Fabrication of nanoporous palladium by dealloying and its thermal coarsening

Article abstract of DOI:10.1016/j.jallcom.2008.12.078

Nanoporous Pd with an average ligament size of 5-20 nm was fabricated by dealloying Pd_0.2Co_0.8 alloy. An electrochemical potential of +0.5 V (vs saturated calomel electrode) on the initial alloy led to Co dissolution in H₂

Full text of DOI:10.1016/j.jallcom.2008.12.078

Journal of Alloys and Compounds 479 (2009) 326–329
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Fabrication of nanoporous palladium by dealloying and its thermal coarsening
Masataka Hakamadaa,∗, Mamoru Mabuchib
a Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora,
Shimoshidami, Moriyama, Nagoya 463-8560, Japan
b Department of Energy Science and Technology, Graduate School of Energy Science, Kyoto University, Yoshidahonmachi, Sakyo, Kyoto 606-8501, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 July 2008
Received in revised form 15 December 2008
Accepted 15 December 2008
Available online 25 December 2008
Nanoporous Pd with an average ligament size of 5–20 nm was fabricated by dealloying Pd0.2Co0.8 alloy.
An electrochemical potential of +0.5 V (vs saturated calomel electrode) on the initial alloy led to Co
dissolution in H2SO4 aqueous solution. However, unlike nanoporous Au fabrication from Au–Ag alloy, the
free corrosion of Pd0.2Co0.8 alloy in H2SO4 did not result in a nanoporous structure; the electrochemical
potential must have been applied to generate a nanoporous structure from the Pd–Co alloy. Thermal
coarsening of nanoporous Pd is also investigated. Heating nanoporous Pd increased the ligament size up
to 650 nm. It is suggested that a solid state process like recrystallization, rather than melting, is responsible
for the thermal coarsening of nanoporous Pd.
Keywords:
Metals and alloys
Nanostructured materials
Corrosion
© 2008 Elsevier B.V. All rights reserved.
Thermal analysis
1. Introduction
composed of coagulated Pd nanoparticles having several hundred
nanometer diameters. Also, Masuda and Fukuda [15] fabricated
Nanoporous metals with an open-cell structure and nanosized
ligaments can be fabricated by dealloying (or electrochemically
etching) binary alloys [1,2]. Recent research studies involving deal-
loying mostly focus on the nanoporous Au fabricated by dealloying
Au–Ag alloy [1–8]. The nanoporous Au fabricated by dealloying
offers interesting properties, such as the dependence of mechan-
ical strength on ligament size [3,4], surface-enhanced Raman
spectroscopic properties [5], electrocatalytic properties [6], piezo-
electricity [7], electrical resistance variation due to adsorbates [8]
and so on.
Other metals can also be made nanoporous through dealloying;
for example, Pugh et al. [9] and Liu et al. [10] fabricated nanoporous
Pt by dealloying Pt–Cu alloy. Hayes et al. [11] succeeded in homoge-
neous nanoporous Cu fabrication by dealloying Cu–Mn alloy. These
studies (including nanoporous Au fabrication from Au–Ag alloy)
suggest that bimetallic but single-phase alloys are preferable start-
ing materials for the fabrication of final nanoporous metals of good
quality. Undesirable intermetallic phases probably cause hetero-
geneity in composition distribution in starting materials, which
leads to heterogeneous porosity during dealloying.
nanoporous Pd using anodic aluminum oxide (AAO) as a template.
Dealloying can create much smaller pores and ligaments (several
nanometers in size) than those for Pd black and AAO-templated Pd.
There have been several studies on corrosion properties of Pd–Cu
[16], Pd–Ag [17] and Pd–Ni [18] alloys, but nanoporous structure has
not been reported. Kabius et al. [19] studied the dealloying of Pd–Cu
and showed electron micrographs of nanoporous Pd; however, the
study mainly discussed the nanoporosity formation in the low cur-
rent region which may not be favorable for efficient production of
nanoporous Pd. Yu et al. [20] has recently reported nanoporous Pd
fabrication via selective dissolution of Pd–Ni–P metallic glass. Nev-
ertheless, the fabrication of nanoporous Pd by dealloying of a simple
binary alloy has not been reported so far.
In the present study, the authors demonstrate the facile pro-
duction of nanoporous Pd by dealloying Pd–Co alloy in acidic
electrolytes. Co is selected as a sacrificial element because it forms
single-phase solid solution with Pd [21] and its standard electrode
potential (E (Co2+/Co) = −0.277 V vs standard hydrogen electrode
(SHE)) is much lower than that for Pd (E (Pd2+/Pd) = +0.987 V vs
SHE) [22], which suggests the easy dissolution of Co without the
loss of Pd.
Nanoporous Pd is a promising material for catalytic application
[12], hydrogen sensing [13] and hydrogen storage [14], offering a
large surface area. Typical nanoporous Pd is Pd black, which is
One of the features of nanoporous metals, which is not observed
in porous metals with a pore size greater than several micrometers,
is the thermal coarsening of their nanoporous structure [3,4,9]. That
is, the pore and ligament sizes of nanoporous metals increase, main-
taining an open-cell porous structure, when subjected to heating at
high temperatures. This thermal coarsening is useful for pore size
∗
Corresponding author. Tel.: +81 52 736 7556; fax: +81 52 736 7406.
E-mail address: masataka-hakamada@aist.go.jp (M. Hakamada).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.12.078

M. Hakamada, M. Mabuchi / Journal of Alloys and Compounds 479 (2009) 326–329
327
Fig. 3. Scanning electron micrograph of Pd0.2Co0.8 alloy immersed in 3 mol/L H2SO4
for 17 days without electrochemical potential.
2. Experimental
Fig. 1. Relationship between current and time during dealloying of Pd0.2Co0.8 alloy
in 0.1 mol/L H2SO4.
Pd0.2Co0.8 alloy ingot was prepared by arc melting 99.9% pure Co and 99.9% pure
Pd together on a water-cooled copper hearth under Ar atmosphere. Then, the alloy
ingot was heated at 1173 K for 24 h under Ar atmosphere for homogenization. After
homogenization, the alloy was water-quenched to prevent the formation of an addi-
tional phase ␧-Co [21], because single-phase (␣-Co, Pd) alloy is desired for achieving
good rolling of the alloy and homogeneous porosity in the resulting nanoporous
Pd. The surface oxide scale generated by water quenching was removed mechani-
cally. Then, the alloy was rolled to an approximately 0.4 mm thickness and cut to
5 mm × 8 mm, followed by grinding with SiC paper and polishing with 3 ␮m dia-
mond paste and 0.04 ␮m Al2O3 powder to obtain a mirror-finished surface. The
electrochemical etching (or dealloying) of the rolled alloy sheet in 0.1 mol/L H2SO4
solution was carried out in a three-electrode electrochemical cell (500 mL) with a Pt
plate electrode as a counter electrode, a saturated calomel electrode as a reference
electrode, and the alloy sheet as a working electrode. An electrochemical potential of
+0.5 V relative to the reference electrode was applied to the alloy sheet for up to 42 h
at 298 K. For comparison, the free corrosion (simple immersion without the electro-
chemical potential) of the alloy sheet in 500 mL of 3 mol/L H2SO4 was attempted at
298 K for 17 days. Resulting materials were observed with a field-emission scanning
electron microscope (FE-SEM, S-4300 by Hitachi High-Technologies Corp.).
Thermal coarsening behavior was conducted under Ar atmosphere in a furnace
at 573, 773 and 973 K. To determine the thermal behavior of the nanoporous Pd,
differential scanning calorimetry (DSC, DSC-50 by Shimadzu Corp.) was performed
on the nanoporous Pd sheet in a standard Pt pan container. The sample mass was
12.61 mg and the heating rate was 20 K/min. DSC was conducted under constant Ar
flow.
control, but undesirable for high-temperature application. There-
fore, the thermal coarsening behavior should be understood for the
practical use of the present nanoporous Pd.
Metallic nanoparticles are also thermally less stable than their
bulky counterpart; that is, the melting temperature of metal-
lic nanoparticles is lower than that (Tm,bulk) of the parent bulky
metal, which is known as melting point depression, owing to
the large surface effect or the radius of curvature of nanoparti-
cle surface [23–26]. When the nanoporous Pd is regarded as an
architecture composed of thermally instable Pd nanoparticles, the
following coarsening mechanism via melting is possible. First, lig-
aments in nanoporous Pd have a lower melting temperature Tm,np
(<Tm,bulk) owing to their small size and melt at Tm,np. The small
melted ligaments then assemble in a liquid state and become
larger to minimize surface area. The increase in melted ligament
size leads to an increase in the melting temperature of the lig-
aments (approaching Tm,bulk). When the melting temperature of
the melted ligaments becomes as high as the heating tempera-
ture, the melted and coarsened ligaments solidify, coarsening the
nanoporous structure. Another possible mechanism for coarsening
is recrystallization in the solid state; that is, the extraordinarily large
surface of nanoporous Pd, as a high-density area defect, induces
nucleation and grain growth via volume (and possibly surface) dif-
fusion.
3. Results and discussion
3.1. Nanoporous Pd formation
During dealloying, the H2SO4 electrolyte turned dilute red, sug-
gesting that Co dissolves in the electrolyte as Co2+. Fig. 1 shows the
variation in current as a function of time during the 42-h dealloying
in 0.1 mol/L H2SO4. The current was high at the initial stage of deal-
loying and decreased gradually as the dealloying time increased.
In the latter part of this paper, the coarsening mechanism
via melting or recrystallization is confirmed by comparing the
experimental results with previous models for the melting of Pd
nanoparticles.
Fig. 2. Scanning electron micrographs of nanoporous Pd fabricated by dealloying of Pd0.2Co0.8 alloy. Dealloying was conducted in 0.1 mol/L H2SO4 for (a) 6 h and (b) 42 h.

328
M. Hakamada, M. Mabuchi / Journal of Alloys and Compounds 479 (2009) 326–329
Fig. 4. Scanning electron micrograph of nanoporous Pd heated at (a) 573 K, (b) 773 K and (c) 973 K.
No current was substantially detected after 6 h. This trend is simi-
lar to that of the dealloying of Au–Ag alloy to prepare nanoporous
Au [27].
and 973 K were 15, 180 and 650 nm, respectively, which were
obtained by FE-SEM observation. Heating at 573 K gave a very small
change in ligament size, while the pore and ligament sizes were
significantly increased by heating at 773 and 973 K.
The resulting nanostructures of dealloyed Pd0.2Co0.8 are
observed as shown in Fig. 2(a and b). Nanoporous structures
with an open-cell structure and nanosized ligaments were clearly
observed. Mass and dimension measurement of rolled Pd0.2Co0.8
sheets revealed that, if no volume shrinkage occurred, the result-
ing porosity would be 76%. However, the 42-h-dealloyed samples
had the porosity of approximately 62% (determined by mass and
dimension measurement). Volume shrinkage would take place dur-
ing the dealloying and lead to porosity reduction [28]. For 6 and
42 h dealloyed samples, average ligament sizes were 5 and 20 nm,
respectively. That is, the pores and ligaments coarsened during
dealloying for 6–42 h, although the current was negligible (Fig. 1).
The surface diffusion in an electrolyte may reduce the surface area,
resulting in increases in pore and ligament sizes [29].
The coarsening temperature of the nanoporous Pd during the
thermal treatment was compared with the lowered melting tem-
perature of Pd nanoparticles [32] in Fig. 5. In this figure, the vertical
axis shows the heating temperature normalized by the melting
temperature of bulky Pd (=1828 K), and the horizontal axis shows
the reciprocal of the ligament radius (=one-half the ligament diam-
eter) of the nanoporous Pd. The relationship between the melting
temperature and radius of Pd nanoparticles [32] is shown by a solid
line in Fig. 5 for comparison. The melting temperature of metal-
lic nanoparticles is approximately proportional to the reciprocal
Fig. 3 shows the FE-SEM image of the sample after 17-day
immersion. Even after17-day immersionin theconcentratedH2SO4
electrolyte, no nanoporous structure (like Fig. 2(a and b)) was
observed. The sample mass also remained unchanged, suggesting
that Co hardly dissolves from the Pd0.2Co0.8 alloy under free cor-
rosion conditions. These results are different from those for the
free corrosion of Au–Ag alloy [29,30], in which Ag readily dis-
solves in HNO3 or HClO4 without the electrochemical potential and
nanoporous Au is spontaneously obtained. It is well known that
the surface of pure Co is always covered with oxide passive film;
however, the passive film is unstable and dissolves in acidic solu-
tions, resulting in the corrosive dissolution of Co [31]. However, no
Co dissolution from the present Pd0.2Co0.8 alloy was observed. The
surface segregation of Pd may be responsible for the inertness of
the Pd0.2Co0.8 alloy, although further investigation is required to
elucidate the mechanism for the inertness.
3.2. Thermal coarsening
Fig. 5. Relationship between heating temperature normalized by melting temper-
ature of bulky Pd and reciprocal of ligament radius of heated nanoporous Pd. The
melting temperature of Pd nanoparticles [32] is also shown for comparison.
FE-SEM images of heated nanoporous Pd are shown in Fig. 4.
Average ligament diameters of nanoporous Pd heated at 573, 773

M. Hakamada, M. Mabuchi / Journal of Alloys and Compounds 479 (2009) 326–329
329
The simple immersion of the alloy in the electrolyte did not
lead to nanoporous structure formation; electrochemical poten-
tial must be applied to generate the nanoporous structure. Heating
nanoporous Pd above 573 K coarsened the nanoporous structure
through a solid state process like recrystallization, rather than melt-
ing which is typical for metallic nanoparticles.
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Nanoporous Pd with pore sizes of 5–20 nm was successfully
fabricated by dealloying rolled Pd0.2Co0.8 alloy in acid electrolyte.