NANO LETTERS
Nanoporous Gold Prism Microassembly through a Self-Organizing Route
2006 Vol. 6, No. 4 882-885
Masataka Hakamada* and Mamoru Mabuchi Department of Energy Science and Technology, Graduate School of Energy Science, Kyoto UniVersity, Yoshidahonmachi, Sakyo, Kyoto, 606-8501 Japan Received February 1, 2006; Revised Manuscript Received March 3, 2006
ABSTRACT We report a simple and spontaneous synthesis of a nanoporous gold prism microassembly with highly dense skins, which is achieved just by immersing nanoporous gold into concentrated hydrochloric acid. The ligament size was coarsened to several hundred nanometers, but the crystal face orientation was still preserved. The same trends were seen in the case of coarsening by annealing; however, the morphology of the nanoporous gold prism microassembly was significantly different from the annealed nanoporous gold.
It has been known that nanoporous metals can be selforganized by dealloying or selective dissolution of a less noble component from a binary alloy.1-3 In particular, nanoporous gold is obtained by dealloying gold-silver binary alloy, where the silver atoms selectively dissolve into the etching solution.1,4-14 The obtained specimens have connected pores and ligaments whose sizes are approximately 10 nm or smaller. In addition, the apparent volume of a specimen hardly varies during dealloying, and a porosity of 70% or higher is attained. When an electrochemical potential is applied to promote dealloying, we can obtain nanoporous Au with dimensions as large as 2 mm,4 which may be a realistic size for various practical applications. Moreover, postprocessing treatments of the nanoporous Au, such as annealing and immersion in an acid solution, markedly vary the material’s volumetric porosity and pore (or ligament) size.4,5 There are some studies that attain more complex nanostructures by preparing a nanostructured Au-Ag alloy prior to dealloying.6-8 However, complicated techniques, such as nanowire fabrication by electrodeposition of a binary alloy6,7 and electroless plating of Ag using a reducing gas,8 are required in those procedures. These procedures may diminish the advantage of dealloying, that is, spontaneity. In this study, we report a simple and spontaneous synthesis of a nanoporous gold prism microassembly with highly dense skins, which is achieved just by immersing the nanoporous gold into a concentrated acid, inducing the surface diffusion of gold atoms. The pore (or ligament) size was coarsened to several hundred nanometers, but the crystal face orientation was still preserved during the process. The same trends were seen in the case of coarsening by annealing; however, the morphology of the nanoporous gold prism microassembly * To whom correspondence may be addressed. E-mail: hakamada@ g03.mbox.media.kyoto-u.ac.jp. 10.1021/nl0602443 CCC: $33.50 Published on Web 03/15/2006
© 2006 American Chemical Society
was significantly different from the annealed nanoporous gold coarsened by annealing. The flowing behavior of the acid solution into the nanoporous gold specimen seems to influence the peculiar morphology like tissue in organisms. We prepared nanoporous Au by typical dealloying of a 0.2-mm-thick Au30Ag70 alloy chip and subjected it to acid treatment as follows. Commercially available gold (>99.999%) and silver (>99.999%) shots were melted together by heating at 1373 K for 1 h to make a precursor Au30Ag70 alloy. Subsequently, the alloy ingot was annealed at 1173 K for 100 h for homogenization. It was then pressed and mechanically cut, and chips with the thickness of 0.2 mm were obtained. The alloy chips were then annealed at 1173 K for 12 h to relieve any residual stresses that may have occurred during pressing and cutting. All melting and annealing were conducted in an Ar flow. The electrochemical etching (or dealloying) of the alloy chips in 0.1 mol/L HClO4 solution was carried out in a three-electrode electrochemical cell with a platinum electrode as a counter electrode, a saturated calomel electrode as a reference electrode, and the alloy as a working electrode. An electrochemical potential of 1.1 V with respect to the reference electrode was applied to the alloy chips for 18 h at 298 K. The prepared nanoporous Au specimen observed by field-emission scanning electron microscopy (FE-SEM) is shown in Figure 1a. The ligament size of the nanoporous Au was approximately 10 nm, agreeing with the previous works.4 The dealloyed nanoporous Au chip was immersed in 50 mL of 35 mass % concentrated HCl solution in a beaker for 24 h at room temperature. As a result, nanoporous Au prisms with dense skins were spread throughout the entire specimen (panels b-e of Figure 1). The morphological features differ between the top and bottom surfaces (here, top means the surface exposed more to an acid solution and bottom means the surface which
Figure 1. FE-SEM images of nanoporous Au structures. (a) Nanoporous Au fabricated by dealloying of Au30Ag70 alloy. (b-e) Nanoporous Au coarsened by immersion into concentrated HCl solution for 24 h. (b) Top surface divided into “plots” by coarsened nanoporous walls. The typical side length of the “plots” was approximately from several micrometers to 20 µm. (c) Nanoporous walls at top surface. Ligament size was coarsened to 300-500 nm. (d) Bottom surface showing nanoporous structure enclosed in dense walls. (e) Tilted bottom surface showing prismlike structure.
touches the bottom of the beaker). Nanoporous walls, whose ligament sizes were increased to 300-500 nm, separated the top surface of the specimens into “plots” (panels b and c of Figure 1). The inside of the plots was hollow. On the other hand, at the bottom surface of the specimens, enclosure of the nanoporous structure inside highly dense walls was observed (Figure 1D). The ligament size of the inner porous structure was 300-500 nm, and the thickness of the enclosing walls was approximately 1 µm. Auger electron spectroscopy confirmed that both the walls and the internal nanoporous structures mainly consisted of pure gold. Typical side lengths of the plots were 5-20 µm at both surfaces. The FE-SEM image of a tilted specimen (Figure 1E) revealed that each plot had some depth toward the inside of the specimens, forming the nanoporous prismlike structure with dense walls. These prisms were approximately perpendicular to the specimen surface. Figure 2 shows low-magnification FE-SEM images of the top surface of the nanoporous Au before and after acid treatment, respectively. The as-dealloyed specimen had a network of cracks with a crack interval of several to 20 µm, reflecting the brittle nature of dealloyed nanoporous Au.4,15 Nano Lett., Vol. 6, No. 4, 2006
The plots of the acid-treated specimen had a geometrical distribution that is very similar to that of the network of cracks of the as-dealloyed specimen. For example, some plots of the acid-treated specimens had a very long side, which was also found in the plots surrounded by cracks of the asdealloyed specimen. Therefore, it is surmised that the nanoporous prism microassembly produced by acid treatment takes over the geometric features from the initial network of cracks of as-dealloyed nanoporous gold. Annealing as well as acid treatment causes coarsening of the porous structure or an increase in the ligament size of nanoporous gold.4 The as-dealloyed nanoporous gold with a network of cracks was annealed to examine the difference between the two coarsening processes. Observed FE-SEM images of the annealed specimens are shown in Figure 3. By annealing, ligament size was increased to 500-600 nm (Figure 3a), which is comparable to that of the acid-treated specimens. However, the effect of the annealed specimen on the initial cracks significantly differed from that of the acid-treated specimen. In the annealed specimen, cracks became vague (Figure 3b), and ligaments were bonded across the cracks (Figure 3c). That is to say, annealing tends to relieve the cracks. On the 883
Figure 2. FE-SEM images of nanoporous Au structures at lower magnification. (a) Nanoporous Au fabricated by dealloying of Au30Ag70 alloy. A network of cracks with a crack interval of 5-20 µm was observed. (b) Nanoporous Au coarsened by immersion in concentrated HCl solution for 24 h. Geometric features of the “plots” were very similar to the network of cracks observed in (a).
other hand, as shown in parts b-e of Figure 1 and Figure 2b, acid treatment clearly divided the specimen into prisms, enhancing the initial cracks. To examine the crystallographic feature of the obtained specimens, we conducted X-ray diffraction (XRD) analyses of the precursor Au-Ag alloy, as-dealloyed nanoporous Au and nanoporous Au coarsened by annealing or acid treatment (Figure 4). The crystal plane in a precursor Au30Ag70 was oriented to (220), probably due to pressing and annealing during its preparation. Only a broad diffraction peak between 75° and 80° was detected in the XRD pattern of as-dealloyed nanoporous Au, indicating that nanocrystallization of Au occurs during dealloying.16 Coarsened specimens showed XRD patterns similar to the precursor Au-Ag alloy. That is, they inherited the (220) crystal plane orientation from their precursor Au-Ag alloy, although dealloying markedly reduces the crystal grain size to the nanometer scale. The as-dealloyed sample still seems to have the (220) crystal face orientation; namely, the initial crystal face orientation is preserved during dealloying and coarsening. Inspection of Figure 4 also reveals that acid treatment and annealing, which induce significantly the different geometric self-organization of nanoporous Au, have almost the same trends as for crystal face orientation. Therefore, fundamental mechanisms of the two coarsening processes may be the same, at least from the viewpoint of crstallography. Coarsening of the porous structure can be attributed to the rapid surface diffusion of gold.4,5 Therefore, evolution of the 884
Figure 3. FE-SEM images of annealed nanoporous Au structures. The as-dealloyed nanoporous Au specimen was annealed at 873 K for 3 min in an Ar flow. (a) Porous structure coarsened to several hundred nanometers. (b) Vague trace of initial cracks which seem to be relieved. (c) Ligaments bonded across initial cracks.
nanoporous Au prisms may be caused by an anisotropic and dynamic change in permeability within the prisms during the immersion in a concentrated acid. It is conceivable that the coarsening acid solution flows along the initial cracks of the as-dealloyed specimen and that the outer sides of the prisms are initially and rapidly coarsened to prevent the following solution from penetrating into the prisms, resulting in the dense walls along the cracks and inner porous structure. The difference in geometric features between the top and the bottom surfaces (panels b and c of Figure 1) also supports the hypothesis that the behavior of acid solution flow influences the morphology of the nanoporous Au prisms. There is much room for study of this peculiar selforganization; effects of the concentration or viscosity of acid solution and immersion time are to be investigated. Gold is a face-centered-cubic metal ,and its lattice structure is very simple; therefore, its atomic bonding is essentially isotropic. Thus, it is very interesting to discover that the morphology of nanoporous Au can be anisotropically modified in such a simple and spontaneous way. The peculiar Nano Lett., Vol. 6, No. 4, 2006
chemical microreactors,22 microelectrodes,23 filters,24 microactuators,25 and catalyst support.26 Acknowledgment. We thank Dr. Y. Chino (National Institute of Advanced Industrial Science and Technology) for a component of specimen preparation and Dr. Y. Yamada (National Institute of Advanced Industrial Science and Technology) for FE-SEM observation. References
Figure 4. X-ray diffraction patterns of Au-Ag alloy and nanoporous Au specimens. Standard card data of gold (JCPDS) are shown together. The precursor Au-Ag alloy had a (220) crystal face orientation, which may be caused by pressing and annealing. The orientation was preserved during dealloying, annealing, and acid treatment, although the (220) peak in as-dealloyed specimens was significantly broadened by nanocrystallization.
structure is similar to tissue in organisms, for example, tissue in a cardiac muscle.17 It is worth noting that the complex tissuelike morphology was obtained through self-organization even in such a pure metal in this work. Therefore, the nanoporous Au prism microassembly has a potential for a variety of bioapplications, taking into account that gold is biocompatible. Moreover, nanoporous gold is mechanically far stronger than is supposed on the basis of a scaling law, because the ligament size in the nanometer range presumably causes a dislocation starvation.18-20 The nanoporous prisms with dense walls may obtain further strength, similar to a foam-filled tube.21 The oriented geometry and large specific surface area of the prism assembly are also appropriate for
Nano Lett., Vol. 6, No. 4, 2006
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