Nanoporous Gold Ribbons with Bimodal Channel Size Distributions

Jan 8, 2009 - We present a novel and simple strategy to synthesize nanoporous gold (NPG) ribbons with bimodal channel size distributions. ... Hierarch...
0 downloads 0 Views 3MB Size
Download PDF
1308

J. Phys. Chem. C 2009, 113, 1308–1314

Nanoporous Gold Ribbons with Bimodal Channel Size Distributions by Chemical Dealloying of Al-Au Alloys Zhonghua Zhang,†,* Yan Wang,†,‡ Zhen Qi,† Jikui Lin,† and Xiufang Bian† The Key Laboratory of Liquid Structure and Heredity of Materials, Shandong UniVersity, Jingshi Road 73, 250061 Jinan (P.R. China), and School of Materials Science and Engineering, UniVersity of Jinan, Jiwei Road 106, Jinan 250022, P.R. China ReceiVed: September 27, 2008; ReVised Manuscript ReceiVed: NoVember 19, 2008

We present a novel and simple strategy to synthesize nanoporous gold (NPG) ribbons with bimodal channel size distributions. The NPG ribbons can be fabricated from Al-Au alloys through rapid solidification and chemical dealloying. The microstructure of these NPG ribbons was characterized using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, transmission electron microscopy, and highresolution transmission electron microscopy. These NPG ribbons are composed of large-sized channels (hundreds of nanometers) with highly porous channel walls (tens of nanometers). Both large- and small-sized channels are 3D, open, and bicontinuous. The length scales of the large-sized channels can be modulated by changing the alloy composition, and those of small ligaments/channels in the channel walls can be tuned by changing the dealloying solution. Introduction Nanoporous metals have recently attracted considerable interest in a wide variety of applications including catalysis, sensors, actuators, fuel cells, microfluidic flow controllers, and so forth.1-4 Template methods are commonly used to fabricate these materials through the replication of porous alumina or liquid-crystal templates.5-7 These methods have the advantages of precise control over the pore size and microstructure periodicity but normally result in materials with 1D porosity, such as an array of tubes.8 Recently, dealloying has received significant attention because certain particular systems exhibit nanoporosity evolution upon dealloying.9 Dealloying refers to the selective dissolution of one or more components out of an alloy. Although dealloying has been observed in several systems including Cu-Au,10 Pt-Cu,11 Pt-Si,12 and so forth, most attention has been paid to the prototypical Ag-Au system. Chemical or electrochemical dealloying of Ag-Au alloys results in the formation of nanoporous gold (NPG) with an open, bicontinuous interpenetrating ligament-channel structure.8,9,13-16 This characteristic is favorable for applications such as sensing and catalysis. In addition, a bimodal channel size distribution composed of large-sized channels with highly porous channel walls is desirable for microfluidic-based sensors to achieve fast response time and high sensitivity. Ding and Erlebacher17 reported a two-step dealloying strategy to create NPG with multimodal pore size distributions. We recently developed a new route to synthesize nanoporous metal ribbons.18 Here, we show that this route can be used to fabricate NPG ribbons with bimodal channel size distributions through rapid solidification and chemical dealloying of Al-Au alloys. Furthermore, the length scales of ligaments/channels in NPG can be modulated by changing the dealloying solution and alloy composition. * To whom correspondence should be addressed. E-mail: zh_zhang@ sdu.edu.cn. † Shandong University. ‡ University of Jinan.

Figure 1. a) Macrograph of the RS Al80Au20 alloy (left) and NPG (right) ribbons. b) Corresponding XRD patterns.

Experimental Section The Al80Au20 and Al70Au30 (nominal compositions, atom %) alloys used in this work were prepared from elemental Al (purity, 99.95 wt %) and Au (purity, 99.9 wt %) in a quartz crucible using a high-frequency induction furnace. Using a single roller melt spinning apparatus, the prealloyed ingots were remelted by high-frequency induction heating in a quartz tube and then melt-spun onto a copper roller with a diameter 0.35 m at a speed of 1000 rpm (rpm) in a controlled argon atmosphere. The ribbons obtained were typically 20-50 µm in thickness, 2-4 mm in width, and several centimeters in length. The rapidly solidified (RS) Al80Au20 and Al70Au30 ribbons were dealloyed

10.1021/jp808569g CCC: $40.75  2009 American Chemical Society Published on Web 01/08/2009

Nanoporous Gold Ribbons

J. Phys. Chem. C, Vol. 113, No. 4, 2009 1309

Figure 2. SEM images showing the microstructure of a,b) free surface, c,d) quenching surface, and e,f) cross-section of the NPG ribbons by dealloying of the RS Al80Au20 alloy in the 20 wt % NaOH aqueous solution. The large-sized channels (hundreds of nanometers) can be clearly seen. a,c,e) Low-magnification images. b,d,f) High-magnification images show the nanoporous structure of the channel walls in NPG. g) A typical EDX spectrum shows the composition of NPG.

in a 20 wt % NaOH or 5 wt % HCl aqueous solution under free corrosion conditions. When the 20 wt % NaOH solution was used, the dealloying of the RS Al-Au alloys was first performed at room temperature until no obvious bubbles emerged. Then the dealloying was continuously carried out in the alkali solution with a temperature of 90 ( 5 °C to further leach out the residual Al in the samples. The typical dealloying time was less than 1 h. For the 5 wt % HCl solution, the dealloying of the RS Al-Au alloys was performed at 90 ( 5 °C until no bubbles emerged. The typical dealloying time was

less than 30 min. The as-dealloyed samples were rinsed using distilled water and dehydrated alcohol. Nanoporous gold (NPG) ribbons were then obtained through rapid solidification and chemical dealloying. The phases present in the RS Al80Au20 and Al70Au30 alloys and NPG ribbons were identified using an X-ray diffractometer (XRD, Philips X’Pert) with Cu KR radiation. The free surface (air side), quenching surface (contacting with the copper roller), and cross section of the NPG ribbons were observed using a scanning electron microscope (SEM, LEO 1530 VP). The

1310 J. Phys. Chem. C, Vol. 113, No. 4, 2009

Figure 3. a) TEM and b-d) HRTEM images showing the nanoporous structure and lattice defects (stacking faults, twins, and dislocations) of NPG by dealloying of the RS Al80Au20 alloy in the 20 wt % NaOH aqueous solution. The inserts show the corresponding a) SAED and b-d) FFT patterns.

chemical compositions of the NPG ribbons were determined using an energy dispersive X-ray (EDX) analyzer, which is attached to SEM. The microstructures of the NPG ribbons were also characterized using a transmission electron microscope (TEM, Philips CM 20) with selected-area electron diffraction (SAED) and a high-resolution TEM (HRTEM, FEI Tecnai G2). TEM specimens were prepared using a Gatan ion mill at 5 kV. The fast Fourier transform (FFT) patterns were obtained from the corresponding HRTEM images using a Gatan software. Results and Discussion Part a of Figure 1 shows the macrograph of the rapidly solidified (RS) Al80Au20 alloy (left) and corresponding NPG (right) ribbons. The NPG ribbons were prepared by dealloying of the RS Al80Au20 alloy in a 20 wt % NaOH aqueous solution (for details see the Experimental Section). The X-ray diffraction (XRD) results show that the RS Al80Au20 alloy is composed of two phases: R-Al and Al2Au (part b of Figure 1). It is obvious that the Al2Au phase is dominant in the RS alloy. After dealloying, however, the NPG ribbons comprise a single face centered cubic (fcc) Au phase (part b of Figure 1). The nanoporous structure of the NPG ribbons is verified by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) (Figures 2 and 3). Part a of Figure 2 shows the SEM image of the free surface of the NPG ribbons. The free surface exhibits a porous structure with length scales of hundreds of nanometers. The SEM image at a higher magnification shows that the channel walls exhibit an open, bicontinuous interpenetrating ligamentchannel structure with length scales of 10-20 nm (part b of Figure 2). The quenching surface of the NPG ribbons is also porous but its morphology is different from that of the free surface due to different cooling rates of the free and quenching surfaces during rapid solidification (part c of Figure 2). In addition, the channel walls are also nanoporous, similar to those of the free surface (part d of Figure 2). The section view of the

Zhang et al. NPG ribbons shows that the large-sized channels (hundreds of nm) penetrate the whole ribbons and a typical SEM image is shown in part e of Figure 2. The fracture surface of the channel wall of the NPG ribbons clearly exhibits an open, bicontinuous interpenetrating ligament-channel structure, suggesting that the nanoporous structure of the channel walls is 3D (part f of Figure 2). Furthermore, the length scales of some ligaments are less than 10 nm. The walls of some large sized channels form dendrites, and one dendrite is highlighted by an arrow in part e of Figure 2. It is reasonable to assume that the morphology and size of these dendrites in NPG inherit from the Al2Au grains in the master Al80Au20 alloy. Obviously, the NPG ribbons have bimodal channel size distributions composed of large-sized channels with highly porous channel walls. In addition, the energy dispersive X-ray (EDX) analysis shows that only Au can be identified and all of Al was removed during dealloying (part g of Figure 2). NPG (by dealloying of Ag-Au alloys) normally contains some residual Ag (several atom percent). The residual Ag is expected to be trapped inside the Au ligaments on the basis of the dealloying mechanism.19,20 Moreover, the residual Ag can not be removed but asymptotically reaches a limit at exhaustively long etching times (up to 100 h).19 Part a of Figure 3 clearly shows the nanoporous structure of the NPG ribbons. The length scales of ligaments/channels are in agreement with the SEM observation. The inset of part a of Figure 3 shows the corresponding selected-area electron diffraction (SAED) pattern. The SAED pattern is from the fcc Au [110] zone axis, indicating a single crystalline characteristic of the selected area. The nanoporous structure of the NPG ribbons can be clearly seen in the HRTEM image (part b of Figure 3). The HRTEM image and corresponding Fast Fourier Transform (FFT) pattern further verify the single crystalline nature of the entire frame of observation (part b and inset of Figure 3). We can see lattice fringes of Au {200} extending throughout all the ligaments shown in the HRTEM image. The present results are consistent with the established notion that the crystal lattice orientation is retained during dealloying of Ag-Au alloys with the conservation of the grain size of the master alloys.8,13,15,21 It should be noted that the Al-Au system in this work is different from the prototypical Ag-Au system. In addition, some lattice defects are present in the NPG ribbons. We can see some stacking faults in the HRTEM image, as marked by some ellipses in part b of Figure 3. Part c of Figure 3 shows another HRTEM image of the ligaments in the NPG ribbons. Twins appear in the lattice, as marked by a broken zigzag line in part c of Figure 3. The corresponding FFT pattern further confirms the appearance of twins, and the twin plane is the Au {111} (part c and inset of Figure 3). Some lattice dislocations and stacking faults can also be observed, as marked by ellipses in part c of Figure 3. Part d of Figure 3 clearly shows stacking faults in another area. The corresponding FFT pattern of Au [110] zone axis is shown as an inset in part d of Figure 3. Parida et al.15 have reported that the formation of nanoporous gold by electrochemical dealloying of Ag-Au alloys is accompanied by the creation of a large number of lattice defects including dislocations, stacking faults, and twins. However, only slight lattice distortion is frequently seen in NPG by the slower electronless process (dealloying under free corrosion conditions) of Ag-Au.13 Our results are in agreement with the previous report by Parida et al., but the dealloying of Al-Au was performed under free corrosion conditions. In our work, the length scales of ligaments/channels in the NPG ribbons can be modulated by simply changing the dealloying solution. Figure 4 shows the microstructure of the

Nanoporous Gold Ribbons

J. Phys. Chem. C, Vol. 113, No. 4, 2009 1311

Figure 4. a-d) SEM and e) TEM images of NPG by dealloying of the RS Al80Au20 alloy in the 5 wt % HCl aqueous solution. a,c) Lowmagnification images. b,d) High-magnification images. The SEM images show the large sized channels in a,b) free surface and c,d) cross-section of the NPG ribbons. The TEM image and the high-magnification SEM images clearly show the nanoporous structure of the channel walls in NPG. f) A typical EDX spectrum shows the composition of NPG.

NPG by dealloying of the RS Al80Au20 alloy in a 5 wt % HCl aqueous solution. The large sized channels (hundreds of nanometers) can be clearly seen in the free surface of the NPG ribbons (part a of Figure 4). The section view of the NPG ribbons demonstrates that the large sized channels penetrate across the whole ribbons (part c of Figure 4). The channel walls also exhibit an open, bicontinuous interpenetrating ligamentchannel structure, as clearly shown in the SEM images at a higher magnification (parts b and d of Figure 4). The length scales of ligaments/channels are 60-80 nm, larger than those of the NPG ribbons by dealloying in the alkali solution. TEM observations further prove the nanoporous structure of the NPG ribbons (part e of Figure 4). Part f of Figure 4 shows a typical EDX spectrum of the NPG ribbons, indicating that all of Al was etched away during the dealloying of the RS Al80Au20 alloy by the 5 wt % HCl aqueous solution. Here, we show that the microstructure of the NPG ribbons also depends upon the initial composition of RS Al-Au alloys.

The RS Al70Au30 alloy is composed of R-Al and Al2Au phases as indicated by the XRD results (not shown). The volume fraction of the R-Al phase is less than that in the RS Al80Au20 alloy due to the increasing Au content. Figure 5 shows the microstructure of the NPG ribbons by dealloying of the RS Al70Au30 alloy in the 20 wt % NaOH aqueous solution. Largesized channels are visible and penetrate through the whole ribbons (parts a and c of Figure 5). The length scales of the large-sized channels range from tens to hundreds of nanometers, less than those of the NPG ribbons by dealloying of the RS Al80Au20 alloy. The SEM images at a higher magnification clearly show that the walls of the large-sized channels are highly nanoporous (parts b and d of Figure 5). It should be noted that the length scales of ligaments/channels (10-20 nm) are comparable to those of the NPG ribbons by dealloying of the RS Al80Au20 alloy in the same alkali solution. The nanoporous structure of the NPG ribbons is further confirmed by the TEM and HRTEM images (parts e and f of Figure 5). The SAED

1312 J. Phys. Chem. C, Vol. 113, No. 4, 2009

Zhang et al.

Figure 5. a-d) SEM, e) TEM, and f) HRTEM images of NPG by dealloying of the RS Al70Au30 alloy in the 20 wt % NaOH aqueous solution. a,c) Low-magnification images. b,d) High-magnification images. The SEM images show the large-sized channels in a,b) free surface and c,d) cross-section of the NPG ribbons. The high-magnification SEM images in combination with the TEM and HRTEM images clearly show the nanoporous structure of the channel walls in NPG. The insert in e) shows the SAED pattern corresponding to the TEM microstructure.

pattern verifies the single crystalline nature of the whole frame of observation (part e and the inset of Figure 5). Furthermore, we can clearly see lattice fringes of Au {111} extending across the whole ligaments in the HRTEM image (part f of Figure 5). Similarly, the length scales of ligaments/channels of the NPG ribbons can be modulated to 60-80 nm by changing the dealloying solution into the 5 wt % HCl solution (Figure 6). The large-sized channels (tens to hundreds of nanometers) penetrate through the whole ribbons as shown in parts a and c of Figure 6. Both-large sized channels and nano-sized channels/ ligaments can be clearly seen at a higher magnification (parts b and d of Figure 6). 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/electrochemical dealloying. Porosity evolution thus forms dynamically during dissolution and is not due to one phase simply being

excavated out of a two-phase material.22 It is well established that the evolution of the porous structure during dealloying involves etching of the less-noble metal coupled with coarsening of the more noble metal by surface diffusion.23 The present results show that dealloying of a two-phase Al-Au system leads to the formation of NPG with bimodal channel size distributions. According to the binary Al-Au phase diagram,24 primary Al2Au dendrites first precipitate during rapid solidification of the Al80Au20 and Al70Au30 alloys. The R-Al solid solution phase thus forms from the remaining liquid, suppressing the occurrence of the eutectic reaction. In the RS Al80Au20 and Al70Au30 alloys, the R-Al phase surrounds the primary Al2Au phase. During etching, the excavation of the R-Al phase out of the alloy contributes to the formation of large-sized channels in the NPG ribbons. The dealloying of the Al2Au phase results in the nanoporous structure of the walls of the large-sized channels. Moreover, the morphology, size, and orientation of the Al2Au

Nanoporous Gold Ribbons

J. Phys. Chem. C, Vol. 113, No. 4, 2009 1313

Figure 6. SEM images showing the bimodal porous structure of NPG by dealloying of the RS Al70Au30 alloy in the 5 wt % HCl aqueous solution. a,b) Free surface and c,d) cross-section.

phase can be conserved into the resulting NPG ribbons. From the cross-section SEM images shown in part c of Figure 4, part c of Figure 5, and in part c of Figure 6, we can know the morphology, size, and distribution of the R-Al and Al2Au phases in the starting RS Al80Au20 and Al70Au30 alloys. The R-Al and Al2Au phases interpenetrate throughout the whole ribbons. In addition, the length scales of the large-sized channels in NPG can be modulated by simply changing the alloy composition of Al-Au because the amount of the Al phase decreases with increasing Au concentration (up to 33.3 atom % Au corresponding to a single Al2Au phase) in the Al-Au alloys. In fact, the dealloying of a single-phase Al2Au alloy results in the formation of NPG with a single modal channel size distribution. We can also increase the length scales of the large-sized channels in NPG by decreasing the Au concentration in the RS Al-Au alloys. There is a lower limit of the Au concentration, however, below which the channels will collapse and the shape of the ribbons cannot be preserved. For example, the dealloying of a RS Al90Au10 alloy results in the formation of NPG powders (like Raney metals25) instead of monolithic ribbons. Normally, alkali dealloying of Al-based alloys (such as Al-Ni, Al-Cu, etc.) is used for preparation of Raney catalysts.25 It is astonishing that the Al2Au phase (intermetallic compound) can be dealloyed, resulting in the formation of NPG. The physical mechanisms of the dealloying process (especially the dealloying of Ag-Au) have been discussed according to the corrosion disordering/diffusion reordering model,14 the dynamic roughening transition model,26 and the kinetic Monte Carlo model.9,22 It is well established that the dealloying of Ag-Au alloys (for example, Ag70Au30) does not require either nucleation of new crystallites nor the formation or removal of lattice sites.13,15 Al2Au has an fcc CaF2-type structure. In a unit cell, Au atoms occupy all fcc lattice sites and all eight Al atoms are encaged inside the cell. During dealloying, all Al atoms are selectively dissolved and the residual Au atoms form an fcc lattice with a lattice constant larger than that of bulk Au. The

formation of NPG only needs the shrinkage of the lattice accompanied by the rearrangement of the Au atoms and the formation/removal of lattice sites. The formation/removal of lattice sites during dealloying of Al-Au may be one reason for the creation of a large number of lattice defects in NPG even if the dealloying is performed under free corrosion conditions. In addition, the typical dealloying time is less than 1 h for the Al-Au ribbons with thickness of 20-50 µm, much shorter than that of Ag-Au (normally several hours under free corrosion conditions). The fast dissolution kinetics during the dealloying of Al-Au is also responsible for the formation of many lattice defects in NPG. Parida et al.15 have reported that there is a clear trend for more defects in NPG at higher dealloying potentials (faster dissolution rate). For NPG by dealloying of Ag-Au, the length scales of ligaments/channels depend upon the initial alloy composition, applied dealloying potential, and electrolyte composition.13,21 The structure can also be coarsened to larger-length scales (up to micrometers) by annealing for short times at elevated temperatures or by storage for long periods at room temperature.21 In our work, the large sized channels in NPG can be adjusted by changing the alloy composition of Al-Au, but the length scales of ligaments/channels (smaller ones) are independent of the alloy composition for a given dealloying solution. For a given Al-Au alloy, however, the length scales of smaller ligaments/channels in NPG can be modulated from 10-20 to 60-80 nm by just replacing the 20 wt % NaOH solution with the 5 wt % HCl. In such a case, the change of the large-sized channels in NPG is negligible. It has been reported that the adsorption of Cl- increases the diffusion rate during the coarsening process of NPG.27 Coarsening of the porous structure in NPG into nanoporous gold prism microassembly can be attributed to the rapid surface diffusion of Au induced by a concentrated hydrochloric acid.28 Therefore, it is reasonable to assume that Cl- can accelerate the surface diffusion of Au and

1314 J. Phys. Chem. C, Vol. 113, No. 4, 2009 plays an important role in the coarsening of ligaments/channels during the dealloying of Al-Au. The nonmonolithic characteristic of Raney metals is inconsequential to their applications as a catalyst because the main property required is high surface area. However, the mechanical integrity of nanoporous metals is important for their structural properties because uniformity and continuity are key requirements. Although minor cracks are observed on the surface of the ribbons (part a of Figure 6), the present NPG ribbons with bimodal channel size distributions are monolithic and can be handled and loaded. These NPG ribbons 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. Moreover, the mechanical integrity of these NPG ribbons is crucial for their applications as microfluidic-based sensors. Conclusions In summary, nanoporous gold ribbons with bimodal channel size distributions can be fabricated from Al-Au alloys by rapid solidification and chemical dealloying. The present route is simple and effective compared to the dealloying-platingannealing-dealloying strategy reported in the literature. Furthermore, the length scales of ligaments/channels in NPG can be tuned by simply changing the dealloying solution and alloy composition. In addition, the length, width, and thickness of these NPG ribbons can be easily modulated by altering rapid solidification parameters. Our findings provide a maximum flexibility in tailoring the macroscale, microstructure, and thus properties of NPG ribbons with bimodal channel size distributions, and these NPG ribbons will find wide applications in catalysis, sensing, micro/nanofluidic control, and so forth. Moreover, this ribbonlike NPG can be a good candidate for investigating physical, chemical, and mechanical properties associated with random porous structures of nanoporous metals. This strategy can also be developed to exploit new alloy systems suitable for dealloying and to synthesize novel nanoporous metals with mulimodal channel size distributions. Acknowledgment. We gratefully acknowledge financial support by the National Natural Science Foundation of China

Zhang et al. under grant 50701028, Excellent Middle-age and Young Scientists Research Award Foundation of Shandong Province under grant 2007BS04024. Z. Zhang thanks financial support by DFG-SFB 459 (Germany) for a guest visit. The experimental assistance from Ruhr University Bochum (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) Joo, S. H.; Choi, S. J.; Kwa, K. J.; Liu, Z. Nature 2001, 412, 169. (4) Rintoul, M. D.; Torquato, S.; Yeong, C.; Keane, D. T.; Erramilli, S.; Jun, Y. N.; Dabbs, D. M.; Aksay, I. A. Phys. ReV. E 1996, 54, 2663. (5) Masuda, H.; Fukuda, K. Science 1995, 268, 1466. (6) Attard, G. S.; Bartlett, P. N.; Coleman, N. R. B.; Elliott, J. M.; Owen, J. R.; Wang, J. H. Science 1997, 278, 838. (7) Kijima, T.; Yoshimura, T.; Uota, M.; Ikeda, T.; Fujikawa, D.; Mouri, S.; Uoyama, S. Angew. Chem., Int. Ed. 2004, 43, 228. (8) Ding, Y.; Mathur, A.; Chen, M. W.; Erlebacher, J. Angew. Chem., Int. Ed. 2005, 44, 4002. (9) Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Nature 2001, 410, 450. (10) Fritz, J. D.; Pickering, H. W. J. Electrochem. Soc. 1991, 138, 3209. (11) Pugh, D. V.; Dursun, A.; Corcoran, S. G. J. Electrochem. Soc. 2005, 152, B455. (12) 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. (13) Ding, Y.; Kim, Y. J.; Erlebacher, J. AdV. Mater. 2004, 16, 1897. (14) Forty, A. J. Nature 1979, 282, 597. (15) Parida, S.; Kramer, D.; Volkert, C. A.; Ro¨sner, H.; Erlebacher, J.; Weissmu¨ller, J. Phys. ReV. Lett. 2006, 97, 035504. (16) Zielasek, V.; Ju¨rgens, B.; Schulz, C.; Biener, J.; Biener, M. M.; Hamza, A. V.; Ba¨umer, M. Angew. Chem., Int. Ed. 2006, 45, 8241. (17) Ding, Y.; Erlebacher, J. J. Am. Chem. Soc. 2003, 125, 7772. (18) Zhang, Z. H.; Wang, Y.; Qi, Z.; Bian, X. F. In preparation. (19) Dixon, M. C.; Daniel, T. A.; Hieda, M.; Smilgies, D. M.; Chan, M. H. W.; Allara, D. L. Langmuir 2007, 23, 2414. (20) Sieradzki, K.; Corderman, R. R.; Shukla, K. Phil. Mag. A 1989, 59, 713. (21) Forty, A. J.; Durkin, P. Philos. Mag. A 1980, 42, 295. (22) Erlebacher, J. J. Electrochem. Soc. 2004, 151, C614. (23) Ji, C. X.; Searson, P. C. J. Phys. Chem. B 2003, 107, 4494. (24) Murray, J. L.; Okamoto, H.; Massalski, T. B. Bull. Alloy Phase Diagrams 1987, 8, 20. (25) Raney, M. U.S. Patent No. 1563587, 1925. (26) Sieradzki, K. J. Electrochem. Soc. 1993, 140, 2868. (27) Newman, R. C.; Sieradzki, K. Science 1994, 263, 1708. (28) Hakamada, M.; Mabuchi, M. Nano Lett. 2006, 6, 882.

JP808569G