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http://www.paper.edu.cn source materials were heated to between 60 and 200 C. The pressure which porosity results naturally from open molecular scaf- in the source vessels was between 80 and 196 hPa. The carrier gas was folds, as in zeolites; or synthetic, in which porous materials are Ar and the flow rate was in the range 70±300 sccm, depending on the made by employing self-assembly processes at some stage of source materials, which were loaded individually into the source vessels. The pipes connected to the reactor were heated to above fabrication. Examples of synthetic porous materials include 200 C in order to avoid vapor condensation. Oxygen was supplied to those made by casting into the interstices ofmicrophase-sepa- the reactor at 36 hPa and the total pressure in the reactor was 65 hPa. rated block copolymers,[1] colloidal crystals,[2] self-assembled The deposition temperature was 800 C. The as-prepared thin films surfactants,[3] and even biologically formed porous skeletal had a cation composition ofBi/Sr/Ca/Cu = 1:1:1:(1.5±1.7). The compo- [4] sition and thickness ofthe thin filmswere determined by inductively structures. Materials that take advantage ofpattern-forming coupled plasma atomic emission spectroscopy (ICP-AES) (SPS 7700, instabilities during etching may also be termed synthetic. Such Seiko Instruments Inc.). X-ray diffraction patterns (D500, Siemens) materials include porous alumina[5] and porous silicon;[6] we have shown that that films were epitaxial, c-axis aligned, and formed may include Vycor[7] and other porous glasses in the latter from the Bi2Sr2Ca2Cu3O10±x phase. The morphology ofthe thin films was inspected by optical microscopy on large areas, and by atomic category because they are formed by three-dimensional (3D) force microscopy (AFM) (SPA 300, Seiko Instruments Inc.) locally. spinodal decomposition phase separation into a mesoscale The superconductivity ofthe thin filmswas checked by the standard bicontinuous structure prior to selective etching ofone glass four-probe method. phase. All ofthese techniques result in precise control over Received: May 14, 2004 the pore size and microstructure ofthe porous material of Final version: September 9, 2004 interest, but it is generally true that the dominant length scale ± ofthe finalporous structure is ªburned-inº, and dynamic con- trol ofthe length scale is virtually impossible. In addition, [1] K. Endo, H. Sato, H. Akoh, Physica C 2002, 378, 1314. [2] K. Endo, H. Sato, K. Yamamoto, T. Mizukoshi, T. Yoshizawa, there are generally very few methods of making nanoporous K. Abe, P. Badica, J. Itoh, K. Kajimura, H. Akoh, Physica C 2002, metals that do not rely on a sacrificial porous scaffold or tem- 372, 1075. plate on which the metal is plated. Here, we report our [3] K. Endo, P. Badica, J. Itoh, Physica C 2003, 386, 323. research into the fabrication and characterization of thin, [4] Y. Ogimoto, M. Izumi, A. Sawa, T. Manako, H. Sato, H. Akoh, metallic, mesoporous membranesÐnanoporous gold leaf M. Kawasaki, Y. Tokura, Jpn. J. Appl. Phys., Part 2 2003, 42, L369. (NPGL). NPGL has an unusual combination ofcharacteristics [5] K. S. Takahashi, A. Sawa, Y. Ishii, H. Akoh, M. Kawasaki, Y. To- kura, Phys. Rev. B 2003, 67, 094 413. for a porous material in that it is formed by a spontaneous [6] K. Endo, S. Hayashida, J. Ishiai, Y. Matsuki, Y. Ikedo, S. Misawa, pattern-forming instability during etching, is metallic, and its S. Yoshida, Jpn. J. Appl. Phys., Part 2 1990, 29, L294. porosity may be adjusted via simple room temperature post- [7] K. Endo, H. Yamasaki, S. Misawa, S. Yoshida, K. Kajimura, Nature processing. 1992, 355, 327. Central to the formation of NPGL is the chemical etching [8] H.Sato, H. Akoh, K. Nishihara, M. Aoyagi, S. Takada, Jpn. J. Appl. Phys., Part 2 1992, 31, L1044. process called dealloying, which is the selective dissolution of one or more components from a metallic solid solution.[8] Dealloying has an ancient history. The Incan civilization deal- loyed copper from the surface of copper-rich Cu/Au alloys to create an illusion ofa pure gold artifact,a process generally Nanoporous Gold Leaf: ªAncient known as depletion gilding. Variations ofdepletion gilding Technologyº/Advanced Material** were independently developed in Europe and used by medie- val artisans.[8±10] Through pioneering work in the 1960s and By Yi Ding, Young-Ju Kim, and Jonah Erlebacher* 1970s, particularly by Forty[9] and Pickering,[11] we now under- stand that depletion gilding ofthe less-noble component from New porous materials are ofcritical importance in many Cu/Au and Ag/Au alloys is a dealloying process that results in technological applications, such as catalysis, sensing, and fil- an open bicontinuous nanoporous microstructure comprised tration, and thus there is a continued interest in the discovery almost entirely ofgold. The unusual aspect ofthe appearance ofnew porous materials, new methods to make them, and ofporosity during dealloying is that the resultant structure is new techniques to process them into useful forms. Generally, formed dynamically during the etching process, and is not due highly porous materials may be categorized as intrinsic, in to simple excavation ofone phase froma two-phase bicontin- uous microstructure, as is the case in the production ofporous glasses. ± Recently, Erlebacher et al.[12,13] presented an analytical at- omistic model that clarified the underlying physics of porosity [*] Prof. J. Erlebacher, Y. Ding, Y.-J. Kim Department of Materials Science and Engineering evolution during dealloying. The model involves a kind of Johns Hopkins University ªinterfacial phase separationº in which gold atoms not dis- Baltimore, MD 21218 (USA) solved from the alloy/electrolyte interface tend to cluster and E-mail: [email protected] form islands, rather than uniformly distribute themselves over [**] We thank Dr. Mingwei Chen for TEM assistance, and Anant Mathur for useful discussions. This work is supported by the NSF under the surface (which would passivate the interface and stop grants DMR-0092756, DMR-0080031, and CTS-0304062. further etching). This process continuously opens up regions

Adv. Mater. 2004, 16, No. 21, November 4 DOI: 10.1002/adma.200400792  2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1897

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ofvirgin alloy, and allows the dissolution frontto penetrate constituent gold and silver materials costs alone are nearly a through the bulk ofthe material. For our purposes here, we factor of ten smaller, USD 0.0008 cm±2, as computed using emphasize two aspects ofthe model. 1) At the metal/electro- current market prices ofgold and silver at the time ofwriting. lyte interface, diffusion of gold atoms is extremely fast com- White-gold leaf is easily transferred from one liquid surface pared to surface diffusion in other environments, such as in to another using graphite rollers, as shown in Figure 1a. Float- vacuum. This is a relatively unappreciated phenomenon, but ing the leafonto concentrated nitric acid (70 %) results in a has been well-characterized.[14] 2) Because porosity formation free-standing nanoporous gold membrane. This membrane is depends only on interfacial kinetics, uniform porosity evolu- tion is predicted for dissolution of even very thin alloy films. This prediction is contrary to earlier models ofporosity evolu- tion that invoke the presence ofanomalous highly mobile bulk defects (ªdi-vacanciesº) that serve to transport silver from deep in the alloy to the surface.[11] Little attention has been paid to the fact that dealloying can be used to generate very useful nanoporous metals.[15,16] The reason for this may lie in the historical focus of dealloying as a corrosion process Ð dealloying ofstainless steels and brasses may lead to stress corrosion cracking and undesirable materi- als failure.[17,18] A second reason may be that the use ofgold alloys is often expensive. In our research, we have been look- ing for highly porous and highly conductive large-area elec- trodes for use in electrocatalysis.[19] In this particular applica- tion, it is beneficial to have a very thin free-standing porous electrode (~ 100 nm thick) that may be attached to a variety ofsubstrates, and we suspected that nanoporous gold would be a good candidate material. We experimented with vapor- deposited alloy films, but vapor deposition is generally incom- patible with some ofthe polymeric substrates we would like to use. Secondly, even though uniform porosity evolution was Figure 1. a,b) Optical and c±e) SEM images of white-gold leaf before observed during dealloying ofthin filmsas predicted by and after dealloying in nitric acid for 15 min. The inset image in (c) shows a regionwhere a solid grainboundaryis located. Very thingold Erlebacher's model, even small thin-film deposition stresses ligaments with diameters smaller than 2 nm are often observed; exam- led to large densities ofgrain-boundary cracks upon dealloy- ples are marked with arrows in(e). ing. Taking inspiration from the historical connection between mechanically robust enough also to be transferred between dealloying and depletion gilding, we discovered that commer- solutions using rollers, but it is still fragile and exhibits brittle cially available 12 carat white-gold decorative leaf(Ag/Au, fracture (consistent with the observations made by Li et al.,[20] 1:1 ratio by weight) can be dealloyed to form our desired who studied a brittle±ductile transition in porous gold as a material: free-standing, large-area, highly conductive, and function of ligament size). It is interesting to note the serendi- ultra-thin nanoporous gold membranes. ªLeafº is a generic pitous property that NPGL floats on aqueous solution. Un- term referring to ultrathin foils made by hammering. In minor dealloyed leaffloatsbecause ofsimple buoyancy, but NPGL detail, a foil is first rolled to a thickness of approx. 50±100 lm, is wet well by water and we have evidence that the solution is then cut into squares and interspersed between pieces oface- wicked up and drawn into the pores.[19,21] It seems as ifonly a tate to make a sandwich structure, usually with a few hundred few microbubbles trapped within or underneath the NPGL layers. This sandwich structure is then pounded until the area are sufficient to supply a buoyancy force. This hypothesis is ofeach metal sheet increases by about a factoroffour(this is supported by our observation that only NPG foils and leaf of called ªbeatingº), at which point the sheets are quartered, thickness less than one micrometer float for extended periods restacked, and beaten again; the process is repeated until oftime. Figure 1b shows a typical NPGL sample floatingon the average foil thickness reaches approximately 100 nm. In the water after being etched for 15 min. In comparison to the comparison with vapor deposition, gold beating has the starting material, NPGL is copper colored and translucent. unique features that there is no loss of material such as by Low-magnification scanning electron microscopy (SEM) deposition on chamber walls, and the process is highly paralle- (Fig. 1c) shows that the NPGL is crack-free over large areas. lized with hundreds ofleaves produced simultaneously. For Thin films formed by vapor deposition on amorphous sub- these reasons, costs for leaf are minimized, and 100 nm thick strates tend to exhibit lateral grain sizes on the order ofthe white-gold leaves measuring 3.375 inch ” 3.375 inch film thickness, i.e., if this membrane were gold or silver depos- (8.57 cm ” 8.57 cm) are commercially available at a cost of ited on a silicon substrate, the in-plane grain size would be less than USD 0.007 cm±2 (for a 100 nm thick sheet). The approximately 100 nm. In contrast, these 100 nm leafmem- COMMUNICATIONS 中国科技论文在线 http://www.paper.edu.cn

branes exhibit extremely large lateral-grain sizes, typically on requires gold atoms to move ~ 1 nm over 1 s, requiring surface the order of10 lm. The presence ofsuch large-aspect-ratio diffusion coefficients at least of the same order or faster than grains seems linked with recrystallization processes associated 10±14 cm2 s±1, a value consistent with those measured by mor- with the latter stages ofhammering that tends to both texture phological relaxation ofrough gold electrodes under applied the leaf[22] and to also segregate a small amount ofgold to electrochemical potential.[14] This value for the room-temper- grain boundaries. The latter conclusion is the result ofthe ob- ature surface diffusion coefficient is at least four orders of servation that grain boundaries are not porous (Fig. 1c, inset), magnitude faster than the magnitude of the surface diffusion indicating that they are comprised ofpure gold. These non- coefficient of gold measured in vacuum extrapolated to room porous grain boundaries add mechanical stability to the over- temperature,[23] and seems to be a generic phenomenon asso- all structure. Within the interior area ofeach grain, high-mag- ciated with surface diffusion of simple face-centered cubic nification SEM images (Figs. 1d,e) clearly demonstrate a very (fcc) metals in non-oxidizing electrolytes. Fast diffusion is eas- uniform random porous structure, with an average ligament ily stopped by simply removing a sample from acid and trans- size around 15 nm for NPGL formed via free corrosion in ferring it to water, quenching the microstructure at the time nitric acid. of transfer. We have verified that this quenched morphology NPGL retains the grain structure ofthe original non-porous is stable in water for at least six months. Also, after rinsing, alloy because gold rearrangement occurs only by diffusion NPGL may be air-dried; the morphology also remains stable along the alloy/electrolyte interface, and does not involve with this treatment. nucleation ofnew grains. Thus, in contrast to the majority of Fast surface diffusion of gold in electrolyte gives NPGL a porous metallic nanostructures that are generally controlled unique ability to have its porosity adjusted using simple room- aggregates ofnanoparticles, NPGL retains a spatially coher- temperature post-processing. Specifically, significant coarsen- ent crystal lattice that extends to the scale ofthe original grain ing occurs simply by leaving NPGL in concentrated nitric acid size. For leaf, the in-plane grain size is on the order of 10 lm, for extended periods of time. Fundamentally, once dealloyed, three orders ofmagnitude larger than the pore size. The sin- surface diffusion is driven by surface-energy reduction, and gle-crystal nature ofNPGL is emphasized by the transmission leads to coarsening ofthe NPGL microstructure to create electron microcopy (TEM) images shown in Figure 2. Fig- self-similar microstructures with ever-increasing ligament size. ure 2a shows a lower-resolution bright-field TEM image in Such self-similar coarsening has been characterized in air dur- which multiple layers ofgold ligaments are clearly apparent, ing elevated-temperature anneals.[20] The same effect drives coarsening in acid, but at experimentally convenient tempera- tures. Figure 3 shows a series ofSEM images ofan NPGL sample left in acid for differing periods of time. After etching for five minutes (Fig. 3a), the sample is completely dealloyed ofsilver and possesses a uniformpore size distribution cen- tered around 8 nm. Compositional analysis using both Auger spectroscopy and energy dispersive X-ray spectroscopy (EDS) shows only residual silver concentration, typically less than 5 %. This sample was etched by floating on nitric acid, and etching proceeded from the lower surface, drawing acid to the upper surface. Continued immersion in acid for 15 min yields significant coarsening, and generates a 15±20 nm sized pore structure (Fig. 3b). After one day of immersion in acid Figure 2. a) TEM and b) HRTEM images of an NPGL sample dealloyed for 10 min in concentrated nitric acid. (Fig. 3d), the porosity length scale approaches the thickness ofthe original membrane, and the structure becomes essentially a two-dimensional (2D) network with a ligament indicative of the 3D network. Selected-area electron-diffrac- size of ~ 40 nm. This effect is even more pronounced after tion patterns (not shown) confirm the entire grain is a single 10 days ofimmersion (Fig. 3f).It is interesting to note the crystal. This is particularly clear in the high-resolution TEM obvious decrease in film thickness for this sample, which is image (Fig. 2b), in which {200} lattice fringes are shown to due to the ªcollapseº ofan originally 3D structure into a 2D extend continuously across a ligament only 6 nm in diameter. structure. Seen from this crystallographic standpoint, NPGL grains are We conclude with a comparison to three other porous mate- single-crystal nanowire networks, and we expect that the lack rials that exhibit porosity as the result ofsome kind of ofgrain boundaries or obvious defectsmake this material par- pattern-forming self-assembly Ð porous silicon from electro- ticularly interesting for electrical-transport studies in nano- chemical etching, porous polymers made from block-copoly- structured media. mer precursors, and porous glass made by spinodal decompo- The observation of fast surface self-diffusion of gold in elec- sition oftwo-phase mixtures. For all these materials, the trolyte allows gold atoms to locally rearrange into the porous degree ofporosity may be controlled to a certain extent. network on experimental timescales. Such rearrangement Usually, this is achieved by pre-processing; for instance, by 中国科技论文在线 http://www.paper.edu.cn

Experimental

All alloy silver/gold leafused here was purchased fromSepp Leaf Products (New York). Leaf-alloy compositions that exhibit porosity evolution upon dealloying range from 9 carat (37 wt.-% Au) to 12 carat (50 wt.-% Au). For all work reported here, 12 carat white- gold leaf(Monarch brand) was dealloyed by floatingon concentrated nitric acid (Fisher). For structure characterization, nanoporous gold (NPG) leaves were floated onto distilled water, and collected with clean silicon wafers and copper grids for scanning electron microscopy (SEM) and trans- mission electron microscopy (TEM) observations, respectively. Those samples were thin enough (100 nm) for direct TEM imaging without further thinning. The morphology of the NPG leaf was observed on a field-emission SEM (JEOL JSM-6700F), equipped with an energy- dispersive X-Ray analyzer (EDAX) Genesis 4000 Microanalysis sys- tem, under an accelerating voltage of20 kV. Crystallographic infor- mation for NPG was acquired with a 300 kV, field-emission Philips CM300FEG TEM at the Electron Microscopy Center at Johns Hop- kins University. Received: May 18, 2004 Final version: July 19, 2004 ± [1] V. Z. H. Chan, J. Hoffman, V. Y. Lee, H. Iatrou, A. Avgeropoulos, N. Hadjichristidis, R. D. Miller, E. L. Thomas, Science 1999, 286, 1716. [2] Y. N. Xia, B. Gates, Y. D. Yin, Y. Lu, Adv. Mater. 2000, 12, 693. [3] C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature 1992, 359, 710. [4] R. Seshadri, F. C. Meldrum, Adv. Mater. 2000, 12, 1149. [5] H. Masuda, K. Fukuda, Science 1995, 268, 1466. [6] R. L. Smith, S. D. Collins, J. Appl. Phys. 1992, 71,R1. [7] D. Enke, F. Janowski, W. Schwieger, Microporous Mesoporous Figure 3. Plan-view and cross-sectional SEM images of NPGL samples Mater. 2003, 60, 19. show significant structure coarsening upon continued immersion in acid for an extended times after the leaf is completely dealloyed (dealloying is [8] R. C. Newman, K. Sieradzki, MRS Bull. 1999, 24, 12. usually complete after 5 min). Structure coarsening may be quenched [9] A. J. Forty, Nature 1979, 282, 597. immediately by removal from the acid and rinsing in deionized water, [10] H. Lechtman, Sci. Am. 1984, 250, 56. making ex situ observation of this dynamic coarsening possible. These [11] H. W. Pickering, Corros. Sci. 1983, 23, 1107. structures are stable, exhibiting no obvious morphological change after [12] J. Erlebacher, M. J. Aziz, A. Karma, N. Dimitrov, K. Sieradzki, remaining in air for several months. The apparent larger sample thick- Nature 2001, 410, 450. ness in (b) comes from sample drift during imaging, and is an artifact. [13] J. Erlebacher, J. Electrochem. Soc., 2004, 151, C614. All images were recorded with same magnification of 100 000; the scale [14] J. M. Dona, J. Gonzalez-Velasco, J. Phys. Chem. 1993, 97, 4714. bar shown in (f) is 100 nm and applicable to all images. [15] M. Stratmann, M. Rohwerder, Nature 2001, 410, 420. [16] D. V. Pugh, A. Dursun, S. G. Corcoran, J. Mater. Res. 2003, 18,216. setting the electrochemical potential or dopant concentration [17] R. C. Newman, S. G. Corcoran, J. Erlebacher, M. J. Aziz, K. Sieradz- ki, MRS Bull. 1999, 24, 24. for porous silicon,[6] the length and chemical character ofthe [18] I. C. Oppenheim, D. J. Trevor, C. E. D. Chidsey, P. L. Trevor, copolymer blocks, or the annealing time prior to phase leach- K. Sieradzki, Science 1991, 254, 687. ing in the porous glasses. Ofthe three, only the length scales [19] Y. Ding, M. W. Chen, J. Erlebacher, J. Am. Chem. Soc. 2004, 126, inherent in porous glass may be modified after formation, and 6876. only by high-temperature annealing.[7] In remarkable con- [20] R. Li, K. Sieradzki, Phys. Rev. Lett. 1992, 68, 1168. trast, the porosity ofthe nanoporous gold may be adjusted [21] Y. Ding, J. Erlebacher, J. Am. Chem. Soc. 2003, 125, 7772. [22] The white-gold leaves used are primarily (100) textured, although from less than 10 nm up to the macroscopic scale of the origi- non-textured leaves were commonly found in our experiments. Ex- nal starting material, and while the porosity does depend to periments show that NPGL dealloyed from non-textured leaves are an extent on the original starting material, the primary degree also non-textured. The preferential (100) texture for gold leaf was ofcontrol is simple bench-top post-processing at room tem- also reported, for example by K. Kitagawa, J. Mater. Sci. 1988, 23, perature by immersion in electrolyte. Also, because nanopo- 2810. [23] E. G. Seebauer, C. E. Allen, Prog. Surf. Sci. 1995, 49, 265. rous gold leafis mechanically rigid, chemically stable, and bio- compatible, it can be used for various applications such as catalysis[19] or sensing, as well as for supplying a conductive ______template for the fabrication of new nanostructured materi- als.[21]