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Electrochimica Acta 63 (2012) 302– 311

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

j ourna l ho me pag e: www.elsev ier .com/ locate /e lec tac ta

ormation and microstructure of nanoporous silver by dealloying rapidlyolidified Zn–Ag alloys

hi Zhang, Junzhe Sun, Junling Xu, Xiaoguang Wang, Hong Ji, Changchun Zhao, Zhonghua Zhang ∗

ey Laboratory for Liquid–Solid Structural Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering,handong University, Jingshi Road 17923, Jinan 250061, PR China

r t i c l e i n f o

rticle history:eceived 22 December 2011ccepted 25 December 2011vailable online 31 December 2011

a b s t r a c t

The electrochemical dealloying of rapidly solidified (RS) Zn–Ag alloys in a 5 wt.% HCl solution and 1 M NaClsolution has been investigated using electrochemical measurements and microstructural analysis. Theresults show that the electrochemical properties of the RS Zn–Ag alloys are associated not only with thealloy composition, but with the phase constitution of the alloys. Nanoporous silver (NPS) ribbons can be

eywords:anoporous silverealloyingn–Ag alloysotentiodynamic polarization

fabricated through potentiostatically dealloying the RS Zn–Ag alloys in both solutions. Moreover, a minordifference in the applied potential has a significant influence on the dealloying process and formationof NPS. The critical potential and polarization behavior of the Zn–Ag alloys have also been discussed, aswell as surface diffusion of Ag adatoms.

© 2011 Elsevier Ltd. All rights reserved.

otentiostatic polarization

. Introduction

Nanoporous metals have recently attracted great attention inide applications including catalysis, sensors, actuators, fuel cells,

nd so forth [1–4]. Chemical or electrochemical dealloying, whichs referred to as selective dissolution of one or more componentsut of an alloy, has become an efficient way to produce nanoporousetals. Compared with chemical dealloying, electrochemical deal-

oying has more advantages, such as low operating temperature,hort dealloying duration, and easy modulation of nanoporoustructures through potential control [5].

Numerous examples of dealloying have been reported. Manyystems such as Ag–Au [6–8], Cu–Au [9], Al–Au [10] and Cu–Ni [11]ave been developed to produce corresponding nanoporous met-ls. Here, we pay attention to Zn-based alloys. Among Zn-basedlloys, brass is the most famous because numerous investigationsn dezincification had been conducted since the phenomenonas discovered in 1866. Several dezincification mechanisms have

een put forward, including preferential dissolution of zinc [12,13],issolution and re-precipitation of copper [14–18], bi-vacancyechanism [19] and percolation mechanism [20,21]. Trace ele-ents like As and B have a pronounced effect on dezincification

f brass [22,23]. In the last decade, some attention has beenaid to dealloying of Zn-based alloys. Zn-based systems includ-

ng Zn–Ag [5], Zn–Pt [24], Zn–Cu [19,25,26], and Zn–Au [27] have

∗ Corresponding author. Tel.: +86 531 88396978; fax: +86 531 88396978.E-mail address: zh zhang@sdu.edu.cn (Z. Zhang).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.12.103

been investigated and corresponding nanoporous metals have beensynthesized by dealloying. In addition, Pourbaix (potential-pH)diagram for the Zn–H2O system [28] and equilibrium diagram ofcorrosion products of Zn in solutions containing chloride [29] canbe used to study the dealloying mechanism of Zn-based alloys.

Nanoporous silver (NPS) is an important kind of nanoporousmetals due to its potential applications in molecular adsorption,ion exchange, medical materials, and heterogeneous catalysis [30].Jia et al. [31] and Yeh et al. [5] have fabricated NPS through electro-chemical alloying (electrodeposition) and subsequent dealloyingof Zn–Ag alloys, but their methods have some drawbacks. First,the composition of zinc during electrodeposition cannot be pre-cisely controlled. Second, the depth that can be alloyed is limitedto several micrometers in the electrochemical alloying/dealloyingprocess [32]. In comparison, the metallurgical method has moreadvantages as the composition, size and shape of precursor alloyscan be easily controlled. Recently, Li et al. [30] have fabricatedporous silver through dealloying Zn–Ag alloys (fabricated by themetallurgical method) in dilute H2SO4 solutions.

The choice of solutions is important in dealloying processes.Neutral solutions have more advantages than acid or alkali solu-tions because of their milder properties. Our group has put forwarda benign route to fabricate nanoporous gold (NPG) through elec-trochemical dealloying of Al–Au alloys in a NaCl solution [33]. Anda neutral AgNO3 solution was employed by Snyder et al. [34] to

obtain NPG film via electrochemical dealloying. It is essential tointroduce economical and ecological solutions to industrialize thefabrication of nanoporous metals by dealloying [33]. In this work,we chose a 5 wt.% HCl solution as the dealloying electrolyte and

imica Acta 63 (2012) 302– 311 303

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hen took a 1 M NaCl solution for comparison, aiming to investigatehe electrochemical dealloying of RS Zn–Ag alloys and formationf NPS. The electrochemical properties of the Zn–Ag alloys haveeen discussed based upon open-circuit measurements, potentio-ynamic and potentiostatic polarization analysis. In addition, theurface diffusivity of Ag adatoms during dealloying has also beenvaluated.

. Experimental

The starting Zn–Ag alloys with nominal compositions ofn80Ag20, Zn70Ag30, Zn60Ag40 and Zn50Ag50 (at.%) were preparedrom pure elements (purity: 99.9 wt.%) in quartz tubes using aigh-frequency vacuum induction furnace. Using a single rollerelt spinning apparatus, the pre-alloyed ingots were remelted by

igh-frequency induction heating in a quartz tube and then melt-pun onto a copper roller with a diameter of 0.35 m at a speedf 1000 revolutions per minute (rpm) in a controlled argon atmo-phere. The melt-spun ribbons obtained were typically 20–50 �mn thickness, 2–5 mm in width and several centimeters in length.n X-ray diffractometer (XRD, Rigaku D/max-rB) with Cu K� radi-tion was used to analyze the phase constitution of the RS Zn–Aglloys.

Electrochemical experiments including open-circuit measure-ents, potentiodynamic polarization and potentiostatic polariza-

ion were performed in a standard three-electrode cell using anK2005A potentiostat. The electrochemical cell consisted of a 100-l beaker, a platinum foil as the counter electrode with a surface

rea of about 5 mm × 5 mm, and a saturated calomel electrodeSCE) as the reference electrode. The Zn–Ag ribbons were directlysed as the work electrode. All measurements were performednder an open ambient condition at room temperature. All poten-ials quoted were on the SCE scale. The electrolytes were 5 wt.%Cl solution and 1 M NaCl solution, which were prepared fromnalytical-grade reagents and nanopure water. In order to ensure

reasonable repeatability, both open-circuit and potentiodynamicolarization measurements were repeated at least three times. Thecanning rate for the potentiodynamic polarization was 5 mV s−1.otentiostatic dealloying was performed at different applied poten-ials to study the dealloying mechanism and fabricate NPS samples.

icrostructural characterization of the as-dealloyed samples wasade by a scanning electron microscope (SEM, LEO 1530 VP) with

n energy dispersive X-ray (EDX) analyzer.

. Results

.1. Electrochemical activities of Zn–Ag alloys

Fig. 1 shows the XRD patterns of the RS Zn–Ag alloys. Ashown in Fig. 1a, the RS Zn80Ag20 and Zn70Ag30 alloys are single-hase and composed of a hexagonal �-AgZn3 (PCPDF#25-1325)hase. The RS Zn60Ag40 alloy is also single-phase and a cubic-Ag5Zn8 (PCPDF#65-1794) phase can be detected (Fig. 1b). Inhe RS Zn50Ag50 alloy, there are two AgZn phases with differenttructures: cubic �-AgZn (PCPDF#29-1155) and hexagonal �-AgZnPCPDF#29-1156). According to the Zn–Ag phase diagram, therehould be only the � phase in the Zn50Ag50 alloy at equilibrium. Butue to rapid solidification, the � phase remains in the RS alloy. Forhe other three alloys, the phases in the RS alloys well correspondo the phase diagram.

Fig. 2 shows typical open-circuit potential (Eocp) vs. time curves

f the RS Zn–Ag alloys in the 5 wt.% HCl and 1 M NaCl solutions.s shown in Fig. 2a, the potential gradually increases with timeecause slight self-corrosion of these Zn–Ag alloys is inevitable

n the 5 wt.% HCl solution. But in the 1 M NaCl solution, the

Fig. 1. XRD patterns of the RS Zn–Ag alloys. (a) Zn80Ag20 and Zn70Ag30, (b) Zn60Ag40

and Zn50Ag50.

open-circuit potentials rapidly reach a steady state after about100 s (Fig. 2b). The average values of potentials during 200–300 swere calculated and listed in Table 1. It can be seen that the Eocp

of the same Zn–Ag alloy in the 5 wt.% HCl solution is comparableto that in the 1 M NaCl solution. In the 5 wt.% HCl solution, theopen-circuit potentials of the Zn80Ag20 and Zn70Ag30 alloys arealmost equal, so are of the Eocp values of the Zn60Ag40 and Zn50Ag50alloys. But in the 1 M NaCl solution, the Eocp of the Zn80Ag20 alloyis apparently lower than that of the other three alloys.

Fig. 3 shows the potentiodynamic polarization curves of theRS Zn–Ag alloys. Here we chose the potential corresponding to acurrent density of 1.0 mA cm−2 as the critical potential (Ecrit) andthe results are given in Table 1. In order to verify that our measure-ments do relate to the actual critical potential, we obtained a seriesof chronoamperometric curves of the Zn80Ag20 and Zn50Ag50 alloysin the 1 M NaCl solution (Fig. 4). As shown in Fig. 4a, the curveskeep level with time at potentials below −0.07 V vs. SCE. When thepotential increases to −0.07 V vs. SCE, an upslope appears on thecurve after 400 s. When the potential is −0.02 V vs. SCE, the currentbegins to rapidly rise after 50 s. Therefore, the actual criticalpotential is between −0.07 and −0.02 V vs. SCE. Analogously, theEcrit of the Zn50Ag50 alloy should be between −0.02 and 0.01 V vs.SCE (Fig. 4b). As shown in Table 1, the Ecrit values of the Zn80Ag20and Zn50Ag50 alloys are −0.07 and 0.01 V vs. SCE respectively,close to those determined by the potential holding experiments.

In addition, the Ecrit increases with increasing Ag content in theZn–Ag alloys for the same electrolyte, but the Ecrit of the Zn80Ag20alloy is more negative than that of the other three alloys (Table 1).

304 C. Zhang et al. / Electrochimica Acta 63 (2012) 302– 311

Table 1The electrochemical activities of the RS Zn–Ag alloys in the 5 wt.% HCl solution and in the 1 M NaCl solution.

Solutions Alloys Open-circuit potential (Eocp, V vs. SCE) Critical potential (Ecrit , V vs. SCE)

5 wt.% HCl Zn80Ag20 −0.40 −0.12Zn70Ag30 −0.39 −0.04Zn60Ag40 −0.33 −0.04Zn50Ag50 −0.32 −0.02

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urthermore, the Ecrit values are comparable for the Zn70Ag30,n60Ag40 and Zn50Ag50 alloys.

As shown in Fig. 3a, every curve is almost level in the cathodeegion, which is different from a general Tafel curve that shows ainear relationship between logi and potential in its cathode andnode regions. Two peaks can be observed on the potentiodynamicolarization curve of the Zn70Ag30 alloy (Fig. 3a). One is located at0.36 V vs. SCE and the other is at −0.08 V vs. SCE. Between the

wo peaks, a level region develops. For the polarization curve ofn70Ag30 in the 1 M NaCl solution, the second peak also occurs at aifferent potential (Fig. 3b). For the Zn80Ag20 alloy, the polarizationurve develops a level region in both the HCl and NaCl solutionsFig. 3). For the Zn60Ag40 and Zn50Ag50 alloys, no passivation regionan be observed in their anode regions of the polarization curvesFig. 3a), and the current density quickly rises when the potential

s swept over the tip of the polarization curves. Taken together,he superposition of four polarization curves arises at about 0.0 Vs. SCE (Fig. 3a). Almost the same phenomenon is observed in the

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ig. 2. The open-circuit potential vs. time curves of the RS Zn–Ag alloys obtained inhe (a) 5 wt.% HCl solution and (b) 1 M NaCl solution.

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polarization curves of the Zn–Ag alloys obtained in the 1 M NaClsolution, and four curves coincide at about 0.1 V vs. SCE (Fig. 3b).

3.2. Potentiostatic polarization of Zn–Ag alloys

Figs. 5 and 6 show the current response of the RS Zn–Ag alloyspotentioststically dealloyed in the 5 wt.% HCl solution and 1 M NaClsolution, respectively. The curves were plotted on the double log-arithmic coordinates with characteristic slopes, and the amplifiedfraction of each curve is shown as an inset. From the insets we cansee that each curve presents the characteristic slopes of −0.25, −0.5and −1, which represent different stages of the dealloying process.It should be mentioned that if the dealloying potential is so posi-tive that dissolution and/or oxidation of Ag may occur, the porous

structure will be destroyed. In order to ensure that dealloying canproceed in a relatively shorter time avoiding the oxidation of Ag,we carried out several potential holding measurements (Fig. 5a).

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Fig. 3. The potentiodynamic polarization curves of the RS Zn–Ag alloys obtained inthe (a) 5 wt.% HCl solution and (b) 1 M NaCl solution.

C. Zhang et al. / Electrochimica

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ig. 4. The chronoamperometric curves of the RS Zn–Ag alloys dealloyed at a seriesf potentials in the 1 M NaCl solution. (a) Zn80Ag20 and (b) Zn50Ag50.

t −0.05 V vs. SCE, the current density was the lowest and it tookearly 10,000 s to finish the dealloying process of Zn70Ag30. At0.03 V vs. SCE, a higher current density occurred and less time was

equired. Compared with the precursor ribbon (Fig. 5b, the left one),he as-dealloyed sample at −0.03 V vs. SCE shows bright metallicuster of silver (Fig. 5b, the middle one). At −0.01 V vs. SCE, the alloyurface quickly became black (Fig. 5b, the right one) due to the oxi-ation of silver. And at −0.01 V vs. SCE, the chronoamperometricurve with long duration of steady-state is different from the othersFig. 5a). The steady-state current density of ∼50 mA cm−2 is quiteigher. Despite the higher current density, it took even longer timehan that at −0.03 V vs. SCE, suggesting the abnormal dealloying ofhe alloy at −0.01 V vs. SCE. Therefore, the dealloying process of then–Ag alloys is quite sensitive to the minor change of the appliedotential. We chose −0.03 V vs. SCE as the dealloying potential forhe Zn70Ag30 alloy in the 5 wt.% HCl solution. For comparison, thether three alloys were also dealloyed at −0.03 V vs. SCE in the HClolution.

Fig. 5c shows the chronoamperometric curve of the Zn80Ag20lloy. The current density increases with time without an appar-nt steady-state. As expected, the curve shows a slope of −0.25uring the time span of 920–1170 s. With the proceeding of deal-

oying, a slope of −0.5 appears during 1200–1320 s. After that, thelope of −1 can be observed, suggesting that the dealloying poten-

ial is below the critical potential of the partially dealloyed sample.inally, the current density sharply falls due to the complete deple-ion of Zn. The Zn70Ag30 alloy shows a comparable curve with then80Ag20 alloy except for the lower current density (Fig. 5d). For

Acta 63 (2012) 302– 311 305

the Zn60Ag40 alloy (Fig. 5e), the current density shows a slight risein the first 200 s. And then the curve exhibits a long-period steady-state, which means that the rate of creation of active sites is equalto the removal of active dissolution sites according to the argumentof Yeh et al. [5]. For the Zn50Ag50 alloy (Fig. 5f), the current densityshows a fall-and-rise phenomenon in the initial 250 s, and then itreaches the steady state. After the steady state, the slopes of −0.25,−0.5 and −1 appear in succession, indicating the formation of aporous structure.

Fig. 6 shows the chronoamperometric curves of the Zn–Ag alloyspotentiostatically dealloyed in the 1 M NaCl solution. Fig. 6a showsthe chronoamperometric curves of the Zn80Ag20 alloy dealloyed at−0.03 and 0.10 V vs. SCE. For these two potentials, the curves showsimilar variation trend and the current density at 0.10 V vs. SCEis slightly higher than that at −0.03 V vs. SCE after 1000 s (Fig. 6a).Moreover, the profile of the curves is similar to that of dealloying inthe 5 wt.% HCl solution. Although the dealloying potential is 130 mVhigher than that in the 5 wt.% HCl solution, the maximum currentdensity is much lower. Despite the minor potential difference of50 mV, the curves of the Zn70Ag30 alloy present quite different fea-tures, including dealloying duration, variation trend and currentdensity (Fig. 6b). At −0.05 V vs. SCE, it takes much more time to fin-ish dealloying, and the maximum current density is about a thirdof that at 0.00 V vs. SCE. For the Zn60Ag40 and Zn50Ag50 alloys (inFig. 6c and d), long duration of steady-state occurs after the ini-tial rising stage. Compared with the dealloying in the 5 wt.% HClsolution, the dealloying of these two alloys takes much longer timein the 1 M NaCl solution at similar applied potentials (−0.03 and0.00 V vs. SCE, respectively).

3.3. Microstructures of as-dealloyed samples

Fig. 7 shows the section-view microstructures of NPS fabri-cated by potentiostatically dealloying the Zn–Ag alloys in the 5 wt.%HCl solution. The dealloying conditions are given in Table 2. Thesection-view microstructure of the as-dealloyed Zn80Ag20 alloyshows a typical bicontinuous nanoporous structure with a liga-ment/channel size of 282 ± 57 nm (Fig. 7a). The inset of Fig. 7aindicates that the microstructure is uniform across the wholesection of the NPS ribbon. Similar nanoporous structures can beobserved in the NPS ribbons dealloyed from the RS Zn70Ag30,Zn60Ag40 and Zn50Ag50 alloys (Fig. 7b–d). The ligament/channelsizes of these NPS samples are different and given in Table 2. Inaddition, the EDX results demonstrate that Zn cannot be detectedin all NPS samples, indicating that all Zn was leached away dur-ing the dealloying of the Zn–Ag alloys in the 5 wt.% HCl solution. Atypical EDX spectrum is shown in Fig. 7e.

Fig. 8 shows the section-view SEM images of NPS fabricated bypotentiostatically dealloying the Zn80Ag20 alloy in the 1 M NaClsolution, and the dealloying conditions are given in Table 2. For theNPS sample obtained at −0.03 V vs. SCE, the nanoporous structureruns throughout the whole ribbon (Fig. 8a and inset). The averageligament size is 187 ± 42 nm. For the NPS sample obtained at 0.10 Vvs. SCE, a similar nanoporous structure with an average ligamentsize of 225 ± 53 nm can be observed (Fig. 8b and inset). In addi-tion, the residual Zn can be detected in the NPS samples by EDX.The residual Zn in the NPS sample obtained at −0.03 V vs. SCE is ashigh as 15.3 at.% (Fig. 8c), much higher than that (2.9 at.%, Fig. 8d)obtained at 0.10 V vs. SCE.

Fig. 9 shows the section-view SEM images of NPS fabricated bypotentiostatically dealloying the Zn70Ag30 alloy at different poten-tials in the 1 M NaCl solution. The microstructure of the NPS sample

obtained at −0.05 V vs. SCE shows a typical bicontinuous inter-penetrating ligament-channel structure with the ligament size of246 ± 50 nm (Fig. 9a). The SEM image shows a uniform porousstructure with a boundary-like characteristic (inset of Fig. 9a).

306 C. Zhang et al. / Electrochimica Acta 63 (2012) 302– 311

Fig. 5. The chronoamperometric curves of the RS Zn–Ag alloys in the 5 wt.% HCl solution. (a and d) Zn70Ag30, (c) Zn80Ag20, (e) Zn60Ag40 and (f) Zn50Ag50. The crucial slopesare denoted by lines and symbolized by k. The insets in (c–f) show corresponding amplified curves. (b) Macrograph showing un-dealloyed and dealloyed ribbons (left:un-dealloyed, middle: dealloyed at −0.03 V vs. SCE, right: dealloyed at −0.01 V vs. SCE).

Table 2Applied potential, dealloying duration, ligament/channel sizes and surface diffusivities of Ag adatoms during the electrochemical dealloying of the RS Zn–Ag alloys.

Solutions Alloys Potential (V vs. SCE) Dealloying duration (s) Ligament/channel size (nm) Surface diffusivities (Ds , cm2 s−1)

5 wt.% HCl Zn80Ag20 −0.03 3915 282 ± 57 5.7 × 10−11

Zn70Ag30 −0.03 3588 320 ± 56 9.9 × 10−11

Zn60Ag40 −0.03 9271 280 ± 56 2.3 × 10−11

Zn50Ag50 −0.03 9750 232 ± 50 1.0 × 10−11

1 M NaCl Zn80Ag20 −0.03 13,011 187 ± 42 3.3 × 10−12

0.10 13,247 225 ± 53 6.9 × 10−12

Zn70Ag30 −0.05 17,892 246 ± 50 7.2 × 10−12

0.00 5424 213 ± 40 1.4 × 10−11

Zn60Ag40 0.00 16,534 262 ± 56 1.0 × 10−11

Zn50Ag50 0.00 13,167 191 ± 46 3.5 × 10−12

C. Zhang et al. / Electrochimica Acta 63 (2012) 302– 311 307

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ig. 6. The chronoamperometric curves of the RS Zn–Ag alloys in the 1 M NaCl soenoted by lines and symbolized by k. The insets in (a–d) show corresponding amp

t is reasonable to assume that these ‘grains’ and ‘grain bound-ries’ inherit from the precursor alloy. The microstructure of NPSbtained at 0.00 V vs. SCE also exhibits a nanoporous structureFig. 9b and inset), but the ligaments are incontinuous and theirizes are not uniform. The average ligament size is 213 ± 40 nm.

Fig. 10 shows the section-view SEM images of NPS fabricatedy potentiostatically dealloying the Zn60Ag40 and Zn50Ag50 alloys

n the 1 M NaCl solution, and the dealloying conditions are givenn Table 2. It is obvious that a typical bicontinuous nanoporoustructure can be observed in these NPS samples. The ligament sizesre different: 262 ± 56 and 191 ± 46 nm for NPS obtained from then60Ag40 and Zn50Ag50 alloys, respectively.

. Discussion

.1. Critical potential and polarization behavior of Zn–Ag alloys

So far there exist ambiguities in defining the critical poten-ial for the onset of bulk dealloying. It is generally accepted thathe Ecrit is determined by locating the onset of the rapidly risingurrent through extrapolation to the baseline current of the pas-ivation region [35]. Sieradzki et al. [36] proposed a method toetermine Ecrit with the potential corresponding to a current den-ity of 1.0 mA cm−2. More recently, Dursun et al. [37] proposed ateady-state method for determining the so-called “true Ecrit” bypplying a series of potential holding experiments. They have foundhat dealloying will occur at potentials as much as 150 mV belowhe potential corresponding to a current density of 1.0 mA cm−2.ere we chose the potential corresponding to a current density of

.0 mA cm−2 as the Ecrit. Moreover, the potential holding experi-ents (Fig. 4) confirm that this method is reasonable to determine

he Ecrit of the Zn–Ag alloys. Indeed, bulk dealloying occurred in theaCl solution even below the Ecrit for the Zn70Ag30 alloy (Table 2

. (a) Zn80Ag20, (b) Zn70Ag30, (c) Zn60Ag40 and (d) Zn50Ag50. The crucial slopes arecurves.

and Fig. 6b, at −0.05 V vs. SCE, 40 mV below the Ecrit). Therefore,more accurate methods should be developed to determine the Ecrit.In addition, the Ecrit value should be sensitive to the scanning rate,and Sieradzki et al. [36] have argued that the Ecrit decreases withincreasing scanning rate.

For the Zn70Ag30 alloy, two peaks are observed on the poten-tiodynamic polarization curves. Similar polarization phenomenonhas been observed by other researchers. Deakin et al. [38] havestudied the polarization curve of 316SS in the 50% NaOH solutionat 140 ◦C and thought that the two peaks may be correlated withactive-passive transitions of Fe [Cr] and Ni. Zberg et al. [39] havefound three zero crossings (peaks) in the potentiodynamic polar-ization curve of Mg60Zn35Ca5, which were generated by the kineticsof corrosion-product formation on the sample surface. The forma-tion of the two-peak polarization behavior of Zn70Ag30 needs to befurther clarified.

Similar to the level regions of the Zn80Ag20 and Zn70Ag30 alloys(Fig. 3), many level regions have been reported in the literatureand are normally taken as passivation regions. Li et al. [30] haveobserved passivation regions in the potentiodynamic polarizationcurves of Zn–Ag alloys in the 0.1 M H2SO4 solution. Pugh et al. [40]have studied the electrochemical characteristics of Cu–Pt alloys inthe 1 M H2SO4 solution, and have argued that passivation regionsdeveloped after the initial dissolution of Cu from the alloy surface.Li et al. [41] have observed the passivation region in the poten-tiodynamic polarization curves of nickel-based superalloys in the1 wt.% (NH4)2SO4 and 1 wt.% citric acid solutions. Gilroy and Con-way [42] have thought that the passivation effect accounts for theinhibition of solute ion discharge by the presence of a passivating

“oxide” produced in a quasi-equilibrium process involving a one-electron change. Baugh [43] has thought that oxide film formationmight occur during the polarization because the double layer capac-ity fell significantly around the corrosion potential of Zn in 1 M NaCl

308 C. Zhang et al. / Electrochimica Acta 63 (2012) 302– 311

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ig. 7. Section-view SEM images showing the microstructures of NPS through potn50Ag50) alloys in the 5 wt.% HCl solution. Insets in (a–d) showing the entire section-f NPS.

queous solution. For the Zn80Ag20 and Zn70Ag30 alloys, the occur-ence of passivation region may be associated with the formationf Ag-rich layer on the alloy surface through surface diffusion of Agollowing the dissolution of Zn.

It should be noticed that the polarization curves of the Zn70Ag30nd Zn60Ag40 alloys are coincident above −0.1 V vs. SCE (Fig. 3a).nd this drives us to believe that the atomic configuration of theuter surface of these two alloys could be similar after the poten-ial was swept over −0.1 V vs. SCE. So a schematic diagram haseen established to interpret the polarization behavior the Zn–Aglloys (Fig. 11). The surface configuration (atomic ratio of Zn:Ag is:3) of the Zn70Ag30 alloy is schematically shown in Fig. 11a. Dur-

ng the potential scan of up to −0.3 V vs. SCE, partial Zn atoms aretripped off the outer surface into the electrolyte (Fig. 11a), and

he atomic ratio of Zn:Ag transforms into 3:2 in the surface layerFig. 11b). It means that the surface composition of the Zn70Ag30lloy is similar to that of the Zn60Ag40 alloy, and thus the passiva-ion region occurs in the polarization curve of the Zn70Ag30 alloy.

tatically dealloying the RS Zn–Ag ((a) Zn80Ag20, (b) Zn70Ag30, (c) Zn60Ag40 and (d)EM images of the NPS ribbons. (e) A typical EDX spectrum showing the composition

When the potential is swept over −0.1 V vs. SCE, the anodic currentdensity sharply increases for both the Zn70Ag30 and Zn60Ag40 alloys(Fig. 3a), and bulk dissolution of Zn in the surface of the alloys takesplace (Fig. 11b). Finally, a nanoporous structure evolves (Fig. 11c).The passivation region of the Zn80Ag20 alloy can also be explainedby this model. Taking into account the complexity of potentiody-namic polarization, the passivation region observed here may alsobe caused by the aggregation of oxidation product of Zn [43].

As shown in Table 1 and Figs. 1–3, the alloy composition,phase constitution and solution have a significant influence on theelectrochemical activities (Eocp and Ecrit) of the RS Zn–Ag alloys.Normally, the Ecrit increases with increasing content of the morenoble element and is proportional to the alloy composition for asolid solution system. It is obvious that both the Eocp and Ecrit of the

present Zn–Ag alloys are not proportional to the alloy composition,but are influenced by the phase constitution. In fact, the presentresults are well consistent with our former work [44,45], where wehave found that for Al-based and Mg–Cu alloys, the open-circuit

C. Zhang et al. / Electrochimica Acta 63 (2012) 302– 311 309

Fig. 8. Section-view SEM images showing the microstructures of NPS through potentiostatically dealloying the RS Zn80Ag20 alloy at (a) −0.03 and (b) 0.10 V vs. SCE in the 1 MNaCl solution. Insets in (a and b) are the entire section-view SEM images of the NPS ribbons. (c and d) EDX spectra of NPS obtained at −0.03 and 0.10 V vs. SCE, respectively.

Fig. 9. Section-view SEM images showing the microstructures of NPS through potentiostatically dealloying the RS Zn70Ag30 alloy at (a) −0.05 and (b) 0.00 V vs. SCE in the1 M NaCl solution. Insets in (a and b) are the entire section-view SEM images of the NPS ribbons.

Fig. 10. Section-view SEM images showing the microstructures of NPS through potentiostatically dealloying the RS (a) Zn60Ag40 and (b) Zn50Ag50 alloys at 0.00 V vs. SCE inthe 1 M NaCl solution.

310 C. Zhang et al. / Electrochimica Acta 63 (2012) 302– 311

F sivatios bove −

pcpsces

osipplTdt

4

oca

D

w�p((lotFdc[

4

v

ig. 11. Schematic illustrations showing the formation and breakdown of the pasolution. (a) Potentiodynamic scan up to −0.3 V vs. SCE. (b) Potentiodynamic scan a

otential and critical potential are associated not only with theontent of the more noble element, but also with the constitutivehases of the alloys. In addition, the Zn60Ag40 and Zn50Ag50 alloyshow similar electrochemical features despite their different phaseonstitution and alloy composition (Fig. 1b), suggesting that thelectrochemical activities of �-Ag5Zn8, �-AgZn and �-AgZn areimilar.

From the electrochemical behavior (Fig. 6a) and microstructuralbservation (Fig. 8), we can see that the applied potential has noignificant influence on the dealloying process of the Zn80Ag20 alloyn the 1 M NaCl solution and the formation of NPS. Despite the samehase constitution, the minor difference (50 mV) of the appliedotentials has a significant influence on the current response (deal-

oying process) of Zn70Ag30 and formation of NPS (Figs. 6b and 9).herefore, it is of great importance to select the applied potentialuring potentiostatic dealloying of the Zn–Ag alloys, consideringhe influence of alloy composition.

.2. Surface diffusivity of Ag adatoms

Surface diffusivity (Ds) is an important parameter in fabricationf nanoporous metals. Based upon the surface diffusion controlledoarsening mechanism, the Ds values of Ag adatoms along thelloy/solution interface can be evaluated by the equation [46]

s = [d(t)]4kT

32�ta4(1)

here d(t) is the ligament size at the dealloying time t, is the surface energy (1.302 J m−2 [47]), a is the latticearameter of Ag (4.09 × 10−10 m), k is the Boltzmann constant1.3806 × 10−23 J K−1), and T is the absolute dealloying temperature298 K for electrochemical dealloying). The calculated Ds values areisted in Table 2. The surface diffusivities of Ag adatoms are on therder of 10−11 to 10−12 cm2 s−1, varying with the compositions ofhe precursor alloys, dealloying solutions and applied potentials.or the same alloy, the higher applied potential leads to greater Ds

uring the dealloying process. In addition, the present Ds data areomparable to the value (1.9 × 10−12 cm2 s−1) reported by Li et al.48].

.3. Electrochemical dealloying mechanism of Zn–Ag alloys

At the initial dealloying stage of the Zn80Ag20 alloy at −0.03 Vs. SCE, the rise of the current density may be related to the

n region in the potentiodynamic polarization curve of Zn70Ag30 in the 5 wt.% HCl0.1 V vs. SCE. (c) The illustration shows the evolution of a nanoporous structure.

formation and development of pitting corrosion [44]. Wagner et al.[49] have pointed out that the slope of −0.25 is indicative of theevaporation–condensation mechanism whereby the more nobleelement is oxidized to soluble cations/anions. Here, the slope of−0.25 is probably correlated with the dissolution of Ag to formsoluble AgCl2− [44]. Then the slope of −0.5 is consistent with theformation of AgCl layer on the alloy surface [50]. Furthermore,the slope of −0.5 has been considered to be associated withthe fact that surface diffusion is the rate-limiting step and theporous structure will evolve [51]. The surface diffusion leads to therearrangement and self-organization of Ag atoms, resulting in theformation of nanoporous structure. Wagner et al. [49] have alsoargued that the current decay with the slope of −1 corresponds tothe exhaustion of activation-controlled dissolution process in thedealloying of Ag0.8Au0.2 and Cu0.8Au0.2 alloys at potentials belowthe critical potential. We think that the slope of −1 is related to adissolution process along with the coarsening of ligaments [44].

During the potentiostatic dealloying of the RS Zn–Ag alloys, alarge amount of hydrogen (H2) bubbles emerged from the surfaceof both the counter electrode and working electrode. This phe-nomenon lasted until the end of dealloying. When the dealloyingprocess takes place, Zn atoms can dissolve from the Zn–Ag ribboninto the electrolyte like pure Zn by the following reaction at theapplied potential.

Zn → Zn2+ + 2e− (2)

And on the counter electrode, the reaction will vary with thechange of the electrolyte. In the 5 wt.% HCl solution, the electrodereaction is

2H+ + 2e− → H2↑ (3)

But in the 1 M NaCl solution, the reaction will be complex as

2H2O + 2e− → 2OH− + H2↑ (4)

As the reaction goes on, more OH− will be generated and enterinto the electrolyte to form insoluble Zn(OH)2. Eventually, whiteflocculent precipitates appeared in the NaCl solution with the deal-loying proceeding. Feitknecht [29] has reported that the corrosionproduct of Zn is ZnCl2·nZn(OH)2 in the 0.5 M NaCl aqueous solution.According to the Cl− concentration and pH value of the 1 M NaCl

solution [29], the precipitates in our case should be ZnCl2·4Zn(OH)2formed through the following reaction.

4Zn(OH)2 + Zn2+ + 2Cl− � ZnCl2·4Zn(OH)2(s) (5)

imica

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[[[[[[[48] Z.Q. Li, D.Y. Wang, B.Q. Li, X. Lu, J. Electrochem. Soc. 157 (2010) K223.[49] K. Wagner, S.R. Brankovic, N. Dimitrov, K. Sieradzki, J. Electrochem. Soc. 144

C. Zhang et al. / Electroch

Because of the enrichment of Zn2+ ions within the dissolutionone caused by reaction (2), the reaction (5) is forced to proceed tohe positive direction to form ZnCl2·4Zn(OH)2. At a relatively lowotential, these precipitates may aggregate on the alloy surface torevent the alloy from dealloying, so it may be the reason for theormation of passivation region in the potentiodynamic polariza-ion curves (Fig. 3). But for the potentiostatic dealloying, the appliedotential was well above the break potential (Eb, to dissolve theassivation layer). For all Zn–Ag alloys, the dissolution of Zn willontinuously proceed in the 1 M NaCl solution following reaction2). According to the literature [29], the ZnCl2·4Zn(OH)2 precip-tates will not be produced in the 5 wt.% HCl solution. With theepletion of Zn, the remaining Ag atoms will reassemble to formPS through surface diffusion.

. Conclusions

1) The open-circuit potentials and critical potentials of the RSZn–Ag alloys are associated not only with the alloy composi-tion, but with the phase constitution of the alloys in both the5 wt.% HCl and 1 M NaCl solutions.

2) The electrochemical properties (open-circuit potential and pas-sivation behavior) of the Zn80Ag20 and Zn70Ag30 alloys arecomparable due to their same phase constitution (single �-AgZn3 phase). The Zn60Ag40 and Zn50Ag50 alloys show similarelectrochemical features despite their different phase constitu-tion and alloy composition, indicating that the electrochemicalactivities of �-Ag5Zn8, �-AgZn and �-AgZn are similar.

3) NPS ribbons with a good mechanical integrity can be fab-ricated through electrochemical dealloying of the RS Zn–Agalloys in both the 5 wt.% HCl and 1 M NaCl solutions. AllNPS ribbons exhibit an open, bi-continuous interpenetratingligament-channel structure.

4) The minor difference in the applied potential has a signifi-cant influence on the current response (dealloying process) ofZn70Ag30 and the formation of NPS in the 1 M NaCl solution, buthas no obvious effect on the dealloying process of Zn80Ag20 andthe formation of NPS.

cknowledgments

The authors gratefully acknowledge financial support by theational Natural Science Foundation of China (50971079), Program

or New Century Excellent Talents in University, and Independentnnovation Foundation of Shandong University (2010JQ015).

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