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Fabrication and characterization of bulk nanoporous copper by dealloying Al–Cu alloy slices
Corrosion Science 90 (2015) 216–222
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Fabrication and characterization of bulk nanoporous copper by dealloying Al–Cu alloy slices Jie Li a, Huawei Jiang a, Nana Yu a, Caixia Xu b, Haoran Geng a,⇑ a b
School of Materials Science and Engineering, University of Jinan, West Nan Xinzhaung Road 336, Jinan 250022, PR China School of Chemistry and Chemical Engineering, University of Jinan, West Nan Xinzhaung Road 336, Jinan 250022, PR China
a r t i c l e
i n f o
Article history: Received 5 September 2014 Accepted 13 October 2014 Available online 18 October 2014 Keywords: A. Alloy B. Potentiostatic B. SEM C. De-alloying C. Alkaline corrosion
a b s t r a c t Bulk NPC (nanoporous copper) were fabricated by dealloying xAl(100 x)Cu (x = 60, 70 at.%) alloy slices in NaOH solutions (10, 20 and 30 wt.%) under free corrosion conditions. The results show that Al–Cu alloy slices can be completely dealloyed. The synergetic dealloying of Al2Cu and AlCu results in the formation of uniform NPC with small-sized channels. The dealloying of a-Al and Al2Cu plays an important role in the formation of NPC with two kinds of structures of both small-sized uniform ligaments–channels and hierarchical skeleton-like (large-sized channels with highly porous walls) structure, and these two structures form layer-cake structure. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Nanoporous materials have attracted extensive attentions because of their promising applications in catalysis, sensors and fuel cells [1–6]. Among various methods including template method [7], hydrothermal method [8], dealloying method [9], etc., dealloying has been approved to be a simple and effective way to fabricate nanoporous materials with a three-dimensional bi-continuous interpenetrating ligaments–channels structure by selective dissolution of the most electrochemically active elements out of crystalline alloys [9–12], amorphous alloys [13] and metallic glasses [14]. And many alloy systems have been applied to prepare nanoporous structures metals through dealloying process such as Au–Ag [9], Ti–Cu [10], Al–Au [15], and Al–Cu [16]. In comparison with expensive nanoporous gold (NPG), platinum (Pt) and palladium (Pd) it is more suitable for mass production of cheaper NPC which also works well in area of catalysis [17]. NPC has been successfully fabricated by dealloying Al–Cu [16,18], Mg–Cu [19] and Zn–Cu [20] alloys. The precursors, however, as reported are usually ultrathin ribbons of about 20–40 lm in thickness. Generally, researchers spin the high-temperature melt onto a copper roller in a single-roller melt spinning apparatus under a controlled argon atmosphere [21,22]. Whereas, large amount of this kind of ribbons would not be fabricated by this method, and industrial application will be limited. ⇑ Corresponding author. Tel./fax: +86 0531 82765314. E-mail address: [email protected] (H. Geng). http://dx.doi.org/10.1016/j.corsci.2014.10.014 0010-938X/Ó 2014 Elsevier Ltd. All rights reserved.
To date, less attention has been paid to the synthesis of nanoporous structures by dealloying bulk alloys. Mao et al. [20] has obtained bulk NPC with a three-dimensional continuous interpenetrating ligament–channel structure by dealloying a 2.0 mm thick Zn–Cu slice that annealed at different temperatures in HCl (or HCl + NH4Cl)solution. In addition, Changchun Zhao also has mentioned that Mg50Cu50 can serve as precursor to fabricate bulk NPC under diluted hydrochloric acid condition but he did not describe the microstructure in detail [23]. In this work, 1.0 mm thick xAl(100 x)Cu (x = 60, 70 at.%) slices were applied to obtain bulk NPC through chemical dealloying at free corrosion conditions. The experimental results show that slices can be completely dealloyed across all given compositions. Uniform NPC can be obtained simply from synergetic dealloying 60Al40Cu slices which comprise two phases of Al2Cu and AlCu, and hierarchical skeleton-like structure (large-sized channels with highly porous walls) can be fabricated by dealloying Al-rich 70Al30Cu. The hierarchical skeletonlike structure forms layer-cake structure with uniform NPC. In addition, the mechanism of the layer-cake structure is studied. 2. Experimental xAl(100 x)Cu (x = 60, 70 at.%) alloy casts were prepared by arcmelting Cu and Al with purities of 99.9 wt.% in an argon atmosphere, then the casts were cut into slices of 1.0 mm in thickness by wire cut electrical discharge machine (WEDM). The slices were cleaned in an ultrasonic cleaner then dried in vacuum chamber. Then chemical dealloying experiments were conducted at room
J. Li et al. / Corrosion Science 90 (2015) 216–222
temperature in the open air, and Al–Cu slices were immersed in NaOH solutions with a concentration of 10 wt.%, 20 wt.% and 30 wt.%, respectively. They were numbered sequentially from sample 1–6 (Table 1). When no obvious bubbles emerged any longer and the solutions became clear again the dealloying process stopped. The residues were taken out of beakers and well rinsed with distilled water and dehydrated alcohol to remove the residual NaOH solutions and metal ions. The phase distributions in precursors were observed using an M-4XC metallographic microscope. The phases present in the Al–Cu alloys were confirmed by X-ray diffraction (XRD, D8 ADVANCE) with a Cu Ka irradiation. The morphology and structure of the dealloyed samples were characterized by a scanning electron microscope (SEM, FEI QUANTA FEG 250) coupled with an energy dispersive X-ray spectroscopy (EDS). Electrochemical experiments were carried out to study the microstructure evolution under a conventional three-electrode electrochemical workstation (GU/07345C) at room temperature. The electrochemical cell consisted of a platinum needle as the counter electrode and saturated calomel electrode (SCE) as a reference electrode, 0.05 M NaOH solution was applied as electrolyte. All potentials quoted are on the SCE scale unless otherwise stated.
3. Results and discussion Fig. 1 shows the XRD patterns of as-prepared Al–Cu alloy slices and a prototypical as-dealloyed bulk sample. The XRD results show that 70Al30Cu alloy is almost totally composed of Al2Cu phase and a trace Al4Cu9 phase also can be detected. However, AlCu phase emerges when the content of Al decreases to 60Al40Cu. It is obvious that the Al2Cu phase is dominant in the 70Al30Cu alloys, and the amount of Al2Cu is comparable to that of AlCu in the 60Al40Cu alloy, and these are in accordance with their diffraction peak intensities [20]. A prototypical XRD pattern of as-dealloyed sample is present at the top of Fig. 1. After dealloying, all of Al2Cu, AlCu and Al4Cu9 can be fully dealloyed in NaOH solution and a face centered cubic (fcc) Cu phase can be identified in the as-dealloyed bulk samples. Additionally, a minor amount of Cu2O that maybe caused by oxidation during preservation is detected in the as-dealloyed samples. The nanoporous structure of the bulk NPC slices is verified by scanning electron microscopy. Fig. 2 shows the plane-view and section-view microstructures of as-dealloyed 60Al40Cu alloy slices after dealloying in 10 wt.%, 20 wt.% and 30 wt.% NaOH solutions (the corresponding products are designed as samples 1–3, and the similar below), respectively. The plane-view of sample 1 shows a porous structure (Fig. 2a), the ligaments–channels structure is not obvious and many Cu nanoparticles with size of 50 nm can be observed. Fig. 2b shows the section-view image of sample 1 at a higher magnification, a typical uniform bi-continuous interpenetrating ligaments–channels structure with size of 50 nm can be observed. Many microcracks (tens of micrometers in length and sub-micrometer in width) can be observed on the fracture surface of the slice (insert image of Fig. 2b). For sample 2, the plane-view
Table 1 Dealloying time (h) of all samples from 1 to 6. NO.
Alloy (at.%)
CNaOH (wt.%)
Dealloying time (h)
1 2 3
60Al40Cu
10 20 30
29.33 31.83 80.0
4 5 6
70Al30Cu
10 20 30
20.33 20.33 44.0
217
Fig. 1. XRD patterns of as-prepared Al–Cu alloy slices and a prototypical asdealloyed bulk NPC.
and section-view images show a similar microstructure characteristic to that of sample 1. For the sample 3, Fig. 2e shows a porous structure, the ligaments–channels structure is slightly larger than that of samples 1 and 2, but the pores are still not well distinguishable. In addition, microcracks can also be observed on the surface of sample 3. However, the section-view microstructure shows a quite different structure compared with the former two samples i.e. a large sum of pores is blocked. Fig. 2 verifies that both Al2Cu and AlCu phases can be dealloyed throughout 60Al40Cu alloy slices and nanoporous structure can be obtained. Combining with XRD result, bulk NPC can be obtained via dealloying 60Al40Cu alloy slices in NaOH solutions. Table 1 shows the dealloying time of different alloys in various NaOH solutions. Samples 1 and 2 have a similar dealloying time of around 30 h, however, that of sample 3 is 90 h, much longer than that of samples 1 and 2. Fig. 2g and h shows the microstructures of sample 3 for dealloying 70 h. Fig. 2g exhibits a prototypical open, bi-continuous interpenetrating ligaments–channels structure with length scales of 100 nm, and the pores are obvious. A three-dimensional nanoporous structure can be observed in Fig. 2h, it is extremely different from that of sample 3. Therefore, significant coarsening occurs at later stage in concentrated alkali solution during such a long time dealloying, and that can be attributed to the fast surface diffusion [20,24]. Zhang et al. and Qi et al. have reported that length scales of ligaments–channels in nanoporous metals could be tuned by simply changing the dealloying solutions [16,21]. In this work, it is obvious that the ligaments–channels size increases with the increase of concentration (from 10 wt.% to 20 wt.%) of NaOH solution. For 60Al40Cu, finer microstructure can be obtained in 20 wt.% NaOH solution and a moderate dealloying time is required. Fig. 3 shows the microstructure of the bulk NPC by dealloying 70Al30Cu alloy slices in 10 wt.%, 20 wt.% and 30 wt.% NaOH solutions (samples 4–6), respectively. Obviously, traditional threedimensional bi-continuous interpenetrating ligaments–channels structure is fabricated throughout the whole alloy slices (Fig. 3b2, d1, d2 and f2). It is clear that ligaments coarse then bond to adjacent ones and the pores become vague as shown in the regions marked by the dotted ellipses in the inset image in Fig. 3a and outer parts of the irregularity closed curves in Fig. 3c. Obviously, the ligaments–channels of samples 4–6 in Fig. 3 are larger than those of samples 1–3 in Fig. 2. That indicates that the effect of alloy compositions (of the starting Al–Cu alloys) on the microstructure of bulk NPC outweighs that of dealloying solutions in case of the free corrosion of the Al–Cu alloys in the 10 wt.%, 20 wt.% and 30 wt.% NaOH solutions. It is in accordance with the
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(a)
(b)
400 nm (c)
20 μm
500 nm
(d)
500 nm (e)
20 μm
500 nm
(f)
500 nm
4 μm
(g)
500 nm (h)
4 μm
500 nm
Fig. 2. SEM images of the microstructures of as-dealloyed sample 1 (a, b), sample 2 (c, d), sample 3 (e, f) and 70 h-dealloyed sample 3 (g, h) by immersing 60Al40Cu slices in the 10 wt.%, 20 wt.% and 30 wt.% NaOH solutions for a certain time (see Table 1), respectively. (a, c, e and g) are the plane-view images and (b, d, f and h) are the section-view images of the four samples.
findings of Zhang et al. [25]. Changchun Zhao also found that compositions of melt-spun Mg–Cu alloys have an important effect on the dealloying process and microstructure of the NPC ribbons [23]. The sectional views of samples 4–6 indicate that they comprise nanoporous composites of uniform nanoporous structure and hierarchical skeleton-like structure which adheres to the surface of uniform structure (Fig. 3b, d–f). Obviously, the entire skeleton-like structure is porous and composed of lower hierarchy level ligaments (Fig. 3b1 and f1), namely, hierarchical skeleton-like structures are large-sized channels with highly porous walls. The
lower level ligaments–channels in Fig. 3b1 (30–40 nm) is smaller than those in Fig. 3f1 (60–70 nm). The skeleton-like structure obtained here is comparable to that obtained by Qi and Weissmüller [26], but this one-step dealloying is much easier to operate than corrosion-coarsening-corrosion process. In addition, uniform NPC layers are filled with hierarchical skeleton-like structure; the two structures form layer-cake structure as shown in Fig. 3d and insert image of Fig. 3e. It is generally recognized that ideal bi-continuous ligaments– channels, nanoporous structures can be obtained from binary
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(a)
(b)
(b1)
500 nm (b2)
500 nm
400 nm
(c)
4 μm
400 nm
d)
(d1)
2 μm d2)
20 μm
500 nm
(f)
(e)
400 μm
(f1)
500 nm
(f2)
300 μm
20 μm
5 μm
500 nm
Fig. 3. SEM images showing the microstructures of sample 4 (a, b), sample 5 (c, d) and sample 6 (e, f) by immersing 70Al30Cu slices in the 10 wt.%, 20 wt.% and 30 wt.% NaOH solutions for a certain time (see Table 1), respectively. (a, c, e and f) are the plane-view images of the three corresponding samples; (b, d) are the section-view images of samples 4 and 5; (b1, b2, d1, d2, f1 and f2) show the higher magnification images of selected areas in (b, d, f); insert image in (e) shows lower magnification appearance of sample 6.
alloys with a single-phase solid solubility across all compositions by chemical dealloying [9]. It is not one phase simply being excavated out of a two phase material but the dynamical mechanism during dissolution that impulse the porosity evolution [27]. In crystalline alloys systems, the dealloying process is expected to start with the selective dissolution of a single less noble atom on a flat alloy surface along closely-packed direction and leaves a pit. More noble atoms then diffuse about and start to agglomerate into islands. As the dealloying proceeds, dealloying propagates into inner part of the alloys, thus resulting in bi-continuous ligaments– channels structures [9]. In this work, bulk NPC is be obtained by means of selective dissolution of Al atoms and diffusion of Cu atoms from dealloying Al–Cu alloy slices. According to Al–Cu phase diagram [28], 60Al40Cu alloy is composed of Al2Cu and AlCu phases which are in accordance with XRD results. 70Al30Cu primarily comprises Al2Cu phase, moreover, a-Al solid solution is also observed in metallographs as shown in Fig. 4.
From Fig. 4a, 60Al40Cu alloy is of a peritectic microstructure with AlCu inside and Al2Cu outside and they are comparable to each other. From the EDX analysis of the site 1 marked in Fig. 4a, only Al and Cu elements are detected, and the atomic ratio of Al/Cu is almost 1 (Table 2). Therefore, the dark region can be confirmed as AlCu phase and the bright region surrounding AlCu phase is Al2Cu phase [29]. Fig. 4b confirms that Al2Cu phase is predominate and gap region is a-Al solid solution and Al2Cu phase is well split by a-Al layer by layer in some certain directions. In addition, aAl solid solution separates Al2Cu out (Fig. 4c). To learn about how these two alloys change during dealloying, electrochemical experiments are conducted. Potentiostatic dealloying tests were performed on the two precursors alloy with the applied potential of 0.6 V and the results are illustrated in Fig. 5. For both 60Al40Cu and 70Al30Cu alloys, from the view of whole process, current density initially increases then gradually decreases at later course. However, from Fig. 5b, there exist
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(a)
(b)
(c)
1
Fig. 4. Metalloscope photos showing different microstructures of (a) 60Al40Cu and (b, c) 70Al30Cu alloys. (d) Shows EDS spectra of site 1 in (a).
Table 2 EDX analysis of composition of AlCu phase. Elements (at.%)
Site 1
Al Cu
51.35 48.65
Fig. 5. Chronoamperometric curves of (a) 60Al40Cu and (b) 70Al30Cu alloys.
obvious current pulses where current density firstly gradually increases then suddenly drops. In addition, the time interval of each pulse is similar, suggesting that dealloying behavior is analogous about every 2000 s, namely, dealloying processes that possess a same characteristic repeat again and again until alloy is fully dealloyed. Fig. 4 has confirmed alternating layer structures of precursor 70Al30Cu alloy; therefore, it is rational to assume that this current pulse is closely related to distribution of a-Al and Al2Cu phases. According to Zhang’ report, the excavation of a-Al phase out of alloy contributes to the formation of large-sized channel [21]. The Al2Cu phase that is surrounded by a-Al is corroded and leaves small level pores. This accounts for how hierarchical skeleton-like structure forms. Because of its low standard potential ( 1.662 V vs. SHE for Al/Al3+), Al is easily etched. Therefore, the fast dissolution of a-Al should be responsible for the increase of current density in a pulse step. As soon as the a-Al is fully corroded, Al2Cu phase is exposed to solution and results in the sudden drop of current density, because of its larger critical potential ( 0.53 V [25]). When the dealloying proceeds, this cycle occurs iteratively and finally forms layer-cake structure NPC. And Fig. 6 shows the morphology of 70Al30Cu corresponding to this electrochemical test. It is obvious that skeleton-like structure is obtained, and next to it is as-corroded alloy frontier. The left bottom figure shows the morphology of microstructure formed by electrochemical
dealloying alloy front marked in Fig. 6. The porous structure is different from that obtained from chemical dealloying but matches well to Fig. 5b. In former reports, both Al2Cu and AlCu phases can be fully dealloyed in alkaline solutions because of their large potential difference of Al and Cu ( 1.662 V vs. SHE for Al/Al3+ and 0.342 V vs. SHE for Cu/Cu2+), and homogeneous porous structure can be synthesized due to the synergetic dealloying of Al2Cu and AlCu phase [16]. This is also applicable to account for the phenomena of 60Al40Cu alloy in this work. In addition, chronoamperometric curve in Fig. 5a shows a uniform reduction of current density that is caused by the increasing distance of alloy–liquid interface and corrosion frontier and the resultant low material exchange rate [23,30]. Fig. 7 shows chronoamperometric curves that show initial stage of potentiostatic dealloying 60Al40Cu and 70Al30Cu alloys under 0.6 V in 0.05 M NaOH solution. Obviously, both 60Al40Cu and 70Al30Cu process a decreasing current density trend during initial 100 s and current density increases at later period of the initial 10 min. The reason of this phenomenon may be the dissolution of oxide layers and breakdown of passivation films. The valley sitting around 1100 s can be regard as the first current pulse, which is followed by a series of pulses, as shown in Figs. 5b and 7b. Fig. 8 shows cyclic voltammetry (CV) curves of evolutions of dealloying of 60Al40Cu and 70Al30Cu alloys that subjected to 350 cycles under potentials from 1.4 V to 0.05 V and from 1.3 V to 0.9 V, respectively. Form Fig. 8a, in the first 10 cycles, the current density of the forward scan gradually increases then the curves does not change much until the 350th cycle in which
Fig. 6. SEM images show the microstructure of 70Al30Cu which suffered electrochemical dealloying at the potential of 0.6 V vs Ag/AgCl in 0.05 M NaOH solution.
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case of 70Al30Cu the CV curves are quite different (Fig. 8b). It is obvious that from the first cycle to the 5th cycle, current density rapidly increases until the 10th cycle after which current density gradually reduces immediately. Insert Figure in Fig. 8b shows the 1st cycle in larger magnification, and obviously, each of the initial five loop curves processes a crossover which separates the loop into two sections. At lower potential section, current density on the reverse scan is higher than that on the forward scan. At higher potential section, the situation is reverse. In addition, the position of crossover moves down with increasing cycles. However, the current density trend of the CV curves in Fig. 8b does not accord with the pulse that shown in Fig. 5b. Perhaps because of the potential range applied in CV tests is much smaller than the static potential exerted in potentiostatic dealloying test. And further works should be carried out to study the reasons of this asynchronous. 4. Conclusion Fig. 7. Chronoamperometric curves showing initial dealloying stage of (a) 60Al40Cu and (b) 70Al30Cu alloys.
In summary, xAl(100 x)Cu (x = 60, 70 at.%) alloy slices can be fully dealloyed in NaOH solutions (10 wt.%, 20 wt.% and 30 wt.%) under free corrosion conditions and bulk NPC is fabricated. For, 60AlCu40, the synergistic dealloying of Al2Cu and AlCu phases results in uniform porous structure. For 70Al30Cu, the dealloying of alternating a-Al and Al2Cu phases plays an important role in the formation of NPC with both hierarchical skeleton-like and uniform porous structures. In addition, hierarchical skeleton-like and uniform porous structures form layer-cake structure that is favorable to filtration. On the basis of the present results, this work will not only offer a help filling in dealloying framework but also provide guidance to process bulk alloys that contain noble elements. Acknowledgements The authors would like to acknowledge Natural Science Foundation of China (50871047 and 51271087) and National Nature Science Foundation (51301076) for this work. References
Fig. 8. CV curves showing evolutions of dealloying of (a) 60Al40Cu and (b) 70Al30Cu alloys subjected to 350 cycles, insert image in (b) shows details of the 1st cycle of 70Al30Cu.
current density declines clearly. This confirms that dealloying initially grows fast then gradually decreases, which is in coincidence with Fig. 5a. And the decrease of current density in the 350th cycle can be ascribed to the depletion of active Al atoms from the alloy. In reverse scan, reduction peaks is obvious and it can be attributed to the redeposition of dissolved Cu during dealloying [19]. In the
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Fabrication and characterization of bulk nanoporous copper by dealloying Al–Cu alloy slices Jie Li a, Huawei Jiang a, Nana Yu a, Caixia Xu b, Haoran Geng a,⇑ a b
School of Materials Science and Engineering, University of Jinan, West Nan Xinzhaung Road 336, Jinan 250022, PR China School of Chemistry and Chemical Engineering, University of Jinan, West Nan Xinzhaung Road 336, Jinan 250022, PR China
a r t i c l e
i n f o
Article history: Received 5 September 2014 Accepted 13 October 2014 Available online 18 October 2014 Keywords: A. Alloy B. Potentiostatic B. SEM C. De-alloying C. Alkaline corrosion
a b s t r a c t Bulk NPC (nanoporous copper) were fabricated by dealloying xAl(100 x)Cu (x = 60, 70 at.%) alloy slices in NaOH solutions (10, 20 and 30 wt.%) under free corrosion conditions. The results show that Al–Cu alloy slices can be completely dealloyed. The synergetic dealloying of Al2Cu and AlCu results in the formation of uniform NPC with small-sized channels. The dealloying of a-Al and Al2Cu plays an important role in the formation of NPC with two kinds of structures of both small-sized uniform ligaments–channels and hierarchical skeleton-like (large-sized channels with highly porous walls) structure, and these two structures form layer-cake structure. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Nanoporous materials have attracted extensive attentions because of their promising applications in catalysis, sensors and fuel cells [1–6]. Among various methods including template method [7], hydrothermal method [8], dealloying method [9], etc., dealloying has been approved to be a simple and effective way to fabricate nanoporous materials with a three-dimensional bi-continuous interpenetrating ligaments–channels structure by selective dissolution of the most electrochemically active elements out of crystalline alloys [9–12], amorphous alloys [13] and metallic glasses [14]. And many alloy systems have been applied to prepare nanoporous structures metals through dealloying process such as Au–Ag [9], Ti–Cu [10], Al–Au [15], and Al–Cu [16]. In comparison with expensive nanoporous gold (NPG), platinum (Pt) and palladium (Pd) it is more suitable for mass production of cheaper NPC which also works well in area of catalysis [17]. NPC has been successfully fabricated by dealloying Al–Cu [16,18], Mg–Cu [19] and Zn–Cu [20] alloys. The precursors, however, as reported are usually ultrathin ribbons of about 20–40 lm in thickness. Generally, researchers spin the high-temperature melt onto a copper roller in a single-roller melt spinning apparatus under a controlled argon atmosphere [21,22]. Whereas, large amount of this kind of ribbons would not be fabricated by this method, and industrial application will be limited. ⇑ Corresponding author. Tel./fax: +86 0531 82765314. E-mail address: [email protected] (H. Geng). http://dx.doi.org/10.1016/j.corsci.2014.10.014 0010-938X/Ó 2014 Elsevier Ltd. All rights reserved.
To date, less attention has been paid to the synthesis of nanoporous structures by dealloying bulk alloys. Mao et al. [20] has obtained bulk NPC with a three-dimensional continuous interpenetrating ligament–channel structure by dealloying a 2.0 mm thick Zn–Cu slice that annealed at different temperatures in HCl (or HCl + NH4Cl)solution. In addition, Changchun Zhao also has mentioned that Mg50Cu50 can serve as precursor to fabricate bulk NPC under diluted hydrochloric acid condition but he did not describe the microstructure in detail [23]. In this work, 1.0 mm thick xAl(100 x)Cu (x = 60, 70 at.%) slices were applied to obtain bulk NPC through chemical dealloying at free corrosion conditions. The experimental results show that slices can be completely dealloyed across all given compositions. Uniform NPC can be obtained simply from synergetic dealloying 60Al40Cu slices which comprise two phases of Al2Cu and AlCu, and hierarchical skeleton-like structure (large-sized channels with highly porous walls) can be fabricated by dealloying Al-rich 70Al30Cu. The hierarchical skeletonlike structure forms layer-cake structure with uniform NPC. In addition, the mechanism of the layer-cake structure is studied. 2. Experimental xAl(100 x)Cu (x = 60, 70 at.%) alloy casts were prepared by arcmelting Cu and Al with purities of 99.9 wt.% in an argon atmosphere, then the casts were cut into slices of 1.0 mm in thickness by wire cut electrical discharge machine (WEDM). The slices were cleaned in an ultrasonic cleaner then dried in vacuum chamber. Then chemical dealloying experiments were conducted at room
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temperature in the open air, and Al–Cu slices were immersed in NaOH solutions with a concentration of 10 wt.%, 20 wt.% and 30 wt.%, respectively. They were numbered sequentially from sample 1–6 (Table 1). When no obvious bubbles emerged any longer and the solutions became clear again the dealloying process stopped. The residues were taken out of beakers and well rinsed with distilled water and dehydrated alcohol to remove the residual NaOH solutions and metal ions. The phase distributions in precursors were observed using an M-4XC metallographic microscope. The phases present in the Al–Cu alloys were confirmed by X-ray diffraction (XRD, D8 ADVANCE) with a Cu Ka irradiation. The morphology and structure of the dealloyed samples were characterized by a scanning electron microscope (SEM, FEI QUANTA FEG 250) coupled with an energy dispersive X-ray spectroscopy (EDS). Electrochemical experiments were carried out to study the microstructure evolution under a conventional three-electrode electrochemical workstation (GU/07345C) at room temperature. The electrochemical cell consisted of a platinum needle as the counter electrode and saturated calomel electrode (SCE) as a reference electrode, 0.05 M NaOH solution was applied as electrolyte. All potentials quoted are on the SCE scale unless otherwise stated.
3. Results and discussion Fig. 1 shows the XRD patterns of as-prepared Al–Cu alloy slices and a prototypical as-dealloyed bulk sample. The XRD results show that 70Al30Cu alloy is almost totally composed of Al2Cu phase and a trace Al4Cu9 phase also can be detected. However, AlCu phase emerges when the content of Al decreases to 60Al40Cu. It is obvious that the Al2Cu phase is dominant in the 70Al30Cu alloys, and the amount of Al2Cu is comparable to that of AlCu in the 60Al40Cu alloy, and these are in accordance with their diffraction peak intensities [20]. A prototypical XRD pattern of as-dealloyed sample is present at the top of Fig. 1. After dealloying, all of Al2Cu, AlCu and Al4Cu9 can be fully dealloyed in NaOH solution and a face centered cubic (fcc) Cu phase can be identified in the as-dealloyed bulk samples. Additionally, a minor amount of Cu2O that maybe caused by oxidation during preservation is detected in the as-dealloyed samples. The nanoporous structure of the bulk NPC slices is verified by scanning electron microscopy. Fig. 2 shows the plane-view and section-view microstructures of as-dealloyed 60Al40Cu alloy slices after dealloying in 10 wt.%, 20 wt.% and 30 wt.% NaOH solutions (the corresponding products are designed as samples 1–3, and the similar below), respectively. The plane-view of sample 1 shows a porous structure (Fig. 2a), the ligaments–channels structure is not obvious and many Cu nanoparticles with size of 50 nm can be observed. Fig. 2b shows the section-view image of sample 1 at a higher magnification, a typical uniform bi-continuous interpenetrating ligaments–channels structure with size of 50 nm can be observed. Many microcracks (tens of micrometers in length and sub-micrometer in width) can be observed on the fracture surface of the slice (insert image of Fig. 2b). For sample 2, the plane-view
Table 1 Dealloying time (h) of all samples from 1 to 6. NO.
Alloy (at.%)
CNaOH (wt.%)
Dealloying time (h)
1 2 3
60Al40Cu
10 20 30
29.33 31.83 80.0
4 5 6
70Al30Cu
10 20 30
20.33 20.33 44.0
217
Fig. 1. XRD patterns of as-prepared Al–Cu alloy slices and a prototypical asdealloyed bulk NPC.
and section-view images show a similar microstructure characteristic to that of sample 1. For the sample 3, Fig. 2e shows a porous structure, the ligaments–channels structure is slightly larger than that of samples 1 and 2, but the pores are still not well distinguishable. In addition, microcracks can also be observed on the surface of sample 3. However, the section-view microstructure shows a quite different structure compared with the former two samples i.e. a large sum of pores is blocked. Fig. 2 verifies that both Al2Cu and AlCu phases can be dealloyed throughout 60Al40Cu alloy slices and nanoporous structure can be obtained. Combining with XRD result, bulk NPC can be obtained via dealloying 60Al40Cu alloy slices in NaOH solutions. Table 1 shows the dealloying time of different alloys in various NaOH solutions. Samples 1 and 2 have a similar dealloying time of around 30 h, however, that of sample 3 is 90 h, much longer than that of samples 1 and 2. Fig. 2g and h shows the microstructures of sample 3 for dealloying 70 h. Fig. 2g exhibits a prototypical open, bi-continuous interpenetrating ligaments–channels structure with length scales of 100 nm, and the pores are obvious. A three-dimensional nanoporous structure can be observed in Fig. 2h, it is extremely different from that of sample 3. Therefore, significant coarsening occurs at later stage in concentrated alkali solution during such a long time dealloying, and that can be attributed to the fast surface diffusion [20,24]. Zhang et al. and Qi et al. have reported that length scales of ligaments–channels in nanoporous metals could be tuned by simply changing the dealloying solutions [16,21]. In this work, it is obvious that the ligaments–channels size increases with the increase of concentration (from 10 wt.% to 20 wt.%) of NaOH solution. For 60Al40Cu, finer microstructure can be obtained in 20 wt.% NaOH solution and a moderate dealloying time is required. Fig. 3 shows the microstructure of the bulk NPC by dealloying 70Al30Cu alloy slices in 10 wt.%, 20 wt.% and 30 wt.% NaOH solutions (samples 4–6), respectively. Obviously, traditional threedimensional bi-continuous interpenetrating ligaments–channels structure is fabricated throughout the whole alloy slices (Fig. 3b2, d1, d2 and f2). It is clear that ligaments coarse then bond to adjacent ones and the pores become vague as shown in the regions marked by the dotted ellipses in the inset image in Fig. 3a and outer parts of the irregularity closed curves in Fig. 3c. Obviously, the ligaments–channels of samples 4–6 in Fig. 3 are larger than those of samples 1–3 in Fig. 2. That indicates that the effect of alloy compositions (of the starting Al–Cu alloys) on the microstructure of bulk NPC outweighs that of dealloying solutions in case of the free corrosion of the Al–Cu alloys in the 10 wt.%, 20 wt.% and 30 wt.% NaOH solutions. It is in accordance with the
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(a)
(b)
400 nm (c)
20 μm
500 nm
(d)
500 nm (e)
20 μm
500 nm
(f)
500 nm
4 μm
(g)
500 nm (h)
4 μm
500 nm
Fig. 2. SEM images of the microstructures of as-dealloyed sample 1 (a, b), sample 2 (c, d), sample 3 (e, f) and 70 h-dealloyed sample 3 (g, h) by immersing 60Al40Cu slices in the 10 wt.%, 20 wt.% and 30 wt.% NaOH solutions for a certain time (see Table 1), respectively. (a, c, e and g) are the plane-view images and (b, d, f and h) are the section-view images of the four samples.
findings of Zhang et al. [25]. Changchun Zhao also found that compositions of melt-spun Mg–Cu alloys have an important effect on the dealloying process and microstructure of the NPC ribbons [23]. The sectional views of samples 4–6 indicate that they comprise nanoporous composites of uniform nanoporous structure and hierarchical skeleton-like structure which adheres to the surface of uniform structure (Fig. 3b, d–f). Obviously, the entire skeleton-like structure is porous and composed of lower hierarchy level ligaments (Fig. 3b1 and f1), namely, hierarchical skeleton-like structures are large-sized channels with highly porous walls. The
lower level ligaments–channels in Fig. 3b1 (30–40 nm) is smaller than those in Fig. 3f1 (60–70 nm). The skeleton-like structure obtained here is comparable to that obtained by Qi and Weissmüller [26], but this one-step dealloying is much easier to operate than corrosion-coarsening-corrosion process. In addition, uniform NPC layers are filled with hierarchical skeleton-like structure; the two structures form layer-cake structure as shown in Fig. 3d and insert image of Fig. 3e. It is generally recognized that ideal bi-continuous ligaments– channels, nanoporous structures can be obtained from binary
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(a)
(b)
(b1)
500 nm (b2)
500 nm
400 nm
(c)
4 μm
400 nm
d)
(d1)
2 μm d2)
20 μm
500 nm
(f)
(e)
400 μm
(f1)
500 nm
(f2)
300 μm
20 μm
5 μm
500 nm
Fig. 3. SEM images showing the microstructures of sample 4 (a, b), sample 5 (c, d) and sample 6 (e, f) by immersing 70Al30Cu slices in the 10 wt.%, 20 wt.% and 30 wt.% NaOH solutions for a certain time (see Table 1), respectively. (a, c, e and f) are the plane-view images of the three corresponding samples; (b, d) are the section-view images of samples 4 and 5; (b1, b2, d1, d2, f1 and f2) show the higher magnification images of selected areas in (b, d, f); insert image in (e) shows lower magnification appearance of sample 6.
alloys with a single-phase solid solubility across all compositions by chemical dealloying [9]. It is not one phase simply being excavated out of a two phase material but the dynamical mechanism during dissolution that impulse the porosity evolution [27]. In crystalline alloys systems, the dealloying process is expected to start with the selective dissolution of a single less noble atom on a flat alloy surface along closely-packed direction and leaves a pit. More noble atoms then diffuse about and start to agglomerate into islands. As the dealloying proceeds, dealloying propagates into inner part of the alloys, thus resulting in bi-continuous ligaments– channels structures [9]. In this work, bulk NPC is be obtained by means of selective dissolution of Al atoms and diffusion of Cu atoms from dealloying Al–Cu alloy slices. According to Al–Cu phase diagram [28], 60Al40Cu alloy is composed of Al2Cu and AlCu phases which are in accordance with XRD results. 70Al30Cu primarily comprises Al2Cu phase, moreover, a-Al solid solution is also observed in metallographs as shown in Fig. 4.
From Fig. 4a, 60Al40Cu alloy is of a peritectic microstructure with AlCu inside and Al2Cu outside and they are comparable to each other. From the EDX analysis of the site 1 marked in Fig. 4a, only Al and Cu elements are detected, and the atomic ratio of Al/Cu is almost 1 (Table 2). Therefore, the dark region can be confirmed as AlCu phase and the bright region surrounding AlCu phase is Al2Cu phase [29]. Fig. 4b confirms that Al2Cu phase is predominate and gap region is a-Al solid solution and Al2Cu phase is well split by a-Al layer by layer in some certain directions. In addition, aAl solid solution separates Al2Cu out (Fig. 4c). To learn about how these two alloys change during dealloying, electrochemical experiments are conducted. Potentiostatic dealloying tests were performed on the two precursors alloy with the applied potential of 0.6 V and the results are illustrated in Fig. 5. For both 60Al40Cu and 70Al30Cu alloys, from the view of whole process, current density initially increases then gradually decreases at later course. However, from Fig. 5b, there exist
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(a)
(b)
(c)
1
Fig. 4. Metalloscope photos showing different microstructures of (a) 60Al40Cu and (b, c) 70Al30Cu alloys. (d) Shows EDS spectra of site 1 in (a).
Table 2 EDX analysis of composition of AlCu phase. Elements (at.%)
Site 1
Al Cu
51.35 48.65
Fig. 5. Chronoamperometric curves of (a) 60Al40Cu and (b) 70Al30Cu alloys.
obvious current pulses where current density firstly gradually increases then suddenly drops. In addition, the time interval of each pulse is similar, suggesting that dealloying behavior is analogous about every 2000 s, namely, dealloying processes that possess a same characteristic repeat again and again until alloy is fully dealloyed. Fig. 4 has confirmed alternating layer structures of precursor 70Al30Cu alloy; therefore, it is rational to assume that this current pulse is closely related to distribution of a-Al and Al2Cu phases. According to Zhang’ report, the excavation of a-Al phase out of alloy contributes to the formation of large-sized channel [21]. The Al2Cu phase that is surrounded by a-Al is corroded and leaves small level pores. This accounts for how hierarchical skeleton-like structure forms. Because of its low standard potential ( 1.662 V vs. SHE for Al/Al3+), Al is easily etched. Therefore, the fast dissolution of a-Al should be responsible for the increase of current density in a pulse step. As soon as the a-Al is fully corroded, Al2Cu phase is exposed to solution and results in the sudden drop of current density, because of its larger critical potential ( 0.53 V [25]). When the dealloying proceeds, this cycle occurs iteratively and finally forms layer-cake structure NPC. And Fig. 6 shows the morphology of 70Al30Cu corresponding to this electrochemical test. It is obvious that skeleton-like structure is obtained, and next to it is as-corroded alloy frontier. The left bottom figure shows the morphology of microstructure formed by electrochemical
dealloying alloy front marked in Fig. 6. The porous structure is different from that obtained from chemical dealloying but matches well to Fig. 5b. In former reports, both Al2Cu and AlCu phases can be fully dealloyed in alkaline solutions because of their large potential difference of Al and Cu ( 1.662 V vs. SHE for Al/Al3+ and 0.342 V vs. SHE for Cu/Cu2+), and homogeneous porous structure can be synthesized due to the synergetic dealloying of Al2Cu and AlCu phase [16]. This is also applicable to account for the phenomena of 60Al40Cu alloy in this work. In addition, chronoamperometric curve in Fig. 5a shows a uniform reduction of current density that is caused by the increasing distance of alloy–liquid interface and corrosion frontier and the resultant low material exchange rate [23,30]. Fig. 7 shows chronoamperometric curves that show initial stage of potentiostatic dealloying 60Al40Cu and 70Al30Cu alloys under 0.6 V in 0.05 M NaOH solution. Obviously, both 60Al40Cu and 70Al30Cu process a decreasing current density trend during initial 100 s and current density increases at later period of the initial 10 min. The reason of this phenomenon may be the dissolution of oxide layers and breakdown of passivation films. The valley sitting around 1100 s can be regard as the first current pulse, which is followed by a series of pulses, as shown in Figs. 5b and 7b. Fig. 8 shows cyclic voltammetry (CV) curves of evolutions of dealloying of 60Al40Cu and 70Al30Cu alloys that subjected to 350 cycles under potentials from 1.4 V to 0.05 V and from 1.3 V to 0.9 V, respectively. Form Fig. 8a, in the first 10 cycles, the current density of the forward scan gradually increases then the curves does not change much until the 350th cycle in which
Fig. 6. SEM images show the microstructure of 70Al30Cu which suffered electrochemical dealloying at the potential of 0.6 V vs Ag/AgCl in 0.05 M NaOH solution.
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case of 70Al30Cu the CV curves are quite different (Fig. 8b). It is obvious that from the first cycle to the 5th cycle, current density rapidly increases until the 10th cycle after which current density gradually reduces immediately. Insert Figure in Fig. 8b shows the 1st cycle in larger magnification, and obviously, each of the initial five loop curves processes a crossover which separates the loop into two sections. At lower potential section, current density on the reverse scan is higher than that on the forward scan. At higher potential section, the situation is reverse. In addition, the position of crossover moves down with increasing cycles. However, the current density trend of the CV curves in Fig. 8b does not accord with the pulse that shown in Fig. 5b. Perhaps because of the potential range applied in CV tests is much smaller than the static potential exerted in potentiostatic dealloying test. And further works should be carried out to study the reasons of this asynchronous. 4. Conclusion Fig. 7. Chronoamperometric curves showing initial dealloying stage of (a) 60Al40Cu and (b) 70Al30Cu alloys.
In summary, xAl(100 x)Cu (x = 60, 70 at.%) alloy slices can be fully dealloyed in NaOH solutions (10 wt.%, 20 wt.% and 30 wt.%) under free corrosion conditions and bulk NPC is fabricated. For, 60AlCu40, the synergistic dealloying of Al2Cu and AlCu phases results in uniform porous structure. For 70Al30Cu, the dealloying of alternating a-Al and Al2Cu phases plays an important role in the formation of NPC with both hierarchical skeleton-like and uniform porous structures. In addition, hierarchical skeleton-like and uniform porous structures form layer-cake structure that is favorable to filtration. On the basis of the present results, this work will not only offer a help filling in dealloying framework but also provide guidance to process bulk alloys that contain noble elements. Acknowledgements The authors would like to acknowledge Natural Science Foundation of China (50871047 and 51271087) and National Nature Science Foundation (51301076) for this work. References
Fig. 8. CV curves showing evolutions of dealloying of (a) 60Al40Cu and (b) 70Al30Cu alloys subjected to 350 cycles, insert image in (b) shows details of the 1st cycle of 70Al30Cu.
current density declines clearly. This confirms that dealloying initially grows fast then gradually decreases, which is in coincidence with Fig. 5a. And the decrease of current density in the 350th cycle can be ascribed to the depletion of active Al atoms from the alloy. In reverse scan, reduction peaks is obvious and it can be attributed to the redeposition of dissolved Cu during dealloying [19]. In the
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