Potential and Concentration Dependent Electrochemical Dealloying of

Article pubs.acs.org/JPCC

Potential and Concentration Dependent Electrochemical Dealloying of Al2Au in Sodium Chloride Solutions Junling Xu,†,‡ Yan Wang,*,‡ and Zhonghua Zhang*,† †

Key Laboratory for Liquid−Solid Structural Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jingshi Road 17923, Jinan 250061, P. R. China ‡ School of Materials Science and Engineering, University of Jinan, Jiwei Road 106, Jinan 250022, P. R. China ABSTRACT: Electrochemical dealloying of a single-phase Al2Au alloy in sodium chloride solutions has been investigated considering influence of potential and electrolyte concentration. Both opencircuit potentials and corrosion potentials of Al2Au decrease with increasing electrolyte concentration and temperature. The overpotential and electrolyte concentration have a significant influence on dealloying behaviors of Al2Au, such as steady-state current density and dealloying duration. Nanoporous gold (NPG) can be fabricated by potentiostatic dealloying of Al2Au in the NaCl solutions. Moreover, surface diffusion evaluation demonstrates that there exist good linear relationships between the logarithm of surface diffusivities of Au adatoms (Auad) and overpotential, and between the surface diffusivities of Auad and electrolyte concentration. In addition, the activation energy decreases with increasing overpotential or chloride ion concentration.

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INTRODUCTION Indians of pre-Columbian Central America and medieval artisans in the Near East and Europe developed a simple surface modification method (known as depletion gilding or dealloying) to decorate artifacts of alloys.1,2 In the past decade, dealloying has received great attention because some alloy systems exhibit nanoporosity evolution during dealloying, and it can be used to fabricate nanoporous metals with random porous structures. The dealloying mechanism has been discussed in the literature. Forty and Durkin1,3 constructed a terrace-ledge-kink model to explain selective dissolution of the less noble component and surface diffusion of the more noble element during dealloying of binary Ag−Au alloys. Erlebacher et al.4 have succeeded in unveiling nanoporosity evolution during dealloying by a kinetic Monte Carlo model. Among nanoporous metals, nanoporous gold (NPG) is of special interest due to its unique mechanical, chemical, and physical properties. Wittstock et al.5 have found that NPG can effectively catalyze the selective oxidative coupling of methanol to methyl formate with selectivities above 97% and high turnover frequencies at temperatures below 80 °C. Biener et al.6 have achieved surface-chemistry-driven actuation on NPG by converting chemical energy directly into a mechanical response. It is known that properties and performances of NPG are closely correlated with its nanoporous structures (homogeneous, bimodal, or hierarchical) and sizes of ligaments/ channels. For example, Qian et al.7 have reported that the strongest surface-enhanced Raman scattering enhancement of NPG takes place from the samples with an ultrafine nanopore size of ∼5−10 nm. Thus, it is of great importance to control the dealloying kinetics and formation of nanoporous structures. Typically, ultrafine NPG can be obtained by dealloying Ag−Au or Al−Au alloys in strong acid or alkali solutions at low temperatures.8,9 © 2012 American Chemical Society

Besides, the kind of electrolytes plays a key role in the dealloying process and many efforts have been devoted to the selection of electrolytes, ranging from acid to alkali solutions,10 from inorganic to organic media,11 even ionic solutions.12 Snyder et al.13 have used a neutral AgNO3 solution to obtain NPG film via electrochemical dealloying of Ag−Au. Actually, anion species significantly influence the microstructure of NPG, such as size and shape of Au islands.14 Halide ions can induce pitting corrosion on alloy surface, except the case of fluorion,15,16 and promote surface diffusion of more noble atoms during dealloying.17 Recently, Renner et al.18 have confirmed that chloride ions can accelerate Au island formation and dealloying kinetics. To date, however, less attention has been paid to influence of halide ion concentrations on dealloying behaviors and formation of nanoporous metals. During dealloying, another key factor to control dealloying kinetics is the applied potential. The definition of the critical potential has been extensively discussed in the literature.19−23 For dealloying systems, the critical potential marks the transition from a passivated alloy surface to the sustained formation of a bicontinuous porous structure.22 Sieradzki et al.23 have determined the critical potential corresponding to the knee associated with the current/potential curve of an alloy. Dursun et al.22 have defined the critical potential through a steady-state method based upon the analysis of current vs time curves under different applied potentials. Xu et al.24 have fabricated NPG with ligament sizes less than 6 nm through electrochemical dealloying of Ag−Au leaves under potential Received: November 1, 2011 Revised: February 5, 2012 Published: February 9, 2012 5689

dx.doi.org/10.1021/jp210488t | J. Phys. Chem. C 2012, 116, 5689−5699

The Journal of Physical Chemistry C

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control. Dona and González-Velasco25 have analyzed the surface diffusion mechanism of gold adatoms according to the influence of applied potential and temperature on roughness factor of gold electrodes. However, few reports are available on the effect of the applied potential on the dealloying behavior and nanoporosity evolution. In our former work, we have studied electrochemical and chemical dealloying mechanisms of Al−Au alloys in a NaCl aqueous solution26 and further electrochemical dealloying of Al−Au, Ag, Pd, and Cu alloys in the NaCl solution.27 Especially, the influence of halide ions (F−, Cl−, Br−, and I−) on electrochemical dealloying of Al2Au has been considered.28 In this article, we aim to investigate the effect of the applied potential and electrolyte concentration on electrochemical dealloying of a single-phase Al2Au alloy in NaCl solutions. Because of the thermal-activation nature of dealloying,29 the influence of temperature has also been considered. In addition, surface diffusivities of gold adatoms (Auad) during dealloying have been evaluated as well as the activation energy and entropy.

determined manually by identifying a minimum of 50 ligaments/ channels, making measurements across the shortest distance of each ligament/channel and then averaging.

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RESULTS Electrochemical Properties of Al2Au in NaCl Solutions. Figure 1 shows the open-circuit potential vs time curves

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EXPERIMENTAL SECTION The single-phase Al2Au precursor alloy with nominal composition of Al66.6Au33.4 (at %) was prepared from elemental Al (purity, 99.95 wt %) and Au (purity, 99.9 wt %) in a highfrequency induction furnace. Using a single roller melt spinning apparatus, the prealloyed ingots were remelted and rapidly solidified into ribbons of 20−50 μm in thickness. The meltspun Al66.6Au33.4 alloy was composed of a single Al2Au intermetallic compound, which was verified by X-ray diffraction (XRD, Rigaku D/max-rB). All electrochemical measurements were performed in a conventional three-electrode potentiostat (LK 2005A) with the Al2Au ribbon as the working electrode, a platinum foil as the counter electrode, and an Ag/AgCl electrode as the reference electrode. The NaCl aqueous solutions were prepared from analytical-grade reagents and nanopure water. All potentials quoted were on the Ag/AgCl scale unless otherwise stated. Electrochemical activity measurements including open-circuit potential (Eocp) and potentiodynamic polarization were performed in the NaCl solutions with concentrations of 0.1, 1.0, and 5.0 M at 273, 298, and 343 K. Before each run of the tests, the electrodes were stabilized for 5−10 min in the solution, and thus, the curves of open-circuit potential vs time were recorded. Potentiodynamic polarization curves were measured by sweeping the potential from a potential below the Eocp to an appropriate anodic potential at a scan rate of 5 mV·s−1. Each measurement was repeated 3−6 times to ensure a reasonable repeatability and a typical curve was shown in the Results section. Potentiostatic dealloying was carried out in the 1.0 M NaCl solution at different applied potentials with overpotentials (here, the overpotential (ξ) is defined as the margin of the applied potential over the corrosion potential (namely, critical potential)) of −50, 0, 50, 100, and 200 mV at 273, 298, and 343 K. In addition, the Al2Au samples were also dealloyed at constant potentials with the overpotential of 50 mV in the 0.1, 1.0, and 5.0 M NaCl solutions at 273, 298, and 343 K. After dealloying, the samples were rinsed with distilled water and anhydrous alcohol. The microstructures of the asobtained NPG samples were characterized using a scanning electron microscope (SEM, LEO 1530 VP) and an energy dispersive X-ray (EDX) analyzer, which was attached to the SEM. In addition, the average ligament/channel sizes (d(t)), defined by the equivalent diameters of ligaments/channels in NPG, were

Figure 1. Open-circuit potential vs time curves of the single-phase Al2Au alloy in the (a) 0.1 M, (b) 1.0 M, and (c) 5.0 M NaCl solutions at different temperatures.

for the Al2Au alloy in the NaCl solutions, and the corresponding Eocp values are listed in Table 1. At the given solution 5690



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Table 1. Open-Circuit Potentials (Eocp) and Corrosion Potentials (Ecor) Obtained by Open-Circuit Measurements and Potentiodynamic Methods for the Single-Phase Al2Au Alloy in the NaCl Solutions with Different Concentrations at Different Temperatures temperature (K) items

concentration (M)

273

298

343

Eocp vs Ag/AgCl

0.1 1.0 5.0 0.1 1.0 5.0

−0.19 −0.24 −0.29 −0.06 −0.14 −0.13

−0.20 −0.26 −0.31 −0.07 −0.19 −0.20

−0.21 −0.28 −0.32 −0.15 −0.20 −0.25

Ecor vs Ag/AgCl

concentration, the Eocp value slightly decreases with increasing temperature (Figure 1 and Table 1). Although the open-circuit potential vs time curves run steadily with time during measurement, some regular fluctuations appear on the curves at 343 K, as marked by arrows in Figure 1. Böhni and Uhlig30 have reported that the critical pitting potential of pure Al is not greatly sensitive to temperature (273−313 K). Meanwhile, Eocp is easily influenced by trace impurities in the electrolyte.30 As shown in Table 1, the electrolyte concentration has a significant influence on the Eocp values at the given temperature. Eocp markedly decreases with increasing electrolyte concentration. According to the Pourbaix (potential-pH) diagram,31 Al will be passivated in neutral solutions at potentials above the Al3+/Al equilibrium potential, but Al is subjected to pitting corrosion with presence of chloride ion,32,33 which may prevent Al from being passivated. In our former work, a self-acidifying effect has been proposed to explain the pitting corrosion of Al−Au alloys in the NaCl solution.26 Figure 2 shows the potentiodynamic polarization (tafel) curves of the Al2Au alloy in the NaCl solutions. For simplicity, here we take the potential corresponding to the tip of the tafel curve (intersection point of anodic and cathodic branches) as the corrosion potential (Ecor), as marked by an arrow in Figure 2a. Moreover, for the single-phase Al2Au alloy, the corrosion potential is considered as the critical potential.27 All the Ecor values are also given in Table 1. At the given electrolyte concentration, Ecor markedly decreases with increasing temperature. On the whole, Ecor decreases with increasing electrolyte concentration at the given temperature, indicating that the increase of chloride ion concentration can promote the dealloying tendency of Al2Au. Böhni and Uhlig30 have found that pure Al becomes more active with increasing Cl − concentration and that the critical potentials are linear with the logarithm of Cl− activity. Baumgärtner and Kaesche16 have proved that the pitting potential of Al continuously decreases with increasing Cl− concentration in the NaCl solution. It should be noted that the Ecor values for the 1.0 and 5.0 M NaCl solutions are comparable, especially at 273 and 298 K. This means that the change of the electrolyte concentration has no obvious effect on Ecor when it reaches a threshold value. Additionally, it is noteworthy that no passivation appears even in the 0.1 M NaCl solution, and NPG can be fabricated by potentiostatic dealloying (see the following). Dealloying Behaviors of Al2Au at Different Overpotentials, Temperatures, and Electrolyte Concentrations. Potentiostatic polarization measurements are crucial to elucidate the dealloying process and relevant mechanisms based upon the literature.22,27,34,35 Figure 3 displays the current density

Figure 2. Potentiodynamic polarization (tafel) curves of the singlephase Al2Au alloy in the (a) 0.1 M, (b) 1.0 M, and (c) 5.0 M NaCl solution at different temperatures.

vs time curves (chronoamperometric curves in double logarithmic plot) of the Al2Au alloy potentiostatically dealloyed in the 1.0 M NaCl solution at different temperatures and overpotentials. The dealloying durations are given in Table 2. The dealloying process can be divided into three stages: initial rising stage, steady stage (the steady-state current density is listed in Table 2), and decay stage. Taking Figure 3a as an 5691



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example, when the overpotential is −50 mV (50 mV below the corrosion potential), the dealloying process is slow, and the current density is lowest among these five curves. However, an appreciable current density of ∼7 mA cm−2 can be observed at the steady stage, indicating that the Al2Au alloy can be dealloyed even at a subcritical potential and at such a low temperature of 273 K. Furthermore, the dealloying process is smooth and steady compared to the other four processes and the dealloying time is longest. At the overpotential of 0 mV, the current vs time curve shows tremendous discrepancy compared to that at the overpotential of −50 mV. The steady-state current density sharply increases to more than 30 mA cm−2, and the dealloying time reduces obviously at this overpotential (Table 2). At the overpotential of 50 mV, the current response is similar to that at the 0 mV overpotential. With increasing overpotential to 100 and 200 mV, the dealloying behaviors are similar but quite different from that at the 50 mV overpotential. The steady-state current density increases to over 70 mA cm−2, and the dealloying time evidently shortens in comparison to the 50 mV overpotential. It should be noted that there exists a similar current response despite the discrepancy of 100 mV between the applied potentials with the 100 and 200 mV overpotentials. In addition, the time reaching the steady stage obviously reduces with increasing overpotential, suggesting the enhanced dealloying kinetics (faster dissolution of Al and surface diffusion of Au). With increasing overpotential, similar scenarios can be observed in the current density vs time curves of the Al2Au alloy dealloyed at 298 and 343 K (Figure 3b,c). At the given overpotential, however, the dealloying time markedly shortens with increasing temperature (Table 2). Some troughs appear on the chronoamperometric curves as highlighted by arrows in Figure 3b,c, which may be caused by transient passivation.36 Moreover, the higher the dealloying temperature, the more fluctuations occur at the later stage of potentiostatic dealloying owing to faster diffusion of Au adatoms at 343 K (Figure 3c). Figure 3d shows the variation of steady-state current densities with overpotential for the Al2Au alloy potentiostatically dealloyed at different temperatures. It is clear that the steadystate current density exhibits a nonlinear variation with overpotential at the given temperature. On the whole, the steadystate current density increases with increasing overpotential. The current density is lowest at the overpotential of −50 mV, but the current densities are comparable for the overpotentials of 0 and 50 mV. In addition, the current density below the 50 mV overpotential is significantly less than that above the 100 mV overpotential. This means that there exists a critical overpotential (ξc), which is located between 50 and 100 mV, as marked by a dashed line in Figure 3d. There is obvious difference in the dealloying behaviors such as the steady-state current density, reaction kinetics, and dealloying duration for the overpotential below or above ξc. At the overpotential below ξc, the pitting corrosion induced by chloride ion dominates the dealloying process, generating lower current density, corrosion rate, and longer dealloying duration. In comparison, at the overpotential above ξc, bulk corrosion prevails in the dealloying process, associated with higher current density and faster reaction kinetics. However, the intrinsic origin of the critical overpotential needs to be further clarified. To further explore the influence of chloride ion concentration on the dealloying process, potentiostatic dealloying was performed in the NaCl solutions with different concentrations at the given overpotential of 50 mV. The dealloying conditions

Figure 3. Chronoamperometric (current density vs time) curves for the potentiostatic dealloying of the single-phase Al2Au alloy in the 1.0 M NaCl solution at different overpotentials and temperatures of (a) 273 K, (b) 298 K, and (c) 343 K. (d) The variation of steady-state current density with overpotentials. 5692



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Table 2. Ligament Sizes of NPG by Dealloying the Single-Phase Al2Au Alloy in the 1.0 M NaCl Solution at Different Temperatures and Overpotentials for Distinct Durations and Corresponding Surface Diffusivities of Au Adatoms; Corresponding Steady-State Current Density Values Are Also Presented temperature, T (K)

overpotential, ξ (mV)

dealloying time, t (s)

steady-state current density (mA cm−2)

273

−50 0 50 100 200 −50 0 50 100 200 −50 0 50 100 200

7332 3592 4822 1952 1659 6561 3834 3374 1310 736 4568 3803 1650 1007 546

7.1 38.6 31.6 83.7 83.7 7.1 16.0 27.1 68.6 68.5 26.4 43.0 39.6 112.9 212.8

298

343

Table 3. Ligament Sizes of NPG by Dealloying the SinglePhase Al2Au Alloy in the NaCl Solutions with Different Concentrations at the Overpotential of 50 mV and Different Temperatures for Distinct Durations, and Corresponding Surface Diffusivities of Au Adatoms concentration, C (M)

dealloying time, t (s)

273

0.1 1.0 5.0 0.1 1.0 5.0 0.1 1.0 5.0

8219 4822 1625 6573 3374 730 4128 1650 508

298

343

ligament size, d(t) (nm) 24.7 39.1 43.1 28.0 41.1 41.2 39.0 44.0 42.9

± ± ± ± ± ± ± ± ±

3.2 9.0 8.7 4.2 8.3 5.7 9.8 8.1 5.6

surface diffusivity, Ds (m2 s−1) 1.9 2.1 9.0 4.4 3.9 1.8 3.0 1.2 3.6

× × × × × × × × ×

29.9 32.8 39.1 41.3 49.4 37.8 40.7 41.1 41.2 44.8 45.9 49.4 44.0 44.7 47.3

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

4.2 5.4 9.0 6.2 6.1 10.1 7.0 8.3 6.7 7.1 5.4 6.1 8.1 10.3 5.2

surface diffusivity, Ds (m2 s−1) 4.7 1.4 2.1 6.4 1.5 1.4 3.3 3.9 1.0 2.5 5.2 8.4 1.2 2.1 4.9

× × × × × × × × × × × × × × ×

10−19 10−18 10−18 10−18 10−17 10−18 10−18 10−18 10−17 10−17 10−18 10−18 10−17 10−17 10−17

Figure 5 shows the microstructure of NPG obtained by potentiostatic dealloying of Al2Au in the 1.0 M NaCl solution at 273 K and the overpotential of −50, 0, 50, 100, and 200 mV. Figure 6 shows the microstructure of NPG obtained by potentiostatic dealloying of Al2Au in the 0.1 and 5.0 M NaCl solutions at the overpotential of 50 mV and different temperatures of 273, 298, and 343 K. All the NPG samples exhibit an open, three-dimensional, bicontinuous interpenetrating ligament-channel structure. Moreover, the ligament/ channel sizes (d(t)) for all the NPG samples were determined and are listed in Tables 2 and 3. In addition, EDX measurements have been performed on all the as-obtained NPG samples, and a typical EDX spectrum is shown in the inset of Figure 6a. It is obvious that most of Al was removed during dealloying and only several atom percent Al residual can be detected in the resultant NPG.

are presented in Table 3, and the corresponding current density vs time curves are shown in Figure 4. It should be pointed out

temperature, T (K)

ligament size, d(t) (nm)

10−19 10−18 10−18 10−19 10−18 10−17 10−18 10−17 10−17

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DISCUSSION Electrochemical Dealloying Mechanism of Al2Au in the NaCl Solutions. The electrochemical dealloying of Al2Au in the NaCl solutions is associated with the Cl− assisted selfacidifying effect due to the dissolution and instant hydrolysis of Al.26 Some ions are involved in the dealloying process, such as Al3+, Cl−, H+, OH−, AlCl2+, and [Al(OH)mCln]3−m−n.26 Once the nanoporous structure forms, mass transport (diffusion of ions) will be inhibited by the narrow channels and thus affect the dealloying process.10,37 At the initial stage (within 10 s), Al dissolution (to form Al3+) and diffusion transport (in the electrolyte) try to establish one equilibrium, which is accompanied by the rise of current density on the chronoamperometric curves (Figures 3 and 4). Subsequently, the current− time curves show a steady stage, indicating the establishment of the equilibrium between Al dissolution and ion transport. It is interesting that there exist obvious oscillations on the current− time curves at the steady stage for the overpotential of 100 and 200 mV (Figure 3). Compared to the lower overpotential (−50, 0, and 50 mV), the higher overpotential (100 and 200 mV) can accelerate the Al dissolution, but the mass transport is mainly affected by temperature. Therefore, the equilibrium will be destroyed and rebuilt, leading to the occurrence of oscillatory behavior. Similar oscillatory behavior during dealloying has also

that Figure 4b is compiled from the data in Figure 3. For the given solution concentration, the temperature has a minor influence on the steady-state current density. However, the span of the current density reaching the steady stage and the total dealloying duration will sharply reduce with increasing temperature (Table 3 and Figure 4). In addition, the solution concentration has a significant effect on the dealloying process of Al2Au. On the one hand, the steady-state current density markedly increases with increasing chloride ion concentration, and its values are on the order of magnitude of ∼10, ∼ 30, and ∼50 mA cm−2 for the concentrations of 0.1, 1.0, and 5.0 M respectively. On the other hand, the dealloying time visibly decreases with increasing chloride ion concentration at the given temperature. Microstructure of NPG Obtained by Potentiostatic Dealloying. All the as-dealloyed samples have been observed and some typical SEM images are shown in Figures 5 and 6 (the dealloying conditions are given in Tables 2 and 3). The results demonstrate that all the as-dealloyed samples are nanoporous. In other words, the Al2Au alloy can be fully dealloyed under different potentiostatic conditions, and NPG can be fabricated. 5693



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below the critical potential). Wagner et al.20 have reported that at a fixed potential below the critical potential, the timedependent evolution of surface morphology of Ag−Au was correlated with the observed current decay. They have argued that when dealloying proceeds at a potential below the critical potential, there is surface enrichment of the more noble components. The triggering effect of micropitting for crevice corrosion of Al in chloride solutions has been demonstrated even at potentials far below the critical pitting potential.38 In addition, Dursun et al.22,34 have argued that the commonly accepted measurement of the critical potential for alloy dissolution results in an overestimation of this quantity (∼115 mV for Ag0.7Au0.3 in 0.1 M HClO4). Therefore, the present dealloying at the overpotential of −50 mV may be associated with the overestimation of the critical potential. Qian et al.8 have prepared ultrafine NPG by a low-temperature dealloying technique and have reported that temperature can affect the kinetics of nanopore formation and control the size of nanopores. The present results demonstrate that the influence of temperature on the dealloying kinetics can be enhanced or weakened by the applied potential and electrolyte concentration (Tables 2 and 3). For example, at 273 K and the overpotential of 200 mV, the dealloying time is only 1659 s in the 1.0 M NaCl solution, much shorter than that (3803 s) at 343 K and the overpotential of 0 mV (Table 2). The dealloying time is 1625 s in the 5.0 M NaCl solution at 273 K and the overpotential of 50 mV, while the dealloying time is as long as 4128 s in the 0.1 M NaCl solution at 343 K. It is worth noting that the dealloying duration also has an important influence on the ligament/channel sizes, and the later stage of electrochemical dealloying is mainly associated with the coarsening of the nanoporous structure.27 Furthermore, the adsorption of Cl− increases the diffusion rate of adatoms during the coarsening process of NPG.39 Here, it can be seen that the ligament/channel sizes obviously increase with increasing chloride ion concentration in the electrolyte, especially at 273 K (Table 3). Although the dealloying time is as long as 8219 s, the ligament/channel size is only 24.7 ± 3.2 nm in the NPG obtained by dealloying in the 0.1 M NaCl solution at 273 K. In comparison, the ligament/channel size is 43.1 ± 8.7 nm in the NPG prepared by dealloying in the 5.0 M NaCl solution for a shorter time of 1625 s at the same temperature and overpotential. This is due to the enhanced surface diffusion of Auad induced by the increase of chloride ion concentration. At 298 and 343 K, the influence of chloride ion will be weakened as shown in Table 3. Through control over the applied potential, electrolyte concentration, temperature, and dealloying duration, we can manipulate the dealloying kinetics including the dissolution of Al and surface diffusion of Auad during the electrochemical dealloying of Al2Au and further the formation and ligament/ channel sizes of NPG. Moreover, the chloride ion concentration varies from 0.1 to 5 M over a wide range and shows a visible effect on the dealloying behavior of Al2Au. This work may trigger new research into electrochemical dealloying in aqueous salt solutions and NaCl is a promising candidate in choosing the electrolyte. Surface Diffusivity of Auad. It is known that the formation of nanoporous structure is a thermal-activation process29 and that nanoporosity evolution during dealloying is controlled by surface diffusion of the more noble element along the alloy/ solution interfaces. Thus, the evaluation of surface diffusivity

Figure 4. Chronoamperometric curves for the potentiostatic dealloying of the single-phase Al2Au alloy in the NaCl solution with different concentrations at different temperatures and the overpotential of 50 mV.

been reported by Dimitrov et al.10 At the last stage of dealloying, it should be noted that the decay of current density is mainly owing to the depletion of Al in Al2Au (not due to the influence of mass transport). NPG can be obtained at the −50 and 0 mV overpotentials (Figure 5a,b). In combination with the dealloying behavior in Figure 3a, this further confirms that the Al2Au alloy can be completely dealloyed even at the subcritical potential (50 mV 5694



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Figure 5. SEM images showing the microstructure of NPG by potentiostatically dealloying the single-phase Al2Au alloy in the 1.0 M NaCl solution at the temperature of 273 K and the overpotential of (a) −50, (b) 0, (c) 50, (d) 100, and (e) 200 mV.

According to the model of Sieradzki et al.,23 there is a built-in length scale, ξperc, which is associated with clusters comprising interconnected atoms of the less noble species. For a binary alloy, ξperc can be obtained through the equation ξperc = ann(1 + p)/(1 − p) where ann is the nearest-neighbor spacing and p is the mole fraction of the less noble element.23 The initial pore size (diameter), d0, can be approximately given by 2ξperc.17 For Al2Au, d0 is equal to ∼3 nm (r0 = ∼1.5 nm). The present ligament/channel sizes (d(t)) range from 24.7 ± 3.2 to 49.4 ± 6.1 nm (Tables 2 and 3). Because of the tremendous margin between [r(t)]4 and r04 (namely, [d(t)]4 and d04), the r04 term in eq 2 can be ignored. Thus, the following equation can be obtained:8,40

(Ds) of Auad will be beneficial to unveil the dealloying mechanism of Al2Au in the NaCl solutions. The analytical description of surface diffusion can be estimated by the following expression for the time dependence of the ligament/channel radius r(t):25,40

2γa4Ds d(r(t )4 ) = dt kT

(1)

where a represents the lattice parameter, γ is the surface energy, k is the Boltzmann constant (1.3806 × 10−23 J K−1), t is the dealloying duration, and T is the temperature. Integration of eq 1 between r0 and r for t = 0 and t = t leads to 1/4 ⎡ 2γa4Dst ⎤ 4 ⎥ r(t ) = ⎢r0 + ⎢⎣ kT ⎥⎦

Ds =

(2) 5695

[d(t )]4 kT 32γta4

(3)



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Figure 6. SEM images showing the microstructure of NPG by potentiostatically dealloying the single-phase Al2Au alloy in the 0.1 M NaCl solution at the overpotential of 50 mV and the temperature of (a) 273 K, (b) 298 K, and (c) 343 K, and in the 5.0 M NaCl solution at the potential of 50 mV and the temperature of (d) 273 K, (e) 298 K, and (f) 343 K. Inset in panel a is a typical EDX spectrum showing the composition of NPG.

log Ds and ξ. The fitting parameters are listed in Table 4 and all the correlation coefficients (κ) are greater than 98%. This linear relationship can be given as follows:

According to the parameters in Tables 2 and 3 (taking γ = 1 J m−217 and a = 4.08 × 10−10 m), the Ds values of Auad were calculated and are listed in Tables 2 and 3. The Ds values in Table 2 are obtained for the potentiostatic dealloying in the 1.0 M NaCl solution. It is clear that the surface diffusivity of Auad increases sharply with increasing overpotential from −50 to 200 mV at the given temperature. The diffusivity at the 200 mV overpotential is about 1 order of magnitude greater than that at the −50 mV overpotential. The present results are in agreement with the previous report by Dona and González-Velasco.25 They have found that the surface diffusivities of Auad increase with increasing applied potential when the gold electrode is in contact with a 0.5 M H2SO4 solution. To further probe the relationship between overpotential and surface diffusivity of Auad, the plot of log Ds vs ξ has been established and is shown in Figure 7a. It is surprising that there exists a good linear relationship between

log Ds = A + Bξ

(4)

where, A and B are fitting parameters. It can be seen that the parameter B decreases with increasing temperature. The present findings are important for control over surface diffusion of Auad and modulation of ligament/channel sizes of NPG through potential adjustment. Besides, the surface diffusivity of Auad increases with increasing temperature at the given overpotential. It has been extensively reported that chloride ions can effectively accelerate surface diffusion of more noble adatoms (Auad in this work) during chemical/electrochemical dealloying.9,14,39,41 However, no literature is available on the influence of chloride ion 5696



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Activation Energy and Activation Entropy of Dealloying. The Arrhenius equation for surface diffusion is given as follows,25 ⎛ E ⎞ Ds = D0exp⎜ − a ⎟ ⎝ RT ⎠

(6)

where D0 is the pre-exponential factor, R is the gas constant, and Ea is the activation energy for surface diffusion. Figure 8a,b

Figure 7. Plots of log Ds vs ξ (overpotential) and Ds vs C (concentration) showing the influence of (a) overpotential and (b) electrolyte concentration on surface diffusivities of Au adatoms during the potentiostatic dealloying of Al2Au under different conditions.

Table 4. Fitting Values of Parameters in Eqs 4 and 5 and Corresponding Correlation Coefficients (κ and κ′) temperature (K) parameters

273

298

343

A B κ α β κ′

−17.9338 0.00597 0.98343 1.53 × 10−19 1.78 × 10−18 0.99942

−17.5654 0.00494 0.98766 1.96 × 10−19 3.57 × 10−18 0.99991

−17.6880 0.00391 0.99882 3.76 × 10−18 6.51 × 10−18 0.99563

concentration on surface diffusivity of Auad. Here, we give the direct evidence for this point. As listed in Table 3, the Ds values sharply increase with increasing chloride ion concentration at the given temperature. The lowest Ds is 1.9 × 10−19 m2 s−1 for the dealloying in the 0.1 M NaCl solution at 273 K. In comparison, the maximum Ds is 3.6 × 10−17 m2 s−1 for the dealloying in the 5.0 M NaCl solution at 343 K. Moreover, it is interesting to note that there is a good linear relationship between the diffusivity and chloride ion concentration, as shown in Figure 7b. The fitting parameters are listed in Table 4, and the following equation can be obtained: Ds = α + βC

Figure 8. Plots of ln Ds vs 1/T for the estimation of the activation energy (Ea) and activation entropy (ΔS) for the dealloying of Al2Au (a) in the 1.0 M NaCl solution at different overpotentials and (b) in the NaCl solutions with different concentrations at the overpotential of 50 mV. (c) The variation of the activation energy with overpotentials.

(5)

where, α and β are fitting parameters, and C is the electrolyte concentration. 5697



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shows the plot of ln Ds versus 1/T for the 1.0 M NaCl solution at different overpotentials and for the NaCl solutions with different concentrations at the given overpotential of 50 mV, respectively. It is obvious that a good linear relationship can be obtained for all the experimental data. According to the slope (−Ea/R) and the intercept (ln D0) in Figure 8a, Ea and D0 for the potentiostatic dealloying in the 1.0 M NaCl solution at different overpotentials were calculated and are listed in Table 5.

Table 6. Activation Energy (Ea) and Activation Entropy (ΔS) for the Nanopore Formation during Dealloying of Al2Au in the NaCl Solutions with Different Concentrations at the Overpotential of 50 mV concentration, C (M) ln D0 (m2 s−1) Ea (kJ mol−1) ΔS (J mol−1 K−1)

Table 5. Activation Energy (Ea) and Activation Entropy (ΔS) for the Nanopore Formation during Dealloying of Al2Au in the 1.0 M NaCl Solution at Different Overpotentials 0

50

100

1.0

5.0

−29.7 19.8 ± 1.0 −108.9

−30.2 15.1 ± 1.8 −113.1

that the increase of chloride ion concentration will enhance the interactions between Auad and solvent molecules/ions of the supporting electrolyte, facilitate the formation of moving surface entities, accelerate the surface diffusion of Auad, and thus lower the activation energy for the dealloying process. At the same time, the interaction of adatoms with solvent molecules and active ions also has an effect on the activation entropy ΔS. The values of activation entropy ΔS were calculated according to the following equation:25

overpotential, ξ (mV) −50

0.1 −27.3 30.9 ± 4.0 −88.5

200

ln D0 (m2 s−1) −28.1 −30.0 −29.7 −31.4 −30.7 Ea (kJ mol−1) 26.7 ± 1.7 20.0 ± 1.8 19.8 ± 1.0 13.4 ± 0.3 12.9 ± 0.4 ΔS (J mol−1 K−1) −95.4 −110.8 −108.9 −122.9 −117.1

Under free corrosion conditions, the activation energy has been evaluated to be ∼63.4 kJ mol−1 for the dealloying of Ag65Au35 in a 70 wt % HNO3 solution8 and ∼60.1 kJ mol−1 for the dealloying of Al2Au in a 20 wt % NaOH solution.9 For the potentiostatic dealloying at different overpotentials, the present Ea values are in the range of 12.9−26.7 kJ mol−1, markedly smaller than those under free corrosion conditions. This suggests that the applied potential can significantly decrease the activation energy for surface diffusion during the electrochemical dealloying. The activation energy is composed of two parts, EaF and EaD, where the superscripts F and D refer to the activation energy for the formation of moving surface species and that for the surface diffusion process, respectively.25 On the one hand, when the gold surface atoms are involved in interactions with solvent molecules and ions of the supporting electrolyte, a relaxation of the surface layer takes place, which can cause the drastic decrease in Ea, especially EaF. On the other hand, EaF will decrease due to the consequence of accumulation of a positive excess charge on the electrode surface and to the fact that the interaction between partially emptied orbitals belonging to surface atoms and sp3 orbitals corresponding to water molecules results in a loss of metallic character of surface atoms. In addition, the activation energy linearly decreases with increasing overpotential as shown in Table 5 and Figure 8c. A similar linear relationship between Ea and applied potential has also been reported for surface diffusion of gold atoms in contact with the 0.5 M H2SO4 solution by Dona and GonzálezVelasco.25 Moreover, they have argued that the strength of the above interactions grows with increasing applied potential, which lowers EaF. Although both experiments were performed under potential control, it should be noted that the present Ea values are much less than those (50.7−57.0 kJ mol−1) reported by Dona and González-Velasco.25 Besides different electrodes, the kind of electrolytes plays a dominant role and the influence of anions should be taken into consideration. In our former work, we have found that Cl− can more effectively enhance surface diffusion of Auad in comparison to SO42−.42 Similarly, Ea and D0 for the potentiostatic dealloying in the NaCl solutions with different concentrations were also determined and are given in Table 6. It is clear that the activation energy visibly decreases with increasing chloride ion concentration in the electrolyte at the given overpotential. This means

D0 = ζ

⎡ ΔS ⎤ d 2 υ0 exp⎢ ⎣ R ⎥⎦ 4

(7)

where D0 is the diffusion constant or preexponential factor, ζ is the number of contiguous positions at which an adatom can jump, υ0 is vibrational frequency of the lattice, and d is the distance from the position of a superfacial atom to another contiguous. Taking the values υ0 = 1012 s−1, d = 2 × 10−10 m, and ζ = 6 for a face centered cubic metal, the activation entropy values can be obtained from D0 listed in Tables 5 and 6. The value of ΔS is positive (53.4 J mol−1 K−1) for a goldvacuum system but is negative (−30.9 to −10.1 J mol−1 K−1) for the gold-electrolyte (0.5 M H2SO4 solution) system in the potential range for double-layer formation.25 However, in the potential range for gold oxidation, the value of ΔS is as negative as −184.9 J mol−1 K−1. As listed in Tables 5 and 6, the ΔS values are on the order of magnitude of ∼−100 J mol−1 K−1 for the potentiostatic dealloying of Al2Au in the NaCl solutions. The negative values of ΔS are connected with an activated complex state, which should be more ordered than the original state of the electrode surface. The negative activation entropy is interpreted to be caused by the orientation of solvent molecules as a result of the accumulated charges on the atoms and further by the interweavement of molecules into a quasi-lattice by hydrogen bonds.25 On the whole, the absolute value of ΔS shows an increasing tendency with increasing overpotential (Table 5) in the given solution or with increasing chloride ion concentration at the given overpotential (Table 6), suggesting that both chloride ions and applied potential can enhance the ordered state on the electrode surface during the electrochemical dealloying.

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CONCLUSIONS (1) The open-circuit potential of the single-phase Al2Au alloy markedly decreases with increasing electrolyte concentration but is not greatly sensitive to the variation of temperature. As a whole, the corrosion potential of Al2Au decreases with increasing electrolyte concentration at the given temperature and also with increasing temperature at the given electrolyte concentration. (2) Both the overpotential and electrolyte concentration have a significant influence on the dealloying behaviors of

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Al2Au in the NaCl solutions, such as steady-state current density, the time reaching the steady stage and the total dealloying duration. The steady-state current density exhibits a nonlinear variation with overpotential at the given temperature. A critical overpotential (ξc) can be deduced, which is located between overpotentials of 50 and 100 mV. For the given electrolyte concentration, temperature has a minor influence on the steady-state current density during the potentiostatic dealloying, but the total dealloying duration will markedly reduce with increasing temperature. The steady-state current density markedly increases, and the dealloying time visibly decreases with increasing chloride ion concentration in the NaCl solutions. (3) NPG with a three-dimensional bicontinuous interpenetrating ligament-channel structure can be fabricated by potentiostatic dealloying of the single-phase Al2Au alloy in the NaCl solutions with different concentrations at distinct temperatures and overpotentials. NPG can even be prepared by dealloying at the applied potential below the critical potential (overpotential of −50 mV) or in the dilute NaCl solution with a concentration of 0.1 M. All the factors including the applied potential, electrolyte concentration, temperature, and dealloying duration have an important influence on the ligament/ channel sizes of NPG. The present findings are of great importance for the green fabrication of NPG by dealloying in neutral solutions. (4) The surface diffusivities of Auad increase with increasing temperature, electrolyte concentration, or overpotential. Furthermore, there exist two good linear relationships: one is between the logarithm of surface diffusivities of Auad and the overpotential; another is between the surface diffusivities of Auad and the electrolyte concentration. (5) Compared to the dealloying under free corrosion conditions, the applied potential can significantly decrease the activation energy for the surface diffusion during the electrochemical dealloying. The activation energy linearly decreases with increasing overpotential during potentiostatic dealloying of Al2Au in the NaCl solutions. In addition, the activation energy visibly decreases with increasing chloride ion concentration in the electrolyte at the given overpotential.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.W.); [email protected]. cn (Z.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The authors gratefully acknowledge financial support by the National Natural Science Foundation of China (50971079), Open Project of Shanghai Key Laboratory of Modern Metallurgy and Materials Processing (SELF-2011-02), Program for New Century Excellent Talents in University (Ministry of Education), Independent Innovation Foundation of Shandong University (2010JQ015) and National Basic Research Program of China (973, 2012CB932800). 5699


Potential and Concentration Dependent Electrochemical Dealloying of

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