Influence of Alloy Composition and Dealloying Solution on the

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Influence of Alloy Composition and Dealloying Solution on the Formation and Microstructure of Monolithic Nanoporous Silver through Chemical Dealloying of Al-Ag Alloys Xiaoguang Wang, Zhen Qi, Changchun Zhao, Weimin 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 73, Jinan 250061, P. R. China ReceiVed: March 19, 2009; ReVised Manuscript ReceiVed: June 1, 2009

We present a facile and effective route to fabricate monolithic nanoporous silver (NPS) ribbons through chemical dealloying of melt-spun Al-Ag alloys comprising R-Al(Ag) and Ag2Al under free corrosion conditions. The microstructure of the NPS ribbons was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray analysis (EDX). The experimental results show that alloy composition and dealloying solution have a significant influence on the dealloying process and the formation of NPS. The Al-Ag alloys with 15-40 atom % Ag can be fully dealloyed in the 5 wt % HCl solution, but a minor amount of undealloyed Ag2Al can be detected in the as-dealloyed ribbons from the Al-45 and 50 Ag alloys. The existence of R-Al(Ag) can supply penetration paths for the solution and promote the dealloying of Ag2Al in the two-phase Al-Ag alloys. Moreover, the synergetic dealloying of R-Al(Ag) and Ag2Al in the two-phase Al-Ag alloys and fast surface diffusion of Ag result in the formation of NPS with a homogeneous porous structure. The Al-Ag alloys with 15-50 atom % Ag cannot be fully dealloyed in the 20 wt % NaOH solution, leading to the formation of NPS/Ag2Al composites. In addition, the Al-60 Ag alloy containing a single Ag2Al phase does not react with the 5 wt % HCl or 20 wt % NaOH solution even at high temperatures (90 ( 5 °C). Introduction Nanoporous materials have attracted great interest for their wide applications in catalysis,1-3 gas sensors,4-6 heat exchangers,7 supercapacitors,8 and so on, because of their novel physical, chemical, and mechanical properties associated with their high surface-to-volume ratio and low densities. Nanoporous metal materials can be prepared by metal organic deposition and liquid crystal template technique,9,10 but they are generally technically difficult and time-consuming to be implemented. Recently, chemical and electrochemical dealloying is usually used to fabricate nanoporous metals by selective dissolution of less noble elements from an alloy because of its high productivity and controllability. Taking the dealloying of a binary AxB1-x alloy (here, A is a less noble element, and B is a more noble element) in the electrolyte as an example, A will be dissolved from the alloy while B remains. Following the dissolution process of A, the residual B will gradually agglomerate and form a nanoporous structure throughout the dimensions of the sample. Although different alloy systems such as Ni-Al,11 Cu-Au,12 Pt-Cu,13 and Pt-Si14 have been used to study the dealloying process and the formation of nanoporous metals, most attention has been paid to the prototypical Ag-Au system.15-18 In contrast, little information is available on the synthesis of monolithic nanoporous silver (NPS) through dealloying. To date, the research on the preparation of NPS has merely focused on the electrochemical alloying/dealloying * Corresponding author. Tel: +86-531-88396978. Fax: +86-53188395011. E-mail: [email protected].

process. Jia et al.19 and Yeh et al.20 independently succeeded in the fabrication of NPS films through general commercial zinc plating solution and zinc chloride-1-ethyl-3-methyl imidazolium chloride ionic liquid, respectively. Compared with the electrochemical alloying/dealloying method, the metallurgical method has some advantages as follows. First, the preparation of a precursor alloy by the electrochemical method requires one of the alloy constituents (more active metal in the precursor) can be precipitated from the electrolyte. However, the metals that can be precipitated from the electrolyte are confined in several groups. In contrast, the metals can be selected in a wider range for the fabrication of the precursor alloy by the metallurgical method. Second, the composition of the precursor alloy by the metallurgical method can be precisely controlled. Third, because of the limited atomic diffusion distance between the deposition metal and the base metal, the depth that can be alloyed is limited to several micrometers in the electrochemical alloying/dealloying method. For example, the thickness of the alloyed zone was only 4-5 µm in the literature reported by Yeh et al.20 In comparison, a precursor alloy with different shape and size can be easily obtained through the metallurgical method. Moreover, the metallurgical method has obvious feasibility in the preparation of ternary or multiple precursor alloys. Snyder et al.21 prepared a novel Pt-doped nanoporous gold (NPG) by dealloying ternary Ag-Au-Pt precursor and thought that the small fraction of Pt in the Ag-Au precursor alloys resulted in the formation of ultrafine NPG with a mean pore size of less than 4 nm and remarkable stability against coarsening. Yamauchi and his co-workers22,23 succeeded in the fabrication of Co-doped skeletal Co-Ag alloys and

10.1021/jp902490u CCC: $40.75  2009 American Chemical Society Published on Web 07/02/2009

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skeletal Co-Cu alloys by chemical leaching of Al-Co-Ag and Al-Co-Cu ternary alloys in the alkali solution. Compared to the skeleton-like silver24 or Raney silver,25 the mechanical integrity of NPS is important for its structural properties and some applications because uniformity and continuity are key requirements.26,27 Recently, we developed a new route to synthesize nanoporous metals through dealloying of Al-based alloys.28 Here, we show that monolithic NPS ribbons can be fabricated through chemical dealloying of melt-spun Al-Ag alloys in an acidic or alkaline solution under free corrosion conditions. The influence of alloy composition and dealloying solution on the formation and microstructure of NPS has also been investigated. Experimental Section Al-Ag alloys with nominal compositions of 15, 20, 25, 30, 35, 40, 45, 50, and 60 atom % Ag were prepared from pure Al (99.9 wt %) and pure Ag (99.9 wt %). Using a high-frequency induction heating apparatus, the charges were melted in a quartz crucible, and then the melt was cast into an iron chill mold. The prealloyed Al-Ag ingots were remelted by high-frequency induction heating in a quartz tube and then melt-spun onto a copper roller at a circumferential speed of ∼18 m · s-1. The ribbons obtained were typically 20-60 µm in thickness, 2-4 mm in width, and several centimeters in length. The melt-spun Al-Ag alloy ribbons were first dealloyed in a 5 wt % HCl aqueous solution at room temperature until no obvious bubbles emerged. And then, the dealloying was continuously carried out in the same solution at 90 ( 5 °C in order to further leach out the residual Al in the samples. The dealloying of the Al-Ag ribbons was also performed in a 20 wt % NaOH aqueous solution first at room temperature and then at 90 ( 5 °C. The dealloying time was typically 0.5-2 h, depending upon the alloy composition and the thickness of the melt-spun Al-Ag ribbons. In addition, the Al-60 Ag alloy was also dealloyed in a 37 wt % HCl solution at 90 ( 5 °C. The as-dealloyed samples were rinsed with distilled water and then with dehydrated alcohol. Finally, the monolithic NPS ribbons were obtained through rapid solidification and chemical dealloying. Microstructural characterization and analysis of the Al-Ag alloys and as-dealloyed samples were carried out using an X-ray diffractometer (XRD, Rigaku D/max-rB) with Cu KR radiation, a scanning electron microscope (SEM, LEO 1530 VP) with an energy dispersive X-ray analyzer (EDX), and a transmission electron microscope (TEM, Philips CM 20). In order to evaluate specific surface areas of the NPS samples, the N2 adsorption/ desorption experiments were carried out at 77 K on an ASAP2000 automatic surface area and pore radius distribution apparatus. Results and Discussion Phase Constitution of the Melt-spun Al-Ag Alloys. Figure 1 shows the XRD patterns of the melt-spun Al-Ag alloy ribbons with 15-60 atom % Ag. According to the XRD results, it is clear that, when the Ag content is less than 60 atom %, the melt-spun Al-Ag alloys consist of two distinct phases, namely R-Al(Ag) solid solution and Ag2Al intermetallic compound. As expected from the equilibrium Al-Ag alloy phase diagram, the maximum solubility of silver in the R-Al phase was 23.5 atom %.29 For the Al-15 and 20 Ag alloys, however, a small amount of Ag2Al can be detected, even under rapid solidification conditions (Figure 1a). On the whole, the amount of R-Al(Ag) decreases, but that of Ag2Al increases with increasing Ag

Figure 1. XRD patterns of the starting melt-spun Al-15-60 Ag alloy ribbons. (a) Al-15-35 Ag, (b) Al-40-60 Ag.

content in the melt-spun alloys, as indicated by the variation of diffraction peak intensities in Figure 1. When the Ag content is e30 atom % in the precursor alloys, it is obvious that the R-Al(Ag) phase is dominant. For the melt-spun alloys with 35 and 40 atom % Ag, the amount of R-Al(Ag) is comparable to that of Ag2Al. When the Ag content is more than 40 atom % in the precursor alloys, the diffraction peak intensities of the Ag2Al phase are significantly stronger than those of the R-Al(Ag) phase, suggesting that the Ag2Al phase has turned out to be the main part in the precursor alloys (Figure 1b). When the Ag content reaches 60 atom % in the precursor alloy, the XRD pattern shows that only the single Ag2Al phase exists, and the R-Al(Ag) phase completely disappears (Figure 1b). Effect of Silver Content in the Al-Ag Precursor Alloys on the Morphology and Structure of the NPS Ribbons by Acidic Dealloying. Figure 2 shows the XRD patterns of asdealloyed Al-Ag alloys in 5 wt % HCl aqueous solution. As can be seen from the XRD patterns in Figure 2a, the asdealloyed ribbons with the Ag content ranging from 15 to 40 atom % in the precursor alloys are composed of a single facecentered-cubic (fcc) Ag phase, suggesting that, not only was the R-Al(Ag) phase completely dealloyed, but the Al content in the Ag2Al phase was also leached out at the same time. For the Al-45 and 50 Ag alloys, the as-dealloyed samples dominantly comprise the fcc Ag phase with a small amount of undealloyed Ag2Al, as shown in Figure 2b. Figure 3 shows the microstructure of the NPS ribbons through chemical dealloying of the Al-15 Ag alloy in the 5 wt % HCl

Alloy Composition/Dealloying Solution Effect on NPS

Figure 2. XRD patterns of as-dealloyed samples through chemical dealloying of the Al-15-50 Ag alloys in the 5 wt % HCl solution. (a) Al-15-40 Ag, (b) Al-45-50 Ag.

solution. The surface morphology of the resulting NPS ribbons exhibits a homogeneous, bicontinuous, interpenetrating ligamentchannel structure with length scales of several tens to hundreds of nanometers (Figure 3a). Many cracks can be observed on the surface of the ribbons. Some large cracks can reach several micrometers in length and submicrometers in width, as marked by arrows in Figure 3a. The nanoporous structure of NPS can be clearly observed at a higher magnification, as shown in Figure 2b. Figure 3c,d shows the cross-sectional morphology of the NPS ribbons. The thickness of the NPS ribbons can reach up to 20-30 µm, larger than that of the NPS films obtained from the traditional electrochemical alloying/dealloying method. It is clear that the ligament-channel structure runs through the whole ribbons. Many cracks can also be observed across the whole section of the ribbons, and one is highlighted by an upward arrow in Figure 3c. In addition, the ligaments/channels form some columnar islands, as marked by a downward arrow in Figure 3c. The nanoporous structure of these islands can be clearly observed at a higher magnification (Figure 3d). Despite the monolithic characteristic, the NPS ribbons are brittle, soft, and their shape cannot be well preserved. Figure 4 shows the microstructure of the NPS ribbons through chemical dealloying of the Al-20 Ag alloy in the 5 wt % HCl solution. The surface of the NPS ribbons is inhomogeneous and obviously divided into two kinds of regions: crack regions and crack-free regions (Figure 4a). The SEM image at a higher magnification clearly shows the nanoporous structure and the morphology of the cracks in the crack regions, as shown in Figure 4b. The microstructure of the crack-free regions at a higher magnification exhibits a typical bicontinuous interpenetrating ligament-channel structure (Figure 4c). The cross-

J. Phys. Chem. C, Vol. 113, No. 30, 2009 13141 sectional morphology of the NPS ribbons is shown in Figure 4d,e. It is clear that the island-crack structure runs through the whole ribbons (Figure 4d). The ribbons show good continuity and rigidity, although some large cracks appear along the crosssection of the ribbons. The columnar islands exhibit a typical ligament-channel structure, as shown by the high-magnification SEM image in Figure 4e. Figure 5 shows the microstructure of the NPS ribbons through chemical dealloying of the Al-25 Ag alloy in the 5 wt % HCl solution. The plan-view (Figure 5a) and section-view (Figure 5c) SEM images show the uniform nanoporous structure of the NPS ribbons. The bicontinuous, interpenetrating ligamentchannel structure can be clearly observed in the high-magnification SEM images (Figure 5b,d). Moreover, the NPS ribbons exhibit good mechanical integrity and almost no cracks appear in the microstructure. Figure 5e,f shows TEM image and corresponding selected-area electron diffraction (SAED) pattern of the NPS ribbons, respectively. The TEM observation further confirms the homogeneous, bicontinuous, interpenetrating ligament-channel structure of the NPS ribbons (Figure 5e). The corresponding SAED pattern is from the fcc Ag [110] zone axis, indicating the single crystalline nature of the ligaments in the NPS ribbons (here, the selected area is 1 µm in diameter), as shown in Figure 5f. Figure 6 shows the cross-sectional microstructure of the NPS ribbons through chemical dealloying of the Al-30, 35, and 40 Ag alloys in 5 wt % HCl solution. It is obvious that the uniform porous structure runs through the whole section for all the NPS ribbons (Figure 6a,c,e). The whole cross-section of all the NPS ribbons exhibits a homogeneous, bicontinuous, interpenetrating ligament-channel structure, as clearly shown in Figure 6b,d,f. Moreover, these NPS ribbons are monolithic and show good continuity, and no obvious microcracks can be observed. The EDX results show that residual Al (several atom percent) can be detected in the NPS ribbons through dealloying of the Al-15, 20, 25, 30, 35, and 40 Ag alloys in the 5 wt % HCl solution. A typical EDX spectrum is shown in Figure 6g. The former research on NPG by dealloying of Ag-Au alloys confirmed that the NPG normally contains some residual Ag (several atom percent). Moreover, the residual Ag can not be removed but asymptotically reaches a limit at exhaustively long etching times (up to 100 h).30 It should be noted that these NPS ribbons comprise a single fcc Ag phase (Figure 2). Therefore, it is reasonable to assume that the tiny amount of residual Al detected by EDX exists in the Ag crystal lattice in the form of solid solution, which is similar to the case of Ag entrapped in the Au crystal lattice during the formation of NPG. In our former work,31,32 we found that all of Al was removed during the dealloying of Al-Au and Al-Cu alloys in the same HCl solution, and no visible Al can be detected by EDX in the resulting nanoporous metals. Figure 7 shows the microstructure of the NPS ribbons through dealloying of Al-45 and 50 Ag alloys in the 5 wt % HCl solution. A uniform porous structure runs through the whole cross-section of the NPS ribbons, as shown in Figure 7a,c. An ideal bicontinuous interpenetrating ligament-channel structure can be clearly seen from the images at a higher magnification (Figure 7b,d). Moreover, the ligament of the as-dealloyed sample from the Al-50 Ag alloy is apparently larger than that from the Al-45 Ag alloy. The XRD results show the presence of a minor amount of Ag2Al in the as-dealloyed samples (Figure 2b), suggesting that the Ag2Al phase cannot be dealloyed completely in the dilute HCl solution. This is different from other Al-Ag alloys with the Ag content of 15 to 40 atom %. However, it is

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Figure 3. (a,b) Plan-view and (c,d) section-view SEM images showing the microstructure of NPS through dealloying of the Al-15 Ag alloy in the 5 wt % HCl solution.

difficult to distinguish Ag2Al from fcc Ag through SEM observation. The EDX results further verify the presence of the residual Al (in the Ag2Al and/or fcc Ag phases) in the NPS ribbons, and a typical EDX spectrum is shown in Figure 7e. For the melt-spun Al-60 Ag alloy, no obvious bubbles appeared on the surface of the ribbons by imersing the alloy in the dilute HCl aqueous solution (5 wt %) and even adopting a heating treatment (90 ( 5 °C), indicating that the Ag2Al phase cannot be dealloyed in the dilute HCl solution under free corrosion conditions. We tried to immerse the Al-60 Ag alloy into a hot (90 ( 5 °C) concentrated HCl solution and found that the alloy could react with the solution. Figure 8 shows the cross-sectional microstructure of the as-dealloyed samples. It can be surprisingly seen from the images that the porous structure merely forms on the surface of the ribbons (Figure 8a,b). The whole as-dealloyed ribbons were divided into two distinct zones: porous surface zones and inner undealloyed Ag2Al zones. The detailed partial structure can be clearly observed at a higher magnification shown in Figure 8b. Additionally, the grain boundaries can be observed in the section of the ribbons, as marked by arrows in Figure 8a,b. It should be noted that the dealloying of the grain boundaries does not proceed ahead of that of the grains. The EDX spectra of these two distinct zones are presented in Figure 8c,d, corresponding to the porous zones on the surface and the dense zones in the interior of the ribbons. The EDX results demonstrate that almost all of Al was removed in the porous surface zones (Figure 8c), and the composition of the undealloyed zones is close to that of Ag2Al (Figure 8d). The porous structure on the surface originated from the interaction between the leaching of Al from the Ag2Al phase and the rearrangement of released Ag atoms. In addition, the XRD results (not shown here) confirm that both fcc Ag and Ag2Al are present in the as-dealloyed samples. The formation mechanism of nanoporous structures during dealloying has been described in the literature.33 It has been shown that ligaments form as a result of an intrinsic pattern

formation during which aggregation of chemically driven noble metal atoms occurs, and that is Ag in the present case. The process started with selective dissolution of Al atoms from the outermost alloy surface, leaving behind Ag atoms that diffused along alloy/solution interfaces and agglomerated into the ligaments. It may be easy to understand the bicontinuous nanoporous structure resulting from the chemical/electrochemical dealloying of binary alloys with a single phase solid solubility across all compositions. If multiple phases exist in an AxB1-x alloy, however, typically only the A-rich phase would be dealloyed.34 If one phase can be dealloyed and another not, a two-phase alloy can be used to fabricate nanoporous metal composites. Herein, the Al-Ag precursor alloys contain two phases: R-Al(Ag) solid solution and Ag2Al intermetallic compound. As is well-known, the Al element from the R-Al(Ag) solid solution is easy to react with the dilute HCl solution to produce hydrogen gas. At the same time, the Al converts to Al3+ and diffuses into the electrolyte, and the remaining Ag atoms rearrange to form the typical open, interpenetrating ligament-channel structure. However, we have found that the single Ag2Al phase cannot react with the dilute HCl solution at all during the dealloying of the Al-60 Ag alloy ribbons. According to the XRD results of the as-dealloyed NPS ribbons from Al-15-40 Ag alloys (Figure 2a), the Al content from both R-Al(Ag) and Ag2Al can be leached away, and the NPS ribbons are composed of a single fcc Ag phase. Most of the Ag2Al phase in the Al-45 and 50Ag alloy can be dealloyed because only a minor amount of Ag2Al can be detected in the as-dealloyed samples (Figure 2b). It is astonishing that the inert Ag2Al phase can be dealloyed in the dilute HCl solution when the R-Al(Ag) and Ag2Al phases coexist in the ribbons. We think that the synergetic effect of the coexisting of R-Al(Ag) and Ag2Al phases and large quantities of interpenetrating paths for the penetration of the electrolyte play a key role in the dealloying of the inert Ag2Al phase. According to the binary Al-Ag phase diagram29 and


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Figure 4. (a,b,c) Plan-view and (d,e) section-view SEM images showing the microstructure of NPS through dealloying of the Al-20 Ag alloy in the 5 wt % HCl solution.

the present XRD results (Figure 1), it is reasonable to assume that the R-Al(Ag) phase exists in the melt-spun Al-Ag alloys with 15-50 atom % Ag in the form of primary or eutectic (RAl(Ag)/Ag2Al) microstructure. In the process of dealloying in the 5 wt % HCl solution, the more active R-Al(Ag) phase was preferentially dealloyed to form a large number of tiny paths like a three-dimensional (3D) channel network for the penetration of the solution throughout the whole thickness of the ribbons. In addition, the dealloying of R-Al(Ag) promotes that of the Ag2Al phase, and we designate this as a catalytic effect. This viewpoint has been verified because the Al-60 Ag alloy comprising a single Ag2Al phase cannot be dealloyed owing to the absence of R-Al(Ag). In the present work, two scenarios can be postulated for the dealloying of the Al-60 Ag alloy in the concentrated HCl solution at room temperature and 90 ( 5 °C (Supporting Information, Figure S1). Due to the passivation of the Al-60 Ag alloy in the concentrated HCl solution at room temperature, it is reasonable to assume that the 60 atom % Ag composition is infinitely close to the parting limit for the dealloying of the Al-Ag alloys.35 When the Al-60 Ag alloy contacts with the concentrated HCl solution, the Al atoms on the outmost layer of the alloy will be first leached away. The reason for the passivation may be attributed to the formation

of an inert AgCl monolayer, resulting from the exposed Ag atoms and concentrated Cl- ions at the alloy/solution interface. The AgCl layer will passivate the alloy surface in the concentrated HCl solution at room temperature, as schematically shown in Figure S1A. When the Al-60 Ag ribbons are immersed in a hot concentrated HCl solution (90 ( 5 °C), however, the surficial AgCl monolayer will dissolve owing to the formation of soluble [AgCl2]- complexes, and the passivation will be broken down.36 Thus, the following exposed Al atoms will be leached away, and the residual Ag atoms aggregate to form the ligaments. At the initial stage of dealloying, the [AgCl2]complexes can rapidly diffuse into the bulk solution and the porous structure evolves accordingly. Once the porous structure forms, both the [AgCl2]- complexes and Cl- ions have to diffuse through the channels in the porous structure. The diffusion of [AgCl2]- and Cl- will slow down as a result of the limit of the porous structure and the increase of the diffusion length, leading to the increase of the [AgCl2]- concentration along the alloy/ solution interface. The dealloying will cease (passivation reoccurs) when the [AgCl2]- concentration in the solution at the alloy/solution interface is equilibrated with Ag in the undealloyed alloy (Figure S1B).

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Figure 5. SEM and TEM images showing the microstructure of NPS through chemical dealloying of the Al-25 Ag alloy in the 5 wt % HCl solution. (a,b) Plan-view and (c,d) section-view SEM images; (e) TEM bright-field image and (f) corresponding SAED pattern.

In order to investigate the possible phase transformation during the dealloying process, the dealloying of the melt-spun Al-40 Ag alloy was taken as an example. Figure S2 (Supporting Information) shows representative XRD patterns of the asdealloyed samples from the Al-40 Ag alloy at three different stages of the whole dealloying process. When the dealloying proceeded to the midpiont of the whole process (Figure S2A), the R-Al(Ag) phase was almost dealloyed, and the Ag atoms released aggregated to form the Ag ligaments. However, the XRD pattern demonstrates that most of Ag2Al was not incorporated into the dealloying at this stage (Figure S2A). When the dealloying ceased at room temperature (Figure S2B), a little of the Ag2Al phase was dealloyed. Interestingly, when the dealloying proceeded to a certain degree first at room temperature and then at 90 ( 5 °C, most of the Ag2Al phase was dealloyed (Figure S2C). Therefore, we can conclude that the temperature also plays a key role in the dealloying process of the Ag2Al phase. It is worth emphasizing that only the R-Al(Ag), Ag2Al, and fcc Ag phases can be detected, and no other phase transformation (formation of metastable phases) can be observed in the whole dealloying process.

Perplexingly, the XRD results of the NPS ribbons dealloyed from the Al-45 and 50 Ag alloys exhibit the presence of a minor amount of undealloyed Ag2Al phase (Figure 2b). This can be explained as follows. Before dealloying, the melt-spun Al-45 and 50 Ag alloys are composed of primary Ag2Al embedded in the R-Al(Ag)/Ag2Al eutectic matrix, as schematically shown in Figure 9a. During dealloying, the nanoporous structure derives from diffusion and rearrangement of Ag atoms accompanying the dissolution of Al from both the R-Al(Ag) and the surface of Ag2Al. At this stage, the solution can still contact the undealloyed Ag2Al (Figure 9b). The dealloying ceases when the size of the residual Ag2Al grains is less than that of the Ag ligaments and fully embedded in the ligaments (Figure 9c). This mechanism may lead to the case that the Ag2Al cannot be totally dealloyed in the high Ag content ribbons such as Al-45Ag and Al-50Ag, resulting in the formation of Ag/Ag2Al composites. According to the series of microstructure of the NPS ribbons by dealloying of the Al-Ag alloys with different Ag contents, the dependence of the ligament size on the alloy composition shows some regularity, as shown in Table 1. On the whole, the ligament size of the NPS ribbons increases with increasing Ag



Figure 6. Section-view SEM images showing the microstructure of NPS through chemical dealloying of the (a,b) Al-30 Ag, (c,d) Al-35 Ag, and (e,f) Al-40 Ag alloys in the 5 wt % HCl solution. (g) A typical EDX spectrum showing the composition of NPS.

concentration in the Al-Ag precursor alloys. During the dealloying process under free corrosion conditions, no noticeable volume shrinkage can be observed, but macroscopic shrinkage up to 30 vol % was reported during electrochemical dealloying of Ag-Au.37,38 To keep the volume constant during the dealloying, pores or channels must be introduced into the ribbons to compensate the missing volume of the dissolved element. Therefore, it is easy to understand that, the more Ag is contained in the precursor alloys, the less leachable section dissolves away

from the ribbons. The decreasing lattice site from “Al” to “vacancy” and the unchanged total volume of the ribbons jointly result in the decline in porosity and the enlargement of the ligaments in the resulting NPS ribbons. The length scales of ligaments/channels in NPS are 90-500 nm, which are significantly larger than those of the conventional NPG (typically 5-50 nm, depending upon dealloying conditions).39-41 Surface diffusion of more noble elements along alloy/solution interfaces during dealloying plays a key role in the formation

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Figure 7. Section-view SEM images showing the microstructure of NPS through chemical dealloying of the (a,b) Al-45 Ag and (c,d) Al-50 Ag alloys in the 5 wt % HCl solution. (e) A typical EDX spectrum showing the composition of NPS.

of NPG and has a significant influence on the length scales of ligaments/channels.33,42 The surface diffusivity of Ag in electrolyte is on the order of 10-12 cm2 · s-1, much faster than that of Au (on the order of 10-14 cm2 · s-1).43 Generally, the length scales of ligaments/channels in nanoporous metals prepared by the dealloying method increase with increasing surface diffusivity of the noble metal, which has also be revealed by our former work.28 Unlike the bimodal morphology of the NPG dealloyed from the Al-Au ribbons composed of two phases: R-Al and Al2Au,31 the microstructure of NPS is homogeneous throughout the whole ribbons (especially for the NPS ribbons by dealloying of the Al-Ag alloys with 25-50 at % Ag (Figures 5-7) although the precursor ribbons also contain two phases: R-Al(Ag) and Ag2Al. This homogeneous characteristic of the ligaments/channels structure is also observed in the nanoporous copper (NPC) ribbons.32 The synergetic dealloying of two distinct phases and the fast diffusion of the noble metal atoms should be responsible for the uniformity of the nanoporous samples (surface diffusivity of Cu in electrolyte is on the order of 10-10 cm2 · s-1).42 The specific surface area of NPS prepared through dealloying of the Al-25 Ag alloy in 5 wt % HCl solution has been evaluated on the basis of N2 adsorption/desorption

experiments. The Brunauer-Emmett-Teller (BET) surface area of the appointed samples has been determined to be 1.6 ( 0.1 m2 g-1, which is high taking the coarse ligaments/channels (more than 100 nm, Table 1) into consideration. Interestingly, the NPS ribbons prepared from the Al-Ag alloys with low Ag contents exhibit some large cracks (about 1 µm in width and several micrometers in length) (Figures 3 and 4). With increasing Ag content in the precursor alloys, the size and quantity of the cracks in the resulting NPS ribbons diminish correspondingly, especially for large cracks (Figures 5-7). Hayes et al.26 thought that cracking in the monolithic NPC could be caused by several mechanisms: residual stress, capillary forces leading to “mud-cracking”, or coherency stresses between the alloy and the dealloyed Cu. All these mechanisms may cause the cracks in the present NPS ribbons. However, we assume that the main reason for such a regularity of crack appearance relies on the residual stress caused by the large volume vanishing due to the low Ag content in the precursor alloys and the constant volume of the ribbons. The accumulated residual stress releases through the formation of cracks during the dealloying of the Al-Ag alloys with lower Ag contents (Figures 3 and 4). With increasing Ag content in the precursor alloys, less



Figure 8. (a,b) Section-view SEM images showing the microstructure of as-dealloyed Al-60 Ag alloy ribbons in the 37 wt % HCl solution, and corresponding EDX spectra showing the composition of (c) porous surface zones and (d) undealloyed zones of the as-dealloyed samples.

Figure 9. Schematic diagrams showing the dealloying process of the starting Al-45 and 50 Ag alloys in the 5 wt % HCl solution: (a) predealloying, (b) during dealloying (complete dealloying of R-Al(Ag)/ Ag2Al eutectic and partial dealloying of Ag2Al), and (c) after dealloying (the residual undealloyed Ag2Al imbedded in the Ag ligament).

vanishing volume during the dealloying process results in less residual stress existing in the NPS ribbons. Thereby, the tiny amount of residual stress can be retained in the NPS, and the morphology of the as-dealloyed ribbons appears continuous and free of cracks (Figures 5-7). In addition, the present NPS ribbons are monolithic and can be easily handled and loaded, especially for the NPS ribbons dealloyed from the Al-Ag alloys with Ag content exceeding 25 atom %. Microstructure of the NPS Ribbons by Alkaline Dealloying. The Al-based alloys are usually used as the precursors for preparation of Raney catalysts because of the chemical activity of Al in alkaline solutions, such as Raney Ni catalyst by leaching Al-Ni alloys in alkaline solutions.44,45 The Raney Ag catalyst is often prepared from the Al-Ag alloys.25 In order to obtain large specific surface areas in Raney catalysts, the noble metal content in the Al-based precursors is normally less than 50 wt %, resulting in the formation of fine powder-like catalysts (for the Raney Ag, that is less than 20 atom % Ag in the precursor alloys). In the present paper, the melt-spun Al-Ag alloy ribbons mentioned above were also dealloyed in the alkaline solution (20 wt % NaOH aqueous solution) for comparison with NPS through dealloying in the HCl solution. Unlike the traditional Raney Ag catalyst, the resulting NPS ribbons exhibit a monolithic characteristic.

Figure 10 shows the surface and cross-sectional microstructure of the NPS ribbons by dealloying of the Al-25 Ag alloy in the NaOH aqueous solution. It is interesting to note that not only has the open, bicontinuous nanoporous structure formed, but also some skins with the thickness of 100-200 nm have separated the NPS surface into a large amount of cells with a size of approximately 1 µm (Figure 10a,b). In some zones, the cells are surrounded by the dense skins (Figure 10a). In other zones, the cells are separated by some discrete plate-like skins (Figure 10b). Unlike the crack-free morphology of the NPS ribbons by dealloying of the Al-25 Ag in the 5 wt % HCl solution, some of cracks appear along this typical plate structure. As can be seen from the cross-sectional morphology shown in Figure 10c (low magnification), the dealloying has also occurred throughout the whole thickness of the ribbons. In the higher magnification image shown in Figure 10d, the typical plate structure is widespread in the interior of the dealloyed ribbons. In addition, the length scale of the ligaments in the interior of NPS reaches 80-90 nm. The EDX results confirm that both the skins and the internal nanoporous structures mainly consist of Ag, and only a minor amount of Al can be detected. Figure 10e,f shows the TEM image and corresponding SAED pattern of the NPS ribbons by dealloying of the Al-25 Ag in the 20 wt % NaOH solution, respectively. The morphology is consistent with the SEM observation (Figure 10e). The corresponding SAED pattern shows highly regular diffraction spots from the fcc Ag [103] zone axis, verifying the single-crystalline nature of the ligaments in the selected area (∼1 µm). That is to say that, not only is the crystal orientation identical in a single ligament but that of all the ligaments in the selected area are also identical. This phenomenon was also revealed by Zhang et al. in NPG by dealloying of Al-Au alloys.31 It has been reported that the crystal lattice orientation is retained during dealloying with the conservation of the grain size of the master alloy.46 Hakamada and Mabuchi47 found the gold prism phe-

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TABLE 1: Dependence of Ligament Size of As-Dealloyed Samples on the Composition of the Starting Al-Ag Alloys alloy composition (atom % Ag) ligament size (nm)

15 94 ( 5

20 109 ( 5

25 111 ( 5

nomenon by coarsening treatment of NPG in a concentrated HCl solution and thought that the formation of a gold prism might attribute to the initial network of cracks of as-dealloyed NPG. Through comparing the plan-view morphology of the asdealloyed ribbons leached in the 5 wt % HCl solution and 20 wt % NaOH solution, it is clear that the acid-leached samples are homogeneous, and no obvious cracks can be observed (Figure S3A). Contrarily, the alkali-leached ribbons exhibit large quantities of ring-like cracks to connect with each other to form a kind of crack network separating the sample surface into cells (Figure S3B). Moreover, it can be observed that the precipitation of hydrogen bubbles appeared more quickly and intensely during the dealloying in the HCl solution, whereas tiny and turbid bubbles appeared in the NaOH solution. Therefore, it is reasonable to assume that the different inner stress resulting from the different dissolution rate, different surface diffusion

30 230 ( 10

35 207 ( 11

40 258 ( 15

45 228 ( 16

50 448 ( 25

of atoms, viscosity of the dealloying solution, as well as mass transfer rate in the acidic and alkaline solutions probably induce the different geometric self-organization of NPS. The detailed study of the formation mechanism of the Ag skins and plates in the alkaline circumstance will be conducted in the following work. Figure 11 shows the XRD results of the as-dealloyed ribbons from the Al-Ag alloys with 15-50 atom % Ag in the 20 wt % NaOH solution. It can be clearly seen that the R-Al(Ag) phase in the Al-Ag alloys was fully dealloyed, and all the asdealloyed NPS samples are composed of two phases: fcc Ag and Ag2Al, even in the samples with low Ag content in the precursor alloys such as Al-15 and 20 Ag. This is quite different from the NPS samples dealloyed in the 5 wt % HCl solution. Yamauchi and his co-workers24 have reported that the Ag2Al phase in the Al-Ag alloys with 15-39 atom % Ag is stable

Figure 10. SEM and TEM images showing the microstructure of NPS through chemical dealloying of the Al-25 Ag alloy in the 20 wt % NaOH solution. (a,b) Plan-view and (c,d) section-view SEM images; (e) TEM bright-field image and (f) corresponding SAED pattern.


J. Phys. Chem. C, Vol. 113, No. 30, 2009 13149 dealloying process and the formation of NPS. The melt-spun Al-Ag alloys with 15-50 atom % Ag are composed of R-Al(Ag) and Ag2Al phases, but the Al-60 Ag alloy comprises a single Ag2Al phase. The Al-Ag alloys with 15-40 atom% Ag can be fully dealloyed in the 5 wt % HCl solution, and the as-dealloyed ribbons are composed of a single fcc Ag phase. The Al-45 and 50 Ag alloys can also be dealloyed in the HCl solution, but a minor amount of undealloyed Ag2Al can be detected in the as-dealloyed ribbons. All the NPS ribbons exhibit an open, bicontinuous interpenetrating ligament-channel structure with length scales of 90-500 nm. The existence of R-Al(Ag) can supply penetration paths for the solution and shows a catalytic effect on the dealloying of Ag2Al in the twophase Al-Ag alloys. Moreover, the synergetic dealloying of R-Al(Ag) and Ag2Al in the two-phase Al-Ag alloys and the fast surface diffusion of Ag adatoms along alloy/solution interfaces result in the formation of NPS with a homogeneous porous structure. In addition, the Al-Ag alloys with 15-50 atom % Ag cannot be fully dealloyed in the 20 wt % NaOH solution, and the as-dealloyed ribbons are composed of fcc Ag and undealloyed Ag2Al. The dealloying strategy in the alkaline solution for the two-phase Al-Ag alloys can be used to fabricate NPS/Ag2Al composites. In addition, the Al-60 Ag alloy containing the single Ag2Al phase does not react with the 5 wt % HCl or 20 wt % NaOH solution even at high temperatures (90 ( 5 °C). The monolithic NPS ribbons can serve as model materials to investigate mechanical, physical, and chemical properties associated with random porous structures of nanoporous solids, and will find wide applications in catalysis, sensors, and so on.

Figure 11. XRD patterns of as-dealloyed samples through chemical dealloying of the Al-15-50 Ag alloys in the 20 wt % NaOH solution. (a) Al-15-30 Ag, (b) Al-35-50 Ag.

on alkali and cannot contribute to the formation of skeletal Ag. We have found that the Al-60 Ag alloy (single Ag2Al phase) cannot react with the 20 wt % NaOH solution even at 90 ( 5 °C. The present results further confirm the viewpoint of the stability of Ag2Al on alkali and the existence of the R-Al(Ag) phase cannot lead to the complete dealloying of Ag2Al. Comparing the XRD patterns of the as-dealloyed samples (Figure 11) with those of the undealloyed alloys (Figure 1), however, we can find that the Ag2Al phase can be partially dealloyed in the alkali solution, especially for the Al-Ag alloys with 35-50 atom % Ag. If one phase can be dealloyed and another not (or partially), a two-phase alloy can be used to fabricate nanoporous metal composites. For example, nanoporous composites with a nanoporous Pt matrix and [Pt] embeddings were fabricated by dealloying of Pt-Ag alloys composed of [Pt] and [Ag] solid solutions.48 Therefore, the present dealloying strategy in the alkaline solution for the two-phase Al-Ag alloys can be used to fabricate NPS/Ag2Al composites. Conclusions In summary, monolithic NPS ribbons can be fabricated through chemical dealloying of melt-spun Al-Ag alloys in acidic or alkaline aqueous solutions under free corrosion conditions. The alloy composition of the precursor alloys and the dealloying solution have a significant influence on the

Acknowledgment. We gratefully acknowledge financial support by the National Natural Science Foundation of China under Grants 50701028 and 50831003, and the Excellent Middle-age and Young Scientists Research Award Foundation of Shandong Province under Grant 2007BS04024. Z.Z. acknowledges financial support by DFG-SFB 459 (Germany) for a guest visit. The experimental assistance from Ruhr University Bochum (Bochum, Germany) is also acknowledged. Supporting Information Available: Schematic illustrations of the dealloying mechanism of Al-60 Ag, and XRD patterns and SEM images of as-dealloyed samples. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Zielasek, V.; Jurgens, B.; Schulz, C.; Biener, J.; Biener, M. M.; Hamza, A. V.; Baumer, M. Angew. Chem., Int. Ed. 2006, 45, 8241. (2) Yu, C. F.; Jia, F. L.; Ai, Z. H.; Zhang, L. Z. Chem. Mater. 2007, 19, 6065. (3) Yin, H. M.; Zhou, C. Q.; Xu, C. X.; Liu, P. P.; Xu, X. H.; Ding, Y. J. Phys. Chem. C 2008, 112, 9673. (4) Hu, K. C.; Lan, D. X.; Li, X. M.; Zhang, S. S. Anal. Chem. 2008, 80, 9124. (5) Mortari, A.; Maaroof, A.; Martin, D.; Cortie, M. B. Sens. Actuators, B 2007, 123, 262. (6) Yu, F.; Ahl, S.; Caminade, A. M.; Majoral, J. P.; Knoll, W.; Erlebacher, J. Anal. Chem. 2006, 78, 7346. (7) Ertenberg, R. W.; Andraka, B.; Takano, Y. Physica B 2000, 2022, 284–288. (8) Cortie, M. B.; Maaroof, A. I.; Smith, G. B. Gold Bull. 2005, 38, 14. (9) Shen, W. N.; Dunn, B.; Moore, C. D.; Goorsky, M. S.; Radetic, T.; Gronsky, R. J. Mater. Chem. 2000, 10, 657. (10) Luo, H.; Sun, L.; Lu, Y.; Yan, Y. S. Langmuir 2004, 20, 10218. (11) Qi, Z.; Zhang, Z. H.; Jia, H. L.; Qu, Y. J.; Liu, G. D.; Bian, X. F. J. Alloys Compd. 2009, 472, 71. (12) Fritz, J. D.; Pickering, H. W. J. Electrochem. Soc. 1991, 138, 3209. (13) Pugh, D. V.; Dursun, A.; Corcoran, S. G. J. Electrochem. Soc. 2005, 152, B455.

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