A benign route to fabricate nanoporous gold through electrochemical dealloying of Al-Au alloys in a neutral solution

Article abstract of DOI:10.1016/j.electacta.2009.05.089

Nanoporous gold (NPG) ribbons have been fabricated through electrochemical dealloying of melt-spun Al-Au alloys with 20-50 at.% Au in a 10 wt.% NaCl aqueous solution under potential control at room temperature. The microstructures of NPG were characterize

Full text of DOI:10.1016/j.electacta.2009.05.089

Electrochimica Acta 54 (2009) 6190–6198
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A benign route to fabricate nanoporous gold through electrochemical dealloying
of Al–Au alloys in a neutral solution
Qian Zhanga, Xiaoguang Wanga, Zhen Qia, Yan Wanga,b, Zhonghua Zhanga,∗
a Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jingshi
Road 73, Jinan 250061, PR China
b School of Materials Science and Engineering, University of Jinan, Jiwei Road 106, Jinan 250022, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Nanoporous gold (NPG) ribbons have been fabricated through electrochemical dealloying of melt-spun
Al–Au alloys with 20–50 at.% Au in a 10 wt.% NaCl aqueous solution under potential control at room
temperature. The microstructures of NPG were characterized using X-ray diffraction (XRD), field-emission
scanning electron microscopy (FE-SEM) and energy dispersive X-ray (EDX) analysis. The microstructures
of the NPG ribbons strongly depend upon the phase constitutions of the starting Al–Au alloys. The single-
phase Al2Au or AlAu intermetallic compound can be fully dealloyed, resulting in the formation of NPG
with a homogeneous porous structure. The separate dealloying of Al2Au and AlAu in the two-phase Al–45
Au alloy leads to the formation of NPG composites (NPGCs). In addition, the dealloying of the Al–20 Au
alloy comprising ␣-Al and Al2Au leads to the formation of NPG with bimodal channel size distributions.
According to the ligament size, the surface diffusivity of Au adatoms along the alloy/electrolyte interface
has been evaluated and increases with increasing applied potential. The dealloying mechanism in the
neutral NaCl solution has been explained based upon pourbaix diagram and chloride ion effect.
© 2009 Elsevier Ltd. All rights reserved.
Received 20 February 2009
Received in revised form 23 May 2009
Accepted 25 May 2009
Available online 6 June 2009
Keywords:
Nanoporous gold
Electrochemical dealloying
Chloride ion effect
Surface diffusivity
Self-acidifying effect
1. Introduction
Therefore, solid solutions such as Ni–Cu [22], Pt–Cu [23], and
Cu–Au [24] are mainly focused on. Recently, metallic glasses with
Dealloying primarily originated from the phenomenon of corro-
sion (for example, stress corrosion [1,2] and pitting corrosion [3–5]),
has attracted more attention because it has been proved to be a
highly productive and controllable route to fabricate nanoporous
metals with a three-dimensional bicontinuous interpenetrating
ligament-channel structure at the nanometer scale [6]. This inter-
connected ligament-channel structure can significantly enhance its
specific surface area and is most ideal for high-quality catalysts
[7–9], sensors [10–12], actuators [13], and so on. So far, however,
these nanoporous materials have been restricted in the field of
laboratory by several reasons, among which the high cost and com-
plexity of fabrication processes are pronounced. In addition, Raney
catalysts [14,15] and AAO templates [16] are two successful appli-
cation instances of corrosion fundamentals.
While the prototypical Ag–Au alloy system is further exploited
for fundamental principles in such subject as porosity evolution
[17–21], many other alloy systems have been reported. It is gener-
ally recognized that ideal bicontinuous nanoporous structures are
obtained from binary alloys with a single-phase solid solubility
across all compositions by chemical/electrochemical dealloying.
a homogeneous structure including Pt–Si [25], Y–Ti–Al–Co [26]
and Pd–Ni–P [27] have also been reported to exhibit nanoporosity
evolution during dealloying. Intermetallic compound is also a
single phase with a relatively well-defined constitution (stoichio-
metric or non-stoichiometric) and crystal structure but it is more
difficult to be chemically corroded under ambient conditions. In
this respect, it has been reported that nanoporous gold (NPG) can
be synthesized through dealloying of AuCu3 [28] and Al2Au [29] in
acid or alkali aqueous solutions. Unsurprisingly, there are to date
a few reports on fabricating nanoporous materials by selectively
leaching less noble element(s) from multi-phase alloys, where
the aforementioned homogeneity is affected by the disparities in
composition and structure between coexistent phases and this
further leads to an additional difficulty in controlling the disso-
lution process [30]. Lu et al. [31] reported the synthesis of porous
copper from crystalline two-phase Cu–Zr film by electrochemical
dealloying. Two phases namely, Cu8Zr3 and Cu10Zr7 were present
in the parent alloy and the Zr element was simultaneously depleted
from these two phases in the nanocrystalline alloy, suggesting
that electrochemical dealloying is a controllable and powerful
tool in terms of deriving fine porous structure from alloys with
multiple elements and/or phases. Apart from the composition
of the parent alloys, dealloying potential and electrolyte have
been recognized to have a significant effect on the morphology
∗
Corresponding author. Tel.: +86 531 88396978; fax: +86 531 88395011.
E-mail address: zh zhang@sdu.edu.cn (Z. Zhang).
0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.05.089

Q. Zhang et al. / Electrochimica Acta 54 (2009) 6190–6198
6191
of the obtained nanoporous structure. Many efforts have been
devoted to the selection of electrolytes, ranging from acid to
alkali aqueous solutions [32], from inorganic to organic media
[33], and even ionic solutions [34]. This is an illustrative list, but
most of these electrolytes are dangerous to be handled and/or are
relatively expensive. Recently, a neutral AgNO3 aqueous solution
was employed by Snyder et al. [21] to obtain NPG film from a Ag–Au
alloy via electrochemical dealloying. This work may trigger new
research into electrochemical dealloying in benign salt solutions.
In addition, it is essential to introduce economical and ecological
dealloying solutions to industrialize the fabrication of nanoporous
materials by dealloying (chemically or electrochemically).
NaCl aqueous solution to the depth of about 1 cm underneath the
air–solution interface in an open cell, as schematically shown in
Fig. 1. The electrolyte was not deaerated and a two-electrode DC
regulated power supply (QJ 3003S) was set to perform the elec-
trochemical dealloying with the parent alloy ribbon as the anode
(working electrode) and a graphite bar as the cathode at room
temperature (293 K). The potentials mentioned in this paper were
values directly read from the DC power supply except those spe-
cially indicated. To record the current change during the dealloying,
a multimeter was interconnected between the power and the elec-
In this work, we investigate the formation and microstructure
of NPG through electrochemical dealloying of Al–Au alloys with
20–50 at.% Au in a 10 wt.% NaCl aqueous solution under poten-
tial control at room temperature. The present Al–Au alloy system
is composed of a single-phase intermetallic compound or a com-
bination of two phases (one solid solution and one intermetallic
compound or two intermetallic compounds). The influence of alloy
composition and applied potential on the microstructure of the
obtained NPG has also been studied. In addition, the dealloy-
ing mechanism has been discussed based upon pourbaix diagram
(potential–pH diagram) and chloride ion effect.
2. Experimental
The starting Al–Au alloys with nominal compositions of 20, 30,
33.4, 45 and 50 at.% Au were prepared from elemental Al (purity,
99.95 wt.%) and Au (purity, 99.9 wt.%) in a quartz crucible using
a high-frequency induction furnace. Using a single roller melt
spinning apparatus, the pre-alloyed ingots were remelted by high-
frequency induction heating in a quartz tube and then melt-spun
onto a copper roller with a diameter of 0.35 m at a speed of 1000
revolutions per minute (rpm) in a controlled argon atmosphere.
The melt-spun ribbons obtained were typically 20–50 ␮m in thick-
ness, 2–4 mm in width and several centimeters in length. Without
any pretreatment (e.g., annealing), the melt-spun Al–Au alloy rib-
bons were vertically immersed in the electrolyte of the 10 wt.%
Fig. 1. Schematic diagram of the setup employed for the electrochemical dealloying
in this work. M represents the hydrolysis products of [Al(OH)mCln]3−m−n derived
from the anode according to Eqs. (10)–(12).
Table 1
Phase constitutions of the starting melt-spun Al–Au alloys.
Fig. 2. XRD patterns of the starting alloys and NPG through the electrochemical
dealloying of the starting melt-spun alloys of (a) Al–33.4 Au, (b) Al–20 Au and (c)
Al–50 Au in the 10 wt.% NaCl solution at the potential of 1.5 V.
Alloys (at.%) Al–20 Au
Phases ␣-Al + Al2Au ␣-Al + Al2Au Al2Au
Al–30 Au
Al–33.4 Au Al–45 Au
Al–50 Au
Al2Au + AlAu AlAu

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Q. Zhang et al. / Electrochimica Acta 54 (2009) 6190–6198
trode. A constant potential of 1.5 V was set to selectively leach
Al from the starting Al–20, 33.4, 45 and 50 Au alloys. In order
to probe the influence of applied potential on the dealloying of
Al–Au and formation of NPG, two potentials of 0.8 and 2.0 V were
used for the dealloying of the melt-spun Al–33.4 Au alloy. In addi-
tion, the melt-spun Al–33.4 Au alloy was also electrochemically
dealloyed in a 2.5 wt.% NaOH aqueous solution at the potential of
1.5 V. In order to examine the chloride ion effect on dealloying, the
electrochemical dealloying of the melt-spun Al–30 Au alloy was
performed in the 10 wt.% NaCl, 21 wt.% (NH4)2SO4 and 2.5 wt.%
NaOH aqueous solutions at the potentials of 2.0, 2.0 and 1.5 V,
respectively.
The phases present in the starting Al–Au alloys and as-
dealloyed samples were identified using an X-ray diffractometer
(XRD, Rigaku D/max-rB) with Cu K␣ radiation. The microstruc-
tures of the as-dealloyed ribbons were characterized using a
field-emission scanning electron microscope (FE-SEM, JSM-6700F).
In addition, the chemical composition of the as-dealloyed rib-
bons was determined using an energy dispersive X-ray (EDX)
analyzer.
Fig. 3. FE-SEM images showing the microstructures of NPG through the electrochemical dealloying of the melt-spun Al–33.4 Au alloy in the 10 wt.% NaCl solution at the
potentials of (a and b) 0.8 V, (c and d) 1.5 V, and (e and f) 2.0 V. (g) EDX spectrum showing the composition of NPG in (c).

Q. Zhang et al. / Electrochimica Acta 54 (2009) 6190–6198
6193
3. Results and discussion
The XRD results confirm that all the as-dealloyed samples are
composed of a single f.c.c. Au phase. The lower parts of Fig. 2a–c
show the XRD patterns of NPG through the electrochemical dealloy-
ing of the melt-spun Al–33.4, 20 and 50 Au alloys in the 10 wt.% NaCl
solution at the potential of 1.5 V, respectively. Figs. 3 and 4 show the
microstructures of NPG through the dealloying of the melt-spun
Al–Au alloys in the 10 wt.% NaCl solution at different potentials. It
can be seen from Figs. 3 and 4 that nanoporous structures were
obtained in all the as-dealloyed samples regardless of their dispar-
ities in the Au content in the starting Al–Au alloys and subjected
potentials during dealloying. It is obvious that the effect of alloy
compositions (of the starting Al–Au alloys) on the microstructures
of NPG outweighs that of dealloying potentials in the case of the
electrochemical dealloying of the Al–Au alloys in the 10 wt.% NaCl
solution. A recent research into NPG fabrication by dealloying of the
Ag–Au alloy in the AgNO3 solution also confirms that the sizes and
interspacings of ligaments in NPG are almost the same when the
anodic potential was raised from 1.44 to 2.04 V (vs. NHE) [21].
Fig. 3a, c and e shows the section-view microstructures of the
NPG ribbons through the electrochemical dealloying of the melt-
spun Al–33.4 Au alloy in the 10 wt.% NaCl solution at the potentials
of 0.8, 1.5 and 2.0 V, respectively. All NPG ribbons exhibit a homoge-
neous, bicontinuous, interpenetrating ligament-channel structure
3.1. Microstructures of the obtained NPG ribbons
The phases present in the starting melt-spun Al–Au alloys were
identified by XRD and the phase constitutions are listed in Table 1.
The melt-spun Al–33.4 Au alloy is composed of a single-phase
Al2Au intermetallic compound and the corresponding XRD pattern
is shown in Fig. 2a. The melt-spun Al–20 and 30 Au alloys con-
sist of two phases: ␣-Al solid solution and Al2Au (Fig. 2b). With
increasing Au content to 50 at.%, the melt-spun Al–50 Au alloy is
composed of a single-phase AlAu intermetallic compound (Fig. 2c).
In addition, two intermetallic compounds of Al2Au and AlAu are
present in the melt-spun Al–45 Au alloy, as presented in Table 1. The
present results are consistent with the binary Al–Au phase diagram
[35], suggesting that rapid solidification has no significant influ-
ence on the phase constitutions of the Al–Au alloys with 20–50 at.%
Au. It should be noted that Al2Au has a face centered cubic (f.c.c.)
¯
CaF2-type structure (with a space group of Fm3m) where Au atoms
occupy all f.c.c. lattice sites in a unit cell and all eight Al atoms are
encaged inside the cell. In contrast, the AlAu phase has a primitive
monoclinic structure (with a space group of P21/m), which differs
considerably from that of f.c.c. Au.
Fig. 4. FE-SEM images showing the microstructures of NPG through the electrochemical dealloying of the melt-spun (a and b) Al–20 Au, (c and d) Al–45 Au and (e and f)
Al–50 Au alloys in the 10 wt.% NaCl solution at the potential of 1.5 V.

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Q. Zhang et al. / Electrochimica Acta 54 (2009) 6190–6198
Table 2
Correlation between applied potential, dealloying time and ligament size in NPG for the electrochemical dealloying of the melt-spun Al–33.4 Au alloy in the 10 wt.% NaCl and
2.5 wt.% NaOH solutions at room temperature (293 K). The surface diffusivity of Au atoms along the alloy/electrolyte interface was determined by Eq. (1) [45].
Potential (V)
0.8
1.5
1.5
2.0
Dealloying time (t, s)
Ligament size (d(t), nm)
Surface diffusivity (Ds, m2 s−1
28,800 (NaCl)
29.5 1.5
1.2 × 10−19
1800 (NaCl)
15.2 0.7
1.4 × 10−19
2220 (NaOH)
15.5 1.0
1.2 × 10−19
900 (NaCl)
32.6 1.7
5.7 × 10−18
)
with nanoscale length scales. Although the applied potentials have
no significant influence on the microstructures of NPG, the deal-
loying duration is much different at different potentials, as listed
in Table 2. The dealloying duration is 28,800 s at the potential of
0.8 V, but is only 900 s at the potential of 2.0 V. Moreover, the lig-
ament size of NPG obtained at 0.8 V is comparable to that of NPG
obtained at 2.0 V. The dealloying duration at the potential of 1.5 V
is double of that at 2.0 V, but the ligament size of NPG obtained at
1.5 V is about one half of that at 2.0 V, as presented in Table 2. For
the dealloying of Ag–Au alloys in the HNO3 solution, the pore size
decreases with increasing potential and the length scale of porosity
has been suggested to be a power law of the ratio of the dissolution
rate of Ag and the surface diffusion rate of Au [17]. For the dealloying
of Al–Au alloys in the 10 wt.% NaCl solution, a similar competition
should also exist between the dissolution rate of Al and the surface
diffusion rate of Au. With increasing potential from 0.8 to 1.5 V, the
dissolution rate of Al increases more quickly than the diffusion rate
of Au. At the high potential of 2.0 V, however, the promoting effect
on the diffusion rate is more substantial. Hence, the influence of
the applied potentials on the dealloying duration and the ligament
size of NPG is due to the dissolution mechanism of Al and the diffu-
sion kinetics of Au adatoms at the electrolyte/alloy interface during
dealloying, and this will be discussed in the following part.
Fig. 3b, d and f shows the plan-view images of the NPG ribbons
corresponding to the images in Fig. 3a, c and e, respectively. Obvi-
ously, many micro-cracks appear in the NPG ribbons obtained at
the potentials of 1.5 and 2.0 V but there are almost no cracks in
the samples obtained at the low potential of 0.8 V. In addition, it
seems that the cracks originate inside the grains and expand both
through neighboring grains and along grain boundaries. In contrast,
such widespread cracks are scarcely observed in electroless deal-
loying. Hayes et al. [36] thought that cracking in the monolithic
nanoporous copper could be caused by several mechanisms: resid-
ual stress, capillary forces leading to “mud-cracking”, or coherency
stresses between the alloy and the dealloyed Cu. All these mech-
anisms may cause micro-cracks in the present NPG. The residual
stress formed during rapid solidification of the Al–33.4 Au alloy
may release during the dealloying, leading to the formation of
micro-cracks in NPG. Especially, the coherency stress is an impor-
tant reason for the appearance of micro-cracks in NPG, because the
lattice mismatch between the un-dealloyed Al2Au (with a lattice
constant of 0.5997 nm) and the dealloyed Au (with a lattice constant
of 0. 4078 nm) is as high as 47%. Furthermore, this coherency stress
may be readily released by the introduction of micro-cracks rather
than dislocations, since dislocation nucleation and movement are
severely restricted by the nanoscale size of the ligaments [37]. On
the other hand, Parida et al. [38] reported a macroscopic shrinkage
by up to 30 vol% during electrochemical dealloying of Ag–Au and
observed more shrinkage at higher potentials. The authors have
argued that the processes associated with the volume shrinkage
at the dealloying front depend on the potential not only through
the dissolution rate but also through the dependency of formation
enthalpy and diffusion constants of surface adatoms and surface
vacancies on the potential. As the potential is raised, the dissolu-
tion rate accordingly increases, driving the surface at the dealloying
front further away from equilibrium. Therefore, it is expected that
the formation of cracks is governed by the dealloying kinetics which
determines the release rate of the stresses. The EDX results show
that all of Al was removed during the electrochemical dealloying
of the melt-spun Al–33.4 Au alloy, and a typical EDX spectrum
is shown in Fig. 3g. NPG by dealloying of Ag–Au alloys normally
contains some residual Ag (several atom percent). Moreover, the
residual Ag cannot be removed but asymptotically reaches a limit
at exhaustively long etching times (up to 100 h) [39]. The present
results show that NPG with a homogeneous structure can be fab-
ricated by the electrochemical dealloying of a single-phase Al2Au
intermetallic compound in the 10 wt.% NaCl solution at room tem-
perature.
The potential of 1.5 V was chosen to investigate the influence of
alloy compositions of the starting Al–Au alloys on the microstruc-
tures of NPG by the electrochemical dealloying in the 10 wt.% NaCl
solution (Fig. 4). Fig. 4a shows the plan-view image of the NPG
by the dealloying of the melt-spun Al–20 Au alloy. A bicontinu-
ous channel-island structure can be observed, as shown in Fig. 4a.
The size of the channels is about 100 nm. The FE-SEM image at a
higher magnification clearly shows that the metallic islands exhibit
an open, bicontinuous interpenetrating ligament-channel structure
with length scales of 10–20 nm (Fig. 4b). This means that NPG with
bimodal channel size distributions can be fabricated through the
electrochemical dealloying of the Al–20 Au alloy in the 10 wt.% NaCl
solution. In our former work [29], we have synthesized NPG with
bimodal channel size distributions by chemical dealloying of the
Al–20 Au alloy in the 5 wt.% HCl or 20 wt.% NaOH aqueous solu-
tion under free corrosion conditions. NPG with a uniform porous
structure is favorable for applications such as sensing and catalysis.
In addition, bimodal channel size distributions composed of large
sized channels with highly porous channel walls are desirable for
microfluidic-based sensors in order to achieve fast response time
and high sensitivity [40]. Ding and Erlebacher [40] reported a two-
step dealloying strategy to create NPG with multimodal pore size
distributions. It is obvious that the present one-step electrochemi-
cal dealloying route is facile and economical than that reported in
the literature [40].
Fig. 4c and d shows the microstructure of the NPG by the elec-
trochemical dealloying of the melt-spun Al–45 Au alloy in the
10 wt.% NaCl solution. Fig. 4e and f shows the microstructure of
the NPG by the electrochemical dealloying of the melt-spun Al–50
Au alloy. It is clear that NPG with a homogeneous porous struc-
ture can also be fabricated by the electrochemical dealloying of
a single-phase AlAu intermetallic compound in the NaCl solution,
as shown in Fig. 4f. In addition, it can be seen from Fig. 4c and
d that the NPG obtained from the Al–45 Au alloy is composed of
two kinds of nanoporous structures. The metallic islands are sur-
rounded by another ligament-channel structure with length scales
of 60–80 nm. This intraisland structure is similar to that shown in
Fig. 4b. The metallic islands exhibit an interpenetrating ligament-
channel structure with length scales of 10–20 nm. We define the
NPG with two kinds of nanoporous structures as NPG composites
(NPGCs). Therefore, NPGCs can be fabricated by the electrochemical
dealloying of the two-phase Al–45 Au alloy comprising Al2Au and
AlAu. In addition, the EDX analysis (not shown) demonstrates that
all of Al was leached away during the electrochemical dealloying
of the melt-spun Al–20, 45 and 50 Au alloys, and the resulting NPG
contains no detectable Al.

Q. Zhang et al. / Electrochimica Acta 54 (2009) 6190–6198
6195
loying time t, a is the lattice parameter of Au (4.08 × 10−10 m),
and T is the dealloying temperature (293 K). According to the lig-
ament size in Table 2, the surface diffusivity of Au adatoms along
alloy/electrolyte interfaces was calculated for the electrochemical
dealloying of the melt-spun Al–33.4 Au alloy in the 10 wt.% NaCl
solution at different potentials, and also listed in Table 2. The surface
diffusivity of Au adatoms slowly increases with increasing poten-
tial from 0.8 to 1.5 V, but sharply increases with further increasing
potential to 2.0 V. Both the high diffusivity of gold adatoms and
the fast injection of vacancies (caused by dissolution of aluminum
atoms) are believed to be accountable for the mass amount of
micro-cracks in the NPG by the dealloying of the Al–33.4 Au alloy
at 2.0 V. The influence of potential on the diffusivity of adatoms
at the alloy/electrolyte interface has been explained as the conse-
quenceofanaccumulationofchargeontheelectrodesurface, which
modifies the enthalpy of formation of surface moving entities and
the formation of adsorbed bonds between neighboring sites due
to electron transfer reactions [46,47]. It is obvious that the calcu-
lation results are in agreement with the SEM observations (Fig. 3),
indicating substantial coarsening with greater diffusivity at higher
potentials. The sharp increase in the diffusivity of Au adatoms at
2.0 V is caused by strong bonding formed between Au adatoms and
adsorptive anion (chloride ion in this work) in the electrolyte, as
discussed more detailedly in the following section.
To examine the coarsening effect caused by the chloride ion, a
2.5 wt.% NaOH aqueous solution (neutral solutions without halide
ions would probably passivate the alloy under anodization and acid
solutions were found to interfere obviously with the electrochem-
ical process) was chosen for the dealloying of the Al–33.4 Au alloy
at the potential of 1.5 V at room temperature, and the section-view
microstructure of the NPG obtained is shown in Fig. 5. Fig. 5a
shows the low-magnification microstructure of the NPG ribbons.
The FE-SEM image at a higher magnification clearly shows the
bicontinuous interpenetrating ligament-channel structure with
length scales of 10–20 nm, Fig. 5b. The surface diffusivity of Au
adatoms was calculated for the dealloying in the NaOH solution
and also listed in Table 2. The Ds value of Au adatoms in the case
of dealloying in the alkali solution was a little lower than that in
the NaCl solution, in compliance with the activating role played
by halide ions. However, this disparity in Ds is much smaller than
that reported by Dursun et al. [42]. It should be noted that the
starting alloy and electrolyte in this work are different from those
by Dursun et al. [42], especially when considering the different
adsorbabilities of perchloride ions and hydroxide ions. Apart from
the influence of the electrolyte, the surface diffusion of Au adatoms
is possibly affected by the kinetics of dissolution of aluminum into
the electrolyte and the adsorption of H atoms on the anode and
these will be discussed in the following section.
The formation of NPG by dealloying of Al–Au alloys can be
explained on the basis of the solidification process and phase
constitutions of the starting alloys. Due to the large difference
of standard electrode potentials of Al and Au, both Al2Au and
AlAu intermetallic compounds can be fully dealloyed, resulting
in the formation of NPG with a homogeneous structure. The size
of ligaments/channels in the NPG by dealloying of Al2Au is much
smaller than that by dealloying of AlAu (Figs. 3c and 4f). The notable
difference in crystal lattice between these two phases may result
in different obstacles to the dealloying process. During dealloying
of Al2Au, all Al atoms are selectively dissolved and the residual Au
atoms form an f.c.c. lattice with a lattice constant larger than that of
bulk Au. The formation of NPG only needs the shrinkage of the lat-
tice accompanying by the rearrangement of the Au atoms and the
formation/removal of lattice sites [29]. In contrast, a more compli-
cated mechanism, e.g., nucleation and growth instead of spinodal
decomposition, is requiredforthe Auatomsintheinitialmonoclinic
AlAu lattice to reconstruct the f.c.c. structure of Au. The disparity in
the crystal lattice structure of the master alloys, together with the
relatively low surface diffusivity of gold adatoms, may be responsi-
ble for separating Al2Au and AlAu phases as independent scenarios
during the dealloying of the Al–45 Au alloy. According to the binary
Al–Au phase diagram [35], the resulting microstructure of the Al–45
Au alloy is composed of Al2Au and AlAu. Moreover, the AlAu phase
surrounds the primary Al2Au phase. It is reasonable to assume that
the electrochemical dealloying of Al2Au and AlAu in the Al–45 Au
alloy proceeds separately, resulting in the formation of the NPGCs
(Fig. 4d). In the melt-spun Al–20 Au alloy, the ␣-Al phase surrounds
the primary Al2Au phase [29]. During dealloying, the excavation of
the ␣-Al phase out of the alloy contributes to the formation of large
sized channels in the NPG ribbons. The dealloying of the Al2Au
phase results in the formation of nanoporous islands. In addition,
the dealloying duration of 1800–3000 s was required for the fabri-
cation of NPG by the electrochemical dealloying of the Al–20, 33.4,
45 and 50 Au alloys in the 10 wt.% NaCl solution at the potential
of 1.5 V. Therefore, the corresponding dealloying rate across the
thickness (20–50 ␮m) of the alloy ribbons could be on the scale
of 10 nm s−1 at the potential of 1.5 V, which is substantially higher
than that reported by Snyder et al. [21] under potential control
from 1.44 to 2.04 V (vs. NHE). The high efficiency of dealloying in
this work could be attributed to such factors as grain refinement
by rapid solidification [41], the activation of chloride ion [42,43]
and much negative standard electrode potential of Al3+/Al [44].
3.2. Surface diffusivity of Au adatoms during dealloying
It has been widely recognized that applied potentials and the
presence of halide ions have a significant effect on the sizes of
ligaments and channels in nanoporous metals fabricated by elec-
trochemical dealloying. Surface diffusion of more noble elements
along alloy/solution interfaces during dealloying plays a key role in
the formation of NPG and has a significant influence on the length
scales of ligaments/channels [17,20]. Erlebacher [17] found a max-
imally unstable spatial period that scales as ꢀ∞(Ds/V0)1/6, where
ꢀ is the characteristic spacing separated the evolution of gold clus-
ters, Ds is the surface diffusion coefficient, and V0 is the velocity
of a flat alloy surface with no gold accumulated upon it. Based on
the surface diffusion controlled coarsening mechanism, the surface
diffusivity (Ds) of Au adatoms along alloy/electrolyte interfaces can
be estimated by the equation [45]:
3.3. Dealloying mechanism based upon pourbaix and chloride ion
effect
During the electrochemical dealloying of the melt-spun Al–Au
alloysinthe10 wt.%NaClaqueoussolution, alargeamountofhydro-
gen (H2) bubbles emerged from the surfaces of both the cathode
and the anode (Al–Au ribbons), as schematically shown in Fig. 1.
This phenomenon lasted until the end of the dealloying (the for-
mation of NPG). Moreover, the amount of the bubbles emerging
from the anode surface increases with increasing Al content in the
Al–Au alloys and the applied potentials. In addition, white floccu-
lent precipitates appeared in the electrolyte after 3–4 runs of the
dealloying.
[d(t)]4kT
32ꢁta4
Ds =
(1)
According to pourbaix diagram for the Al–H2O system (schemat-
ically shown in Fig. 6a), Al should have been passivated in the
aqueous solution from pH ∼4 to ∼9 at the potential above −1.5 V
(vs. NHE) [44]. Snyder et al. [21] recently reported the relief of pas-
where k is the Boltzmann constant (1.3806 × 10−23 J K−1), ꢁ is the
surface energy (1 J m−2 [42]), d(t) is the ligament size at the deal-

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Q. Zhang et al. / Electrochimica Acta 54 (2009) 6190–6198
sivation of Ag during the electrochemical dealloying of the Ag–Au
alloy in the neutral silver nitrate solution, and introduced a model
of a proton concentration gradient away from the pore front to
explain the porosity evolution and more residual Ag in the resulting
NPG. Since the model of a self-acidifying effect caused by hydrol-
ysis is fundamental to pitting corrosion and Al is often subjected
to this corrosion manner with presence of chloride ion [3,48–53],
it is reasonable to assume that large-scale pitting corrosion caused
by chloride ion may prevent Al from being passivated. A similar
self-acidifying effect existed in our work because hydrogen ions
were released when Al was depleted and hydrolyzed in the anodic
region, as schematically illustrated in Fig. 6b. This will be detailedly
explained in the following.
Once an above-critical potential is imposed to initiate the deal-
loying reaction, it is obvious that hydrogen should evolve on the
cathode under our experimental conditions by the following reac-
tion (Fig. 1).
2H2O + 2e− → 2OH− + H2↑
(2)
As the anodic reaction is concerned, the compact passivation
layer is locally dissolved at potentials above the critical potential
(Eb, break potential) for pitting corrosion to occur. For example,
the Eb value of Al in the 0.1 mol L−1 NaCl aqueous solution at 298 K
is −0.74 V (vs. SCE) [53]. In this work, the Al–Au alloy ribbon was
dipped into the 10 wt.% (∼1.8 mol L−1) NaCl aqueous solution, and
the applied potential was well above the Eb value of Al. Therefore,
the passivation layer of aluminum oxide or hydroxide is difficult to
form due to the fierce and overall attack of chloride ions during the
dealloying process. Instead, the depletion of Al from the Al–Au alloy
Fig. 6. (a) Pourbaix (potential–pH) diagram for Al–H2O system [44]. (b) Schematic
illustration for pore evolution and hydrogen bubble emerging during the electro-
chemical dealloying of Al2Au in the 10 wt.% NaCl aqueous solution. M represents the
hydrolysis products of [Al(OH)mCln]3−m−n derived from the anode according to Eqs.
(10)–(12).
instantly occurs by the reaction [49]:
Al → Al3+ + 3e−
(3)
This reaction leads to rapid enrichment of positive charges
within the dissolution zone, which impose remarkable attraction
to anions for charge balance. The anions in the electrolyte (mainly
Cl− and OH−) readily migrate into the dissolution zone, as shown in
Figs. 1 and 6b. Because the mobility of Cl− is much greater than that
of OH− [48], the enrichment of Cl− within the dissolution zone will
be established. Simultaneously, the hydrolysis of Al3+ takes place
[49] by the following reactions (Fig. 6b).
Al3+ + H2O ↔ [Al(OH)]2+ + H+
[Al(OH)]2+ + Cl− → [Al(OH)Cl]+
[Al(OH)Cl]+ + H2O ↔ Al(OH)2Cl + H+
(4a)
(4b)
(5)
Obviously, the hydrolysis process is accompanied by continuous
release of hydrogen ions. Since reactions (4) and (5) are promoted
by the increase of Al3+ (reaction (3)), the faster depletion of Al from
the Al–Au alloy accelerates the release of hydrogen ions, result-
ing in a lower pH value in the dissolution zone (self-acidifying
effect, Fig. 6b). Hydrochloric acid is automatically concocted at the
alloy/electrolyte interface, and a self-catalyzed evolution of hydro-
Fig. 5. FE-SEM images showing the microstructure of NPG by the electrochemical
dealloying of the melt-spun Al–33.4 Au alloy in the 2.5 wt.% NaOH solution at the
potential of 1.5 V at room temperature.

Q. Zhang et al. / Electrochimica Acta 54 (2009) 6190–6198
6197
gen takes place through the following reaction [50]:
+
2Al + 6H+ + 4Cl− → 3H2↑ + 2AlCl2
(6)
Macroscopically, emerging bubbles can be observed on the sur-
face of the anode, as shown in Figs. 1 and 6b. In addition, since both
kinds of aluminum oxychlorides Al(OH)2Cl and Al(OH)Cl2 were
found to be present during dissolution of Al in an aqueous chlo-
ride solution [51,52], it is reasonable to infer that the hydrolysis of
+
AlCl2 occurs by the following equation.
AlCl2+ + H2O ↔ Al(OH)Cl2 + H+
(7)
The overall reactions during the dealloying can be summarized
as follows:
Cathode : 2H2O + 2e− → 2OH− + H2↑
Anode : Al → Al3+ + 3e−(electrochemically)
Al3+ + H2O + Cl− ↔ [Al(OH)Cl]+ + H+
[Al(OH)Cl]+ + H2O ↔ Al(OH)2Cl + H+
2Al + 6H+ + 4Cl− → 3H2↑ + 2AlCl2+(chemically)
AlCl2+ + H2O ↔ Al(OH)Cl2 + H+
(8)
(9)
Fig. 7. Double logarithmic plot of chronoamperometric curves for the dealloying of
the melt-spun Al–33.4 Au alloy in the 10 wt.% NaCl aqueous solution at different
applied potentials and room temperature (293 K).
(10a)
(10b)
(11)
3.4. The response of Al to potential and resultant effects on gold
diffusion
(12)
Electrolyte : Al3+ + 3OH− → Al(OH)3↓
[Al(OH)mCln]3−m−n + (3 − m)OH− → Al(OH)3↓ + nCl−
(13a)
(13b)
The porosity formation during dealloying above the critical
potential has been acknowledged to be kinetically controlled by
both the diffusion of the more noble component and the dissolution
of the less noble component. Meanwhile, the slope of the double
logarithmic plot of the chronoamperometric response is reported
to reflect the rate-limiting step [55]. Therefore, the current den-
sity evolution during dealloying was investigated and illustrated in
Fig. 7, in which the current density (logarithmic) vs. time behaviors
are displayed for the Al–33.4 Au alloy subjected to the potentials
at 0.8, 1.5, and 2.0 V. Apparently, the dealloying duration dropped
along with the substantial increase of the current density as the
potential was raised, indicating faster kinetics of Al dissolution at
higher potentials. For the potential of 2.0 V, the chronoamperomet-
riccurveshowsaslope(designatedask)closeto0beforethesudden
drop around 200 s, implying an active corrosion [55]. For the poten-
tial of 1.5 V, the curve initially has a slope of −0.5 and then shows a
slope close to 0. This transition is beyond our explanation available
at present. The slope of −0.5 normally denotes a surface diffusion
controlled process [20]. It is interesting to note that the curve cor-
responding to the potential of 0.8 V shows a slope of −0.5 at almost
the whole dealloying stage, implying a porosity formation process
continuously under the control of surface diffusion. In addition, the
high current density at the last stage of the curve for the potential
of 2.0 V is attributed to the ionization of gold atoms with Cl− to
form soluble complexes, as highlighted by an arrow in Fig. 7. And
the sudden drop of the current density appears in all three curves is
believed to be an indication of exhaustion of most Al and the onset
of the post-dealloying coarsening process.
During the electrochemical dealloying of the Al–Au alloys in the
10 wt.% NaCl solution, Al was oxidized into aluminum ion (Al3+
)
instead of passivation. Moreover, a self-acidifying effect was trig-
gered due to its instant hydrolysis assisted by chloride ions, which
accelerated the dealloying process of the Al–Au alloys. Owing to the
depletion of Al from the Al–Au alloys, the remaining Au atoms dif-
fused along the alloy/electrolyte interface and rearranged to form
f.c.c Au islands (ligaments), leading to the formation of the resulting
NPG.
To testify the role of chloride ions during dealloying, the melt-
spun Al–30 Au alloy ribbon was dealloyed in the 21 wt.% (NH4)2SO4
aqueous solution under potential control. No observable bubbles
emerged from the anode and the dealloying current was close to
zero even at the potential of 2.0 V. At this potential, however, the
Al–30 Au alloy ribbon was completely dealloyed in the 10 wt.%
NaCl aqueous solution within 600 s. In addition, no visible bub-
bles appeared on the anode until the end of dealloying, when the
Al–30 Au alloy ribbon was subjected to the 2.5 wt.% NaOH aqueous
solution at the potential of 1.5 V.
In addition to the leading role of chloride ions, the assistant
effect of Au should not be neglected. Because the overpotential
of hydrogen adsorption on gold is less than half of the value on
Al, the reduction of hydrogen ion (H+) is catalyzed by Au and the
depletion of Al is accelerated. On the other hand, the adsorptive
hydrogen atoms (Had) are preferentially attached to the Au surface
before combining into H2 and escaping as bubbles. This so-called
weakly adsorbed hydrogen has been reported to reduce the mobil-
ity of Au adatoms. (Although the change of the applied potential
from 0.8 to 1.5 V tends to enhance the surface diffusion of Au
adatoms, the calculated diffusivity of Au along the alloy/electrolyte
interface slowly increases due to the adsorption of more hydrogen
atoms on Au.) At the higher potential of 2.0 V, however, the surface
coverage of hydrogen atoms decreases while a so-called strongly
adsorbed hydrogen forms, enhancing the mobility of Au adatoms
by an adsorption-induced outward relaxation [54]. It should also be
noted that the influence of adsorbed hydrogen atoms on the mobil-
ityofAuwillbeweakenedatthehigherpotentialduetothestronger
driving force for the precipitation of hydrogen bubbles. Therefore,
the surface diffusivity of Au at the potential of 2.0 V is ∼40 times
greater than that at 1.5 V, as presented in Table 2.
High potentials can effectively shorten the dealloying duration,
but will inevitably accelerate the polarization extent of the elec-
trode. For example, the electron transfer from Al adatoms to the
3+
power (step I) is very fast and a great amount of Alad is always
3+
ready at the potential of 1.5 V. Alad migrates into the electrolyte
to form Al3+ (step II) and instantaneous hydrolysis (step III) follows
(see Eqs. (9) and (10)). Because of the inefficient diffusion inside
the channels and the limiting solubility of the hydrolysis products,
the rate of step II will lag behind that of step I and the accumula-
tion of Alad3+ on the anode can be expected. The Alad3+ ions in turn
attract chloride ions and polarized water molecules, enhancing the
diffusivity of gold adatoms. Obviously, the higher the potential is or
the lower solubility of the species which are in balance with Al3+ in
the electrolyte, the more charge will accumulate on the electrode.

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Q. Zhang et al. / Electrochimica Acta 54 (2009) 6190–6198
Considering this, it is reasonable to yield a moderately high diffu-
sivity of gold adatoms at 1.5 V in NaOH solutions, not only becaus−e
of the adsorption of OH− but owing to the lower solubility of AlO2
in alkali than Al3+ in acid.
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4. Conclusions
In summary, a facile, economical and benign route has been
developed to fabricate NPG ribbons through electrochemical deal-
loying of melt-spun Al–Au alloys with 20–50 at.% Au in a 10 wt.%
NaCl aqueous solution under potential control at room tempera-
ture. The microstructures of the NPG ribbons strongly depend upon
the phase constitutions of the starting Al–Au alloys. The Al–33.4 Au
and Al–50 Au alloys are composed of single-phase Al2Au and AlAu
intermetallic compound, respectively. Both Al2Au and AlAu can be
fully dealloyed, resulting in the formation of NPG with a homo-
geneous porous structure. The dealloying of the Al–20 Au alloy
comprising ␣-Al and Al2Au leads to the formation of NPG with
bimodal channel size distributions. In addition, the dealloying of
Al2Au and AlAu in the two-phase Al–45 Au alloy proceeds sepa-
rately, resulting in the formation of NPGCs. The applied potential
has no significant influence on the ligament-channel structure of
NPG by dealloying of the Al–33.4 Au alloy. In comparison to the low
potentials of 0.8 and 1.5 V, the ligament size markedly increases
and many micro-cracks appear in the NPG at the potential of 2.0 V.
Surface diffusion of Au adatoms along the alloy/electrolyte inter-
face plays an important role in the formation of ligaments/channels
in NPG during dealloying. The surface diffusivity of Au increases
with increasing applied potential and can be interpreted in terms
of interaction with adsorptive species. The dealloying mechanism
in the neutral NaCl solution can be explained based upon pour-
baix diagram and chloride ion effect. During the electrochemical
dealloying of the Al–Au alloys in the 10 wt.% NaCl solution, a self-
acidifying effect is triggered due to the dissolution and instant
hydrolysis of Al3+/Al, which is assisted by chloride ions in the elec-
trolyte. The self-acidifying effect accelerates the dealloying process
of the Al–Au alloys. In addition, the dissolution kinetics of Al at dif-
ferent potentials has some effects on the dealloying process of the
Al–Au alloys and the formation of NPG.
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