Pt NWs
could be observed. Interestingly, the morphology observed in these
damaged regions is reminiscent of the Pt NTs previously observed,
suggesting that the mechanisms responsible for the rough structure
observed at the top of the Pt nanostructures are different from those
previously described. In each case, the composition of the deposit
was checked by energy-dispersive X-ray analysis ͑EDS͒ and no
trace of impurities was detected.
7
−
min for the most negative Vdep values. For Vdep = −0.45 and
0.60 V, the OCP first tends to a value ϳ−0.2 V, which is close to
the reversible H /H O equilibrium potential. The period of time
over which the OCP stays close to −0.20 V is determined by the
amount of dissolved hydrogen ͑and thus by Vdep͒. Then, the OCP
reached the equilibrium potential of the Pt /Pt redox couple, which
is 0.60 V in our conditions͒. Interestingly, Pt deposition still occurs
during the OCP period and the number of moles of Pt deposited
increases with Vdep. For Vdep ഛ −0.30 V, the mass increase during
the OCP period is attributed to the presence of H that chemically
reduces Pt according to Eq. 1
2
2
4
+
2
4
+
39
A combined EQCM and OCP study of Pt deposition during the
latent period at the end of the electrodeposition process was under-
taken to better understand the processes responsible for the previous
observations. In Fig. 7A, the evolution of the OCP following Pt
4+
+
2
H + Pt ↔ Pt + 4H
͓1͔
2
In Fig. 7, the deposition of Pt is observed as long as the OCP
values remained below ca. +0.20 V, which is close to the electrode
potential at which hydrogen underpotential deposition occurs on Pt.
This suggests that adsorbed hydrogen could also reduce Pt accord-
ing to Eq. 2. Obviously, this latter mechanism must be less impor-
tant than the former one because only a monolayer of adsorbed
hydrogen can be present
4
+
4
Hads + Pt4+ ↔ Pt + 4H+
͓2͔
In this respect, a mass increase is also observed for Pt deposition
performed at ca. −0.15 V, where no hydrogen is evolved ͑see the
inset of Fig. 7B͒. However, the mass increase is significantly lower
Figure 6. ͑Color online͒ SEM micrographs for Pt electrochemically depos-
ited through an AAO membrane at −0.15 V in a 0.5 mM Na PtCl , KCl-free
2
6
−
8
͑
i.e., 0.13 ϫ 10 mol of Pt͒ compared to more than 4.00
plating bath. ͑A͒ Electrode removed immediately from the electroplating
solution and rinsed with deionized water. ͓͑B͒ and ͑C͔͒ Electrode left for 5 h
in the electroplating solution under OCP.
−
8
ϫ 10 mol for Pt deposition performed at ca. −0.45 V ͑cf. Fig.
7B͒. The mass increase during OCP period following polarization at
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Journal of The Electrochemical Society, 157 ͑3͒ K59-K65 ͑2010͒
Vdep ജ −0.3 V could thus be attributed to Eq. 2. Indeed, considering
K63
2
the geometrical surface area of the crystal quartz ͑i.e., 0.2 cm ͒,
49
typical roughness factor of 10.0, and surface density of atoms of
15
−2 50
polycrystalline Pt around ca. 1.35 ϫ 10 atoms cm , one can
calculate that 0.44 ϫ 10−8 mol of adsorbed hydrogen are present on
Pt ͑full monolayer͒. Considering Eq. 2 this corresponds to 0.11
−
8
ϫ 10 mol of Pt adsorbed at the cathode surface. This value is in
accordance with the results obtained by EQCM ͑cf. Fig. 7B͒ as
−
8
0
.13 ϫ 10 mol of Pt were deposited after electrode polarization at
−
0.15 V. It seems straightforward to conclude that NWs observed in
Fig. 6B and C are more likely NTs with a Pt “stopper” at their top
end, chemically deposited during the final OCP step according to
Eq. 1 and 2. Therefore, the time duration of the final OCP is also a
crucial parameter if one wants to synthesize ͑or at least observe͒ Pt
NTs.
PtRu nanostructures.— Similar studies were performed to syn-
thesize alloyed PtRu nanostructures, which were recently shown to
28
exhibit interesting electrocatalytic properties, and to demonstrate
the versatility of the above-mentioned template method. Figure 8
shows SEM images of PtRu nanostructures deposited at Vdep
0.3 V through an AAO membrane in a KCl-free plating bath.
The concentrations of Na PtCl6 and RuCl3 varied from
=
−
2
z+
4
:1 mM to 0.5:4.5 mM and, in all cases, the total M ion concen-
tration remained fixed at 5 mM and the plating bath temperature was
T = 25°C ͑similar to the Pt section͒. The morphology of the PtRu
nanostructures is strongly dependent on the concentration ratios of
Na PtCl and RuCl3 in the precursor solution. Indeed, NWs are
2
6
obtained for a platinum salt concentration of 2.5 mM and higher,
whereas NTs are achieved for a lower platinum salt concentration.
Moreover, the interior diameter, D , of the NTs is also dependent on
i
the Pt salt concentration and varies from 65 to 295 nm ͑resulting in
thinner walls, W , from ca. 130 to 15 nm͒ as ͓Na PtCl ͔ varied from
t
2
6
1
to 0.5 mM ͑see Fig. 8C and D͒. Therefore, the formation of PtRu
NTs follows the same type of behavior previously observed for pure
Pt, i.e., NTs are obtained for low platinum salt concentration. Those
results suggest that even in the presence of Ru salt, the key factor to
4
+
achieve PtRu NTs at Vdep = −0.3 V is to limit the Pt mass trans-
port through the pores of the AAO membrane.
A structural analysis was performed on the PtRu deposits, and
Fig. 9 shows the corresponding XRD patterns for the PtRu nano-
structures depicted in Fig. 8. All the peaks can be indexed to the
hexagonal phase of Ti ͑substrate͒, fcc Pt, and the hexagonal phase of
Ru. The lattice parameter, a, for Pt was extracted from the XRD
patterns by performing a Rietveld refinement analysis ͑see the inset
of Fig. 9͒. The Pt lattice parameter decreases as one goes from 0 to
9
0% of RuCl in the plating solution, indicating that Ru is incorpo-
3
rated in the Pt lattice, i.e., a PtRu alloy is formed. Hexagonal Ru is
also formed for a plating bath containing more than 80% of RuCl3.
As will be shown later on, the Ru content of those deposits exceeds
4
0 atom %, which corresponds to the high solubility limit of Ru in
the fcc structure of Pt ͑Ref. 51 and references therein.͒
The atomic Ru content ͑XRu͒ of the nanostructures displayed in
Fig. 8A-D was evaluated by EDS and the results are presented in
Fig. 10 with respect to the RuCl content ͑X
͒ in the precursor
RuCl
3
3
solution. For comparison, PtRu films prepared on Ti substrates with-
out the AAO membrane were also prepared in the same deposition
conditions ͑Vdep = −0.3 V͒. As seen in Fig. 10, the Ru content of
PtRu films follows the same trend as that observed for PtRu nano-
structures, indicating that the presence of the AAO membrane does
not influence the final composition of the deposit. The Ru content of
the PtRu deposits is always lower than the RuCl content of the
3
solution. For example, the Ru content of the deposits is only ca.
Figure 8. SEM micrographs for PtRu electrochemically deposited through
an AAO membrane. The Na PtCl :RuCl concentrations are in the ratio ͑A͒
60% for 90% of Ru metallic ions in the electrolyte. Indeed, as de-
2
6
3
picted in Fig. 2, the deposition rates of Pt and Ru are radically
different and, at Vdep = −0.3 V and T = 25°C, the deposition rate of
Ru is only ϳ15% of that of Pt. So, based on these figures, one
would expect the Ru content of the PtRu deposits made from an
4
:1, ͑B͒ 2.5:2.5, ͑C͒ 1:4, and ͑D͒ 0.5:4.5, with ͓Na PtCl ͔ + ͓RuCl ͔
2 6 3
−2
= 5.0 mM.
In all cases, Q = 40 C cm , Vdep = −0.3 V, and ͓HCl͔
d
= 10 mM ͑KCl-free plating bath͒.
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K64
Journal of The Electrochemical Society, 157 ͑3͒ K59-K65 ͑2010͒
are both justified, depending on the material and the deposition con-
3
.94
.92
.90
.88
.86
ditions. Also, we have shown that NT formation is favored with a
low conductivity plating bath. Moreover, NT formation can be
achieved even if the porous membrane is not functionalized or if an
annular conductive film is not deposited before NT formation. Fi-
nally, we showed that arrays of PtRu alloy NT and NW can be
readily prepared by adjusting the composition of the electroplating
bath. It is expected that electrodeposition, combined with EQCM
measurements of the deposition rates, constitutes a promising tool
for the synthesis of NWs and NTs of most metals, oxides, and alloys
with a wide range of controlled composition and dimensions.
3
3
3
3
0
20
40
60
80 100
XRuCl3 in solution / (%)
D
C
Acknowledgment
B
A
This work was done with the financial support of the National
Science and Engineering Research Council ͑NSERC͒ of Canada and
the Canada Research Chair program.
Pt
Ru
Ti
Institut National de la Recherche Scientifique assisted in meeting the
publication costs of this article.
20
30
40
50
60
70
80
90
100
110
2
ꢀ
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