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J. Phys. Chem. C 2009, 113, 6694–6698
Formation and Characterization of Monolithic Nanoporous Copper by Chemical Dealloying of Al-Cu Alloys Zhen Qi, Changchun Zhao, Xiaoguang Wang, Jikui Lin, Wei Shao, Zhonghua Zhang,* and Xiufang Bian The Key Laboratory of Liquid Structure and Heredity of Materials, Shandong UniVersity, Jingshi Road 73, Jinan 250061, People’s Republic of China ReceiVed: December 6, 2008; ReVised Manuscript ReceiVed: February 20, 2009
Monolithic nanoporous copper (NPC) ribbons can be fabricated through chemical dealloying of melt-spun Al-Cu alloys with 33-50 at % Cu under free corrosion conditions. The microstructure of these NPC ribbons was characterized using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, and transmission electron microscopy. The experimental results show that the melt-spun Al-Cu alloys with 33-50 at % Cu are composed of one or a combination of Al2Cu and AlCu intermetallic compounds. Both Al2Cu and AlCu can be fully dealloyed, and the synergetic dealloying of Al2Cu and AlCu in the two-phase Al-Cu alloys results in the formation of NPC with a homogeneous porous structure. The NPC ribbons exhibit an open, bicontinuous interpenetrating ligament-channel structure. NPC is a promising high strength/low density material due to its high porosity and yield strength of 86 ( 10 MPa. In addition, bulk NPC rods and slices can also be synthesized using the same strategy. These NPC ribbons, rods, and slices can serve as model materials to investigate the mechanical, physical (for example, electrical resistivity), and chemical properties associated with random porous structure of nanoporous solids. Introduction Nanoporous metals with a high surface area have attracted great attention in many technological applications, such as catalysis,1 sensors,2 actuators,3 fuel cells,4 microfluidic flow controllers, and so forth. Nanoporous metals can be fabricated by the process called dealloying, which refers to the selective dissolution of one or more components out of an alloy.5 For example, nanoporous gold (NPG) can be synthesized through chemical/electrochemical dealloying of Ag-Au alloys.5 Historically, dealloying has received significant attention in the context of corrosion but has recently been receiving renewed attention due to the fact that certain particular systems exhibit nanoporosity evolution upon dealloying.6 The dealloying process results in the formation of a bicontinuous structure of metal-and-void with an average metal ligament diameter (or pore diameter) as small as 3 nm.7 In addition, the widely used Raney-type catalysts, treated in an alkaline to extract Al from a binary alloy system, are also manufactured by dealloying.8 Raney copper, which is a well-known catalyst in the water-gas shift (WGS) reaction, is produced by selective removal of Al from CuAl2 or Cu-Al-Zn alloys using a NaOH solution.9 It is generally recognized that nanoporous metals with ideal bicontinuous structures are obtained from binary alloys with a single-phase solid solubility across all compositions (like Ag-Au).5 Recently, researchers have focused on the preparation of nanoporous metals in multiphase and amorphous alloy systems by electrochemical/chemical dealloying. It has been reported that nanoporous metals can be obtained by electrochemical deposition and dealloying of Zn-based surface alloys in a zinc chloride-1-ethyl-3-methyl chloride (ZnCl2-EMIC) ionic liquid.10-13 Thorp et al.14 reported the synthesis of nanoporous Pt thin films on Si by electrochemical dealloying of amorphous * To whom correspondence should be addressed. E-mail: zh_zhang@ sdu.edu.cn.
PtxSi1-x alloys in aqueous HF solutions. Lu et al.15 reported that porous copper can be synthesized from nanocrystalline twophase Cu-Zr films by electrochemical dealloying. To date, however, less attention has been paid to the synthesis of monolithic nanoporous copper (NPC). Hayes et al.16 reported the fabrication of monolithic NPC by chemical/electrochemical dealloying of a single-phase Mn0.7Cu0.3 solid solution. For the synthesis of Raney copper, the composition of the starting Al-Cu alloys is not more than 33.3 at % Cu (Al2Cu).9 Furthermore, the Raney copper is nonmonolithic. Here, we show that monolithic NPC can be fabricated through electronless dealloying of Al-Cu alloys with a composition range of 33-50 at % Cu. Moreover, the uniform porous structure of NPC can be obtained from one or a combination of Al2Cu and AlCu intermetallic compounds by dealloying. In addition, the porous structure of NPC can be modulated by simply changing the dealloying solution. Experimental Section Al-Cu alloys with nominal compositions of 33, 35, 40, and 50 at % Cu were prepared from pure Al (99.9 wt %) and pure Cu (99.9 wt %). High-frequency induction heating was employed to melt the charges in a quartz crucible, and then the melt was cast into ingots in an iron chill mold. By use of a single roller melt spinning apparatus, the Al-Cu ingots were remelted in a quartz tube by high-frequency induction heating and then melt-spun onto a copper roller at a circumferential speed of ∼ 27 m s-1. The ribbons obtained were typically 30-60 µm in thickness, 2-4 mm in width, and several centimeters in length. In addition, the Al-Cu melt was cast into rods (2 mm in diameter) and slices (1 mm in thickness) by blow casting. The dealloying of the melt-spun Al-Cu ribbons was performed in a 5 wt % HCl aqueous solution at 90 ( 5 °C (condition I). The total dealloying time ranged from 2 to 4 h, depending upon
10.1021/jp810742z CCC: $40.75 2009 American Chemical Society Published on Web 03/31/2009
Monolithic Nanoporous Copper
Figure 1. XRD patterns of melt-spun Al-Cu alloy ribbons (A) before and (B) after dealloying (condition I). (a-d) Al 33, 35, 40, and 50 at % Cu, respectively.
the thickness of the ribbons and alloy compositions. The Al-Cu ribbons, rods, and slices were also dealloyed in a 20 wt % NaOH aqueous solution first at room temperature and then at 90 ( 5 °C. After rinsing with distilled water, the samples were continuously treated in the 5 wt % HCl solution at 90 ( 5 °C (designated as two-step dealloying, condition II). The total dealloying duration was 1-2 h for the melt-spun ribbons and more than 10 h for the as-cast rods and slices. All reactions were carried out until no obvious bubbles emerged. After dealloying, the samples were rinsed with distilled water and dehydrated alcohol. The as-dealloyed samples were kept in a vacuum chamber to avoid oxidation. Microstructural characterization and analysis of the Al-Cu alloys and as-dealloyed samples were made using X-ray diffraction (XRD, Rigaku D/max-rB) with Cu KR radiation, scanning electron microscopy (SEM, LEO 1530VP) with an energy dispersive X-ray analyzer (EDX), and transmission electron microscopy (TEM, Philips CM 20). Nanoindentation experiments were carried out to determine yield strength of NPC using a MTS Nanoindenter XP. Results and Discussion Figure 1A shows the XRD patterns of the starting melt-spun Al-Cu alloy ribbons. The filled circles and squares in Figure 1A stand for Al2Cu and AlCu, respectively. The melt-spun Al 33 at % Cu alloy consists of a single Al2Cu phase (part a of Figure 1A). The melt-spun Al 35, 40, and 50 at % Cu alloys are composed of two phases: Al2Cu and AlCu (parts b-d of Figure 1A). The Al2Cu phase is dominated in the Al 35 at % Cu alloy, but a minor amount of Al2Cu is detected in the Al 50 at % Cu alloy. In addition, the amount of Al2Cu is comparable to that of AlCu in the Al 40 at % Cu alloy. After dealloying (condition I), only a face-centered cubic (fcc) Cu phase can be identified in the as-delloyed samples (Figure 1B). Moreover,
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Figure 2. SEM images showing the microstructure of NPC by dealloying of (a) Al 33 at % Cu, (b and c) Al 35 at % Cu, (d and e) Al 40 at % Cu, and (f) Al 50 at % Cu alloys in the 5 wt % HCl solution (condition I). Parts a, c, e, and f are the section view; insert in (b) and part d show the plane view.
the two-step dealloying (condition II) of the Al-Cu alloys also results in the formation of the single fcc Cu phase. Figure 2 shows the porous structure of as-dealloyed samples after delloying in the 5 wt % HCl solution (condition I). For the melt-spun Al 33 at % Cu alloy, the as-dealloyed ribbons are porous and contain nanoparticles with sizes ranging from one to several hundred nanometers (Figure 2a). Moreover, large particles with a size up to one micron can be observed (one is highlighted by an arrow in Figure 2a). Despite the monolithic characteristic, the NPC ribbons are brittle and soft, and their shape cannot be well preserved. Figure 2b shows the porous structure of NPC by dealloying of the melt-spun Al 35 at % Cu alloy. The NPC ribbons consist of nanoparticles and some ligaments (100-300 nm in size, Figure 2c). Furthermore, the shape of the ribbons can be well kept, and no microcracks can be observed in the NPC ribbons as shown in the insert of Figure 2b. For the melt-spun Al 40 and 50 at % Cu alloys, the asdealloyed ribbons exhibit an open, three-dimensional bicontinuous interpenetrating ligament-channel structure with length scales of 100-300 nm (for Al 40 at % Cu) and 300-500 nm (for Al 50 at % Cu), as shown in parts d-f of Figure 2. Figure 2d shows the plane-view microstructure of the NPC ribbons by dealloying of the melt-spun Al 40 at % Cu alloy, and the sectionview microstructure of the NPC ribbons is presented in Figure 2e. It is clear that a uniform porous structure can be obtained in the NPC ribbons through dealloying of the melt-spun Al 40 and 50 at % Cu alloys. In addition, TEM observation further verifies the porous structure of the NPC ribbons and one typical TEM bright-field image is shown in Figure 3a. The selectedarea electron diffraction (SAED) pattern consists of polycrystalline rings, corresponding to fcc Cu (111), (200), (220), and (311) reflections (Figure 3b). Moreover, the pattern from a smaller selected area (1 µm) demonstrates that the ligaments in the NPC ribbons comprise fcc Cu nanocrystals (Figure 3c).
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Figure 4. Section-view SEM images showing the microstructure of nanoporous copper by a two-step dealloying (condition II) of (a and b) Al 35 at % Cu, (c and d) Al 40 at % Cu, and (e) Al 50 at % Cu alloys. Insert in (a) plane-view SEM image. (f) EDX spectrum of NPC by dealloying of the Al 40 at % Cu alloy (condition I).
Figure 3. (a) TEM image showing the porous structure of NPC by dealloying of the Al 40 at % Cu alloy (condition I). (b and c) SAED patterns corresponding to part a. The selected area (in diameter) is 5 µm for b and 1 µm for c.
Figure 4 shows the porous structure of the NPC ribbons by the two-step dealloying (condition II) of the melt-spun Al-Cu alloys. For the Al 35 at % Cu alloy, the porous structure of the as-dealloyed ribbons is different from that of the NPC ribbons by dealloying in the HCl solution (Figure 4a). Many cracks (tens of micrometers in length and sub-micrometers in width) can be observed on the surface of the ribbons (insert of Figure 4a). Moreover, the margin of the ribbons shows a typical ligamentchannel structure, differing from that of the center of the ribbons (Figure 4b). For the melt-spun Al 40 and 50 at % Cu alloys, a uniform ligament-channel structure can be obtained in the asdealloyed samples (parts c-e of Figure 4). It can be seen from Figure 4c that the uniform porous structure runs throughout the whole ribbons. Furthermore, the SEM images at a higher magnification clearly show that the length scales of ligaments/ channels in these two NPC ribbons are comparable to those of NPC by one-step dealloying in the HCl solution (parts d and e of Figure 4). EDX analysis has been performed on all the NPC ribbons, and one typical spectrum is shown in Figure 4f. It is obvious that all of Al was removed from the Al-Cu alloys during dealloying. NPG (by dealloying of Ag-Au alloys) normally contains some residual Ag (several atomic percent). Moreover, the residual Ag can not be removed but asymptotically reaches a limit at exhaustively long etching times (up to 100 h).17 It is generally recognized that ideal bicontinuous nanoporous structures are obtained from binary alloys with a single phase solid solubility across all compositions by chemical/electro-
chemical dealloying.6 If multiple phases exist in an AxB1-x alloy (Here, A is a less noble element and B is a more noble element.), typically only the A-rich phase would dealloy.7 If one phase can be dealloyed and another not, 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.18 If one phase is entirely dissolved and another is partly corroded, the dealloying of a two-phase alloy leads to nanoporous metals with bimodal channel size distributions. In our former work, nanoporous gold ribbons composed of large sized channels (hundreds of nm) with highly porous channel walls were synthesized from Al-Au alloys by dealloying.19 The present results demonstrate that both Al2Cu and AlCu can be fully dealloyed in acid or alkaline solutions due to large standard potential difference of Al and Cu (-1.662 V standard hydrogen electrode (SHE) for Al/Al3+ and 0.342 V SHE for Cu/Cu2+), resulting in the formation of NPC. Furthermore, NPC with a uniform porous structure can be synthesized through dealloying of two-phase Al-Cu alloys composed of Al2Cu and AlCu (parts d-f of Figure 2 and parts c-e of Figure 4). Obviously, the synergetic dealloying of Al2Cu and AlCu in these alloys is important for the formation of NPC with a homogeneous structure. The length scales of ligaments/channels in NPC are much larger than those in NPG (typically 5-50 nm, depending upon dealloying conditions). Surface diffusion of more noble elements along alloy/solution interfaces during dealloying plays a key role in the formation of NPG and has a significant influence on the length scales of ligaments/channels.5,6 The surface diffusivity of Cu in electrolyte is of the order of 10-10 cm2/s, much faster than that of Au (of the order of 10-14 cm2/s).6 Ding et al.20 have reported that significant structure coarsening occurs in NPG upon continued immersion in concentrated nitric acid solution.
Monolithic Nanoporous Copper It should be noted that this acid-induced structure coarsening is obvious during the immersion of the first hour but markedly slows down with further increasing immersion time (up to 10 days). 20 The duration of the one-step dealloying in the 5 wt % HCl solution is normally double that of the two-step dealloying, but the length scales of ligaments/channels are comparable in the NPC ribbons fabricated by these two dealloying processes. In addition, the dealloying solution used in this work is the dilute HCl. Therefore, surface diffusion of Cu plays a dominant role in the formation of NPC and weakens the influence of the dealloying duration on the length scales of ligaments/channels in NPC. Although the porous structure of NPC can be adjusted (compare Figure 2b with Figure 4a), the length scales of ligaments/channels in NPC can not be tuned by changing the dealloying solution. In our former work, we have found that the length scales of ligaments/channels in NPG can be tuned by simply changing the dealloying solution from 5 wt % HCl to 20 wt % NaOH.19 It has been reported that Cl- can increase the surface diffusion rate of Au and plays a significant role in the coarsening of ligaments/channels during dealloying.21,22 For Cu with faster surface diffusion, however, the influence of Clon the length scales of ligaments/channels in NPC is not so significant. The large amount of fine grains and boundaries increase the density of diffusion path for the dissolution of Zr and the rearrangement of Cu and thus play a key role in the formation of a porous Cu through electrochemical dealloying of a nanocrystalline dual-phase 62Cu-38Zr film. In contrast, no porous Cu could be obtained from the coarse-grain 62Cu-38Zr alloy.15 Parida et al.23 have reported that the crystal lattice orientation is retained in the resulting NPG during dealloying and with the conservation of the grain size of the master alloy (Ag-Au alloys). We also found that the morphology, size, and orientation of grains in melt-spun Al-Au alloys can be conserved into the resulting NPG ribbons during dealloying.19,24 Vice versa, the grain size of the melt-spun alloys can be estimated from the NPG ribbons. The electrochemical dealloying of amorphous PtxSi1-x alloys leads to the formation of nanoporous Pt films with Pt nanocrystals of only 3-5 nm.14 The amorphous nature (no long-range order) of the staring PtxSi1-x alloys may has an effect on the formation of ultrafine Pt nanocrystals in the nanoporous films. Ultrafine Pd/Pt nanocrystals of 3-5 nm can also be observed in nanoporous Pd/Pt ribbons through chemical dealloying of melt-spun Al-Pd/Al-Pt alloys.24 The size of grains in the melt-spun Al-Pd/Al-Pt alloys is comparable to that in the melt-spun Al-Au alloys. Therefore, the microstructure (size, morphology, and orientation of grains) in the melt-spun Al-Pd/Al-Pt alloys has no dominant influence on the formation of ultrafine Pd/Pt nanocrystals, and the grain characteristic cannot be preserved into the resulting nanoporous Pd/Pt ribbons. In the present work, the SAED results demonstrate that the ligaments in the NPC ribbons are composed of fcc Cu nanocrystals (tens of nanometers, Figure 3). The grain size of the melt-spun Al-Cu alloys is comparable to that of the melt-spun Al-Au alloys (from sub-micrometers to several micrometers depending upon species and compositions of alloys, rotation speed of the roller, and so forth), 1-2 orders of magnitude larger than that of Cu nanocrystals in the ligaments. It is obvious that the grain characteristic of the starting Al-Cu alloys cannot be preserved into the resulting NPC. In addition, the microstructure of the quenching side (contacting with the roller) is finer than that of the air side of the melt-spun ribbons. If the grain size of the melt-spun Al-Cu alloys had a significant influence on the formation of NPC, the microstructure of NPC
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Figure 5. Macrograph showing the NPC ribbon, rod, and slice by the two-step dealloying of Al 40 at % Cu (from left to right).
should be inhomogeneous. However, a homogeneous porous structure can be observed throughout the whole NPC ribbons (Figure 4c). It is true that fine grains of the melt-spun Al-Cu alloys significantly enhance the dissolution process of Al and the rearrangement of Cu and therefore accelerate the formation of NPC ribbons. The nonmonolithic characteristic of Raney metals is inconsequential to their applications as a catalyst, since the main property required is high surface area. However, the mechanical integrity of nanoporous metals is important for their structural properties and some applications because uniformity and continuity are key requirements. Hayes et al.16 found cracking to be unavoidable in their nanoporous copper. Although cracks are observed in the NPC ribbons by the two-step dealloying of Al 35 at % Cu (Figure 4a and insert), no cracks are found in other NPC ribbons. The present NPC ribbons are monolithic and can be easily handled. In addition, bulk NPC rods and slices can also be fabricated by dealloying of blow-cast Al-Cu alloys (Figure 5). These NPC ribbons and bulk NPC rods/slices can serve as model materials to investigate the mechanical, physical (for example, electrical resistivity), and chemical properties associated with random porous structure of nanoporous solids. In addition, the present NPC ribbons can be loaded and bended up to 180°. For example, the hardness (or yield strength25) was determined by nanoindentation tests to be 86 ( 10 MPa for the NPC ribbons by one-step dealloying of the melt-spun Al 50 at % Cu alloy, higher than that (55 MPa26) of fully annealed polycrystalline Cu. This suggests that NPC is a promising high strength/low density material. Hayes et al.16 reported a yield strength of 128 ( 37 MPa for NPC with a ligament diameter of 135 ( 31 nm. It is obvious that the yield strength of NPC depends upon the length scales of ligaments/channels. The larger length scales (300-500 nm) of ligaments/channels are the main reason for the lower yield strength of the present NPC ribbons, compared to the yield strength of NPC reported by Hayes et al.,16 Mathur and Erlebacher,27 and Biener et al. 25 recently reported the size effects on the mechanical behavior of NPG. Conclusions In summary, monolithic NPC ribbons can be fabricated by chemical dealloying of rapidly solidified Al-Cu alloys with a composition range of 33-50 at % Cu under free corrosion conditions. Moreover, bulk NPC rods/slices can also be obtained through dealloying of Al-Cu alloys. Both Al2Cu and AlCu intermetallic compounds can be completely dealloyed, and the synergetic dealloying of Al2Cu and AlCu in two-phase Al-Cu alloys plays a key role in the formation of NPC with a
6698 J. Phys. Chem. C, Vol. 113, No. 16, 2009 homogeneous porous structure. The fast surface diffusion of Cu along alloy/solution interfaces results in the formation of NPC with large length scales of ligaments/channels (100-500 nm). The monolithic characteristic of the present NPC is crucial for its properties and applications. Nanoporous copper is a promising high strength/low density material, because of its high porosity and yield strength of 86 ( 10 MPa. On the basis of the present results, new alloy systems can be developed to fabricate novel nanoporous metals through chemical or electrochemical dealloying. The dealloying mechanism should be clarified, especially for the dealloying of intermetallic compounds and multiphase alloys. Acknowledgment. We give thanks for financial support from the National Natural Science Foundation of China (50701028 and 50801031), Excellent Middle-age and Young Scientists ResearchAwardFoundationofShandongProvince(2007BS04024), and 43rd China Postdoctoral Science Foundation. The experimental assistance from Ruhr University Bochum (Germany) is acknowledged. References and Notes (1) Bond, G. C.; Thompson, D. T. Catal. ReV. 1999, 41, 319. (2) You, T.; Niwa, O.; Tomita, M.; Hirono, S. Anal. Chem. 2003, 75, 2080. (3) Weissmueller, J. R.; Viswanath, N.; Kramer, D.; Zimmer, P.; Wuerschum, R.; Gleiter, H. Science 2003, 300, 312. (4) Joo, S. H.; Choi, S. J.; Kwa, K. J.; Liu, Z. Nature 2001, 412, 169. (5) Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Nature 2001, 410, 450.
Qi et al. (6) Erlebacher, J. J. Electrochem. Soc. 2004, 151, C614. (7) Pugh, D. V.; Dursun, A.; Corcoran, S. G. J. Mater. Res. 2003, 18, 216. (8) Raney, M. U. S. Patent No. 1563587, 1925. (9) Smith, A. J.; Trimm, D. L. Annu. ReV. Mater. Res. 2005, 35, 127. (10) Huang, J. F.; Sun, I. W. Chem. Mater. 2004, 18, 1829. (11) Yeh, F. H.; Tai, C. C.; Huang, J. F.; Sun, I. W. J. Phys. Chem. B 2006, 110, 5215. (12) Lin, Y. W.; Tai, C. C.; Sun, I. W. J. Electrochem. Soc. 2007, 154, D316. (13) Huang, J. F.; Sun, I. W. AdV. Funct. Mater. 2005, 15, 989. (14) Thorp, J. C.; Sieradzki, K.; Tang, L.; Crozier, P. A.; Misra, A.; Nastasi, M.; Mitlin, D.; Picraux, S. T. Appl. Phys. Lett. 2006, 88, 033110. (15) Lu, H. B.; Li, Y.; Wang, F. H. Scripta Mater. 2007, 56, 165. (16) Hayes, J. R.; Hodge, A. M.; Biener, J.; Hamza, A. V.; Sieradzki, K. J. Mater. Res. 2006, 21, 2611. (17) Dixon, M. C.; Daniel, T. A.; Hieda, M.; Smilgies, D. M.; Chan, M. H. W.; Allara, D. L. Langmuir 2007, 23, 2414. (18) Jin, H.; Kramer, J. D.; Ivanisenko, Y.; Weissmu¨ller, J. AdV. Eng. Mater. 2007, 9, 849. (19) Zhang, Z. H.; Wang, Y.; Qi, Z.; Lin, J. K.; Bian, X. F. J. Phys. Chem. C 2009, 113, 1308. (20) Ding, Y.; Kim, Y. J.; Erlebacher, J. AdV. Mater. 2004, 16, 1897. (21) Newman, R. C.; Sieradzki, K. Science 1994, 263, 1708. (22) Hakamada, M.; Mabuchi, M. Nano Lett. 2006, 6, 882. (23) Parida, S.; Kramer, D.; Volkert, C. A.; Ro¨sner, H.; Erlebacher, J.; Weissmu¨ller, J. Phys. ReV. Lett. 2006, 97, 035504. (24) Zhang, Z. H.; Wang, Y.; Qi, Z.; Frenzel, J.; Bian, X. F. J. Phys. Chem. C xx, xxxx. (25) Biener, J.; Hodge, A. M.; Hayes, J. R.; Volkert, C. A.; ZepedaRuiz, L. A.; Hamza, A. V.; Abraham, F. F. Nano Lett. 2006, 6, 2379. (26) Wang, Y. M.; Chen, M. W.; Zhou, F. H.; Ma, E. Nature 2002, 419, 912. (27) Mathur, A.; Erlebacher, J. Appl. Phys. Lett. 2007, 90, 061910.
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