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Nanoporous Metals by Dealloying Multicomponent Metallic Glasses Jinshan Yu,† Yi Ding,‡ Caixia Xu,‡ Akihisa Inoue,† Toshio Sakurai,† and Mingwei Chen*,† Institute for Materials Research, Tohoku UniVersity, Sendai 980-8577, Japan, and School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, China ReceiVed April 4, 2008 ReVised Manuscript ReceiVed May 29, 2008
Porous metals with a large surface-to-volume ratio, light weight, and excellent electrical/thermal conductivity have attracted much attention because of a wide range of applications in chemistry, mechanics, and nanotechnology.1 A number of approaches have been developed to fabricate porous metals,1a,b,e and among them, chemical or electrochemical dealloying has the advantage of producing bicontinuous open nanoporosity extending in three dimensions. This porous nanostructure is ideal for catalysis, chemical sensors, and biofilteration because the interconnected nanopore channels and metal ligaments allow unlimited transport of medium molecules and electrons. However, to form uniform nanoporosity upon dealloying, an alloy system is required to be a monolithic phase because the nanoporosity is formed by a self-assembly process through surface diffusion, not by the simple excavation of one phase from a preseparated multiphase system.2–4 So far, only limited crystalline alloy systems have been proved to form uniform nanoporous structures and only metals with single pore sizes can be possibly produced through one-step dealloying.5 The technique to fabricate any desired technologically important metals, such as palladium,6 has not been well established. In comparison with crystalline alloys, multicomponent metallic glasses are monolithic in phase with a homogeneous * Corresponding author. E-mail:
[email protected]. † Tohoku University. ‡ Shandong University.
(1) (a) Gibson, L. J.; Ashby, M. F. Cellular Solids: Structure and Properties, 2nd ed.; Cambridge University Press: New York, 1997. (b) Ding, Y.; Chen, M. W.; Erlebacher, J. J. Am. Chem. Soc. 2004, 126, 6876. (c) Qian, L. H.; Yan, X. Q.; Fujita, T.; Inoue, A.; Chen, M. W. Appl. Phys. Lett. 2007, 90, 153120. (d) Qian, L. H.; Chen, M. W. Appl. Phys. Lett. 2007, 91, 083105. (e) Masuda, H.; Fukuda, K. Science 1995, 268, 1466. (f) Xu, C.; Su, J.; Xu, X.; Liu, P.; Zhao, H.; Tian, F.; Ding, Y. J. Am. Chem. Soc. 2007, 129, 42. (2) Forty, A. J. Nature 1979, 282, 597. (3) Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Nature 2001, 410, 450. (4) Rugolo, J.; Erlebacher, J.; Sieradzki, K. Nat. Mater. 2006, 5, 946. (5) (a) Pugh, D. V.; Dursun, A.; Corcoran, S. G. J. Mater. Res. 2003, 18, 216. (b) Ding, Y.; Erlebacher, J. J. Am. Chem. Soc. 2003, 125, 7772. (c) Hayes, J. R.; Hodge, A. M.; Biener, J.; Hamza, A. V.; Sieradzki, K. J. Mater. Res. 2006, 21, 2611. (d) Yu, C.; Jia, F.; Ai, Z.; Zhang, L. Chem. Mater. 2007, 19, 6065. (e) Jia, F.; Yu, C.; Ai, Z.; Zhang, L. Chem. Mater. 2007, 19, 3648. (f) Huang, J.; Sun, I. AdV. Funct. Mater 2005, 15, 989. (6) Kabius, B.; Kaiser, H.; Kaesche, H. In Surfaces, Inhibition, and PassiVation: Proceedings of an International Symposium Honoring Doctor Norman Hackerman on His SeVenty-Fifth Birthday; McCafferty, E., Brodd, R. J., Eds.; Electrochemical Society: Penington, NJ, 1986, p 562.
Figure 1. Formation of nanoporous palladium by electrochemically dealloying Pd30Ni50P20 metallic glass. (a) SEM micrograph of the ribbon surface after dealloying for 200 s; (b) AES spectra of partially dealloyed Pd30Ni50P20 sample in (a); (c) SEM micrograph of a fully dealloyed glassy ribbon; and (d) high magnification SEM micrograph of the fully dealloyed sample in (c).
composition and structure down to subnanoscale. More than thousands of metallic glasses have been found in different alloy systems, mostly from our laboratory.7,8 The changeable component elements and relatively wide composition ranges make metallic glasses interesting systems for fabricating various nanoporous metals that cannot be achieved from conventional crystalline alloy systems. There are some attempts to synthesize nanoporous metals using amorphous materials as the starting alloy. For example, Thorp et al. had fabricated nanoporous Pt thin films by electrochemical dealloying of vapor deposited homogeneous amorphous PtxSi1-x films.9 But to our knowledge, systemic research of synthesizing porous metals by dealloying metallic glasses has not been reported. In the present study, we have explored the synthesis of nanoporous palladium by electrochemically dealloying multicomponent Pd30Ni50P20 metallic glass ribbons. Glassy Pd30Ni50P20 ribbons with the cross section of ∼0.02 mm × 1 mm were produced by single-roller melt-spinning in vacuum which was introduced in detail elsewhere.10 X-ray diffraction (XRD) and transmission electron microscopy (TEM) demonstrated that the as-obtained ribbons are fully amorphous (Figure S1, Supporting Information) with a uniform structure and composition down to subnanoscale. Nanoporous palladium is synthesized by electrochemically dealloying glassy Pd30Ni50P20 ribbons in 1 mol/L sulfuric acid solution using a classical three-electrode setup. Figure 1a shows a scanning electron microscopy (SEM) micrograph of a partially dealloyed Pd30Ni50P20 sample. A (7) Inoue, A. Acta Mater. 2000, 48, 279. (8) Greer, A. L. Science 1995, 267, 1947. (9) 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. (10) Zeng, Y.; Nishiyama, N.; Wada, T.; Louzguine-Luzgin, D.; Inoue, A. Mater. Trans. 2006, 47, 175.
10.1021/cm8009644 CCC: $40.75 2008 American Chemical Society Published on Web 06/18/2008
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Figure 2. (a) Bright-field TEM and (b) HRTEM micrograph of hierarchical nanoporous Pd. The inset is a selected-area electron diffraction pattern. (c) EDX spectrum of nanoporous Pd.
large number of 30-60 nm pores with a dark contrast can be readily observed on the sample surface. With careful inspection, extremely small pores with a size of about 5 nm can be seen in the ligaments. The Auger electron spectrometery (AES) spectra shown in Figure 1b demonstrate that only Pd remains on the etched surface, and after removing the surface layer by argon ions sputtering, the AES spectrum reveals the peaks of Pd, Ni, and P, confirming that selective etching take place by dissolving less noble Ni and P. A fully dealloyed sample is shown in Figure 1c. Nanoporous palladium with a rather uniform structure has been obtained. Figure 1d shows the same sample with a higher magnification, which reveals a bimodal porous Pd structure with two length-scale nanopores at ∼50 and 5 nm. The hierarchical nanoporous Pd structure was further confirmed by TEM in detail. Figure 2a shows a bright-field TEM image of the fully dealloyed sample. The size of the large pores is about 30-60 nm, and the ligaments among those big pores contain a large number of small pores with a size of about 5 nm, which is fairly consistent with the SEM observations (Figure 1). The atomic structure of the nanoporous Pd was revealed by high resolution transmission electron microscopy (HRTEM). The ligaments are found to be comprised of nanocrystals with a grain size of about 5 nm (Figure 2b). Both the selected area electron diffraction pattern (inset of Figure 2b) and energy dispersive X-ray (EDX) spectrum (Figure 2c) prove that the nanocrystalline phase is fcc Pd with a small amount of residual Ni (less than 3 atom %). The electrochemical dealloying process of the Pd30Ni50P20 metallic glass can be monitored by the cross-sectional SEM micrographs (Figure S2, Supporting Information). The dealloying process starts from the sample surface and gradually penetrates into the interior, leaving behind a well-defined interface between the porous region and the metallic glass substrate. The entire thickness of the ribbon can be etched through in ∼3000 s, from which an etching rate can be estimated to be ∼7 nm/s with the applied voltage of 0.83 V at room temperature. Detailed characterization (Figure S2, Supporting Information) revealed a similar morphology for both the middle and the surface regions of the fully dealloyed ribbon, indicating that the nanoporosity formation is mainly controlled by the dissolution process of the less noble Ni
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and P in the metallic glass system rather than by the surface diffusion of Pd in the acid during electrochemical dealloying. The detailed electrochemical process for the dealloying of the multicomponent metallic glass has not been fully understood and requires further investigation. However, based on our preliminary observations, the formation mechanism of nanoporosity in the Pd30Ni50P20 metallic glass appears to be analogous to that in the crystalline systems, such as gold-silver alloys. As the less noble atoms continuously dissolve into the solution, the noble atoms will be driven to agglomerate into clusters and gradually evolve into a 3D network structure.3 The large diversity in the standard electrode potentials between Ni and P results in different dissolution rates during the process of dealloying. Because P has a more negative standard electrode potential than that of Ni, it is expected to dissolve faster than Ni. The loss of P will give rise to nanocrystallization of the metallic glass and result in the formation of a nanocrystalline Pd-Ni solid solution as the ligaments in the initial porous structure. Upon further dealloying, the residual Ni will be selectively removed from the nanocrystalline Pd-Ni alloy and leads to the formation of small nanopores in the ligaments of large pores. Therefore, the observed hierarchical nanoporous structure is produced by the different dissolution rates between P and Ni in the ternary alloy, implying that more complex hierarchical structures could be formed by designing the composition of a multicomponent alloy. Like other noble metals, nanostructured palladium has immense technological applications in chemical sensors,11 catalysis,12 and hydrogen storage.13 In comparison with other nanostructures, nanoporous palladium with nanocrystalline ligaments are expected to have superior performances in those applications due to a unique combination of open porosity, highly stressed surface, and synergistic contribution from residual active elements, such as nickel.14,15 In this study, the internal surface area of the nanoporous Pd was measured by a CO electro-oxidation experiment. The electrochemical active surface area was estimated to be about 13 m2/g (Figure S3, Supporting Information). Their electrochemical properties for formic acid electro-oxidation were evaluated by cyclic voltammetry in dilute HClO4 and mixed HClO4/HCOOH solution. As shown in Figure 3b, the first anodic peak around 0.4 V corresponds to formic acid oxidation, while the second peak around 0.8 V can be attributed to CO oxidation and formic acid oxidation on sites that were previously blocked by CO.16 The formic acid (11) (a) Christofides, C.; Mandelis, A. J. Appl. Phys. 1990, 68, R1. (b) Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, 293, 2227. (c) Ding, D.; Chen, Z. AdV. Mater. 2007, 19, 1996. (12) (a) Tsuji, J. Palladium Reagents and Catalysts; Wiley-VCH: New York, 1995. (b) Huang, J.; Wang, D.; Hou, H.; You, T. AdV. Funct. Mater. 2008, 18, 441. (c) Desforges, A.; Backov, R.; Deleuze, H.; Mondain-Monval, O. AdV. Funct. Mater. 2005, 15, 1689. (13) (a) Lewis, F. A. The Pd-H System; Academic: New York, 1967. (b) Watari, N.; Ohnishi, S.; Ishii, Y. J. Phys.: Condens. Matter 2000, 12, 6799. (14) Koh, S.; Strasser, P. J. Am. Chem. Soc. 2007, 129, 12624. (15) Knudsen, J.; Nilekar, A. U.; Vang, R. T.; Schnadt, J.; Kunkes, E. L.; Dumesic, J. A.; Mavrikakis, M.; Besenbacher, F. J. Am. Chem. Soc. 2007, 129, 6485. (16) Jayashree, R. S.; Spendelow, J. S.; Yeom, J.; Rastogi, C.; Shannon, M. A.; Kenis, P. J. A. Electrochim. Acta. 2005, 50, 4674.
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Figure 4. SEM micrograph of nanoporous Au by dealloying glassy Au35Si20Cu28Ag7Pd5Co5 ribbon; the inset is the EDX spectrum, showing nearly pure Au in the fully dealloyed sample. Figure 3. Electrochemical properties of nanoporous palladium and commercial Pd/C catalyst (Johnson Matthey, 20 wt %). (a) Cyclic voltammograms in 0.1 M HClO4; (b) cyclic voltammograms in mixed solution of 0.1 M HClO4 and 0.1 M HCOOH. Scan rate: 50 mV s-1. The 1000 s sample is partially dealloyed, and the central part of the ribbon is still the glassy alloy. The 6000 s sample is fully dealloyed with open nanoporosity across the whole sample.
oxidation peak shifts to lower potential for dealloyed Pd30Ni50P20 metallic glasses compared to the commercial Pd/C catalyst. This shift suggests that the dealloyed samples exhibit better catalytic activity toward formic acid oxidation than the commercial Pd/C catalyst. The fully dealloyed sample (dealloyed 6000 s) indeed demonstrates extraordinary activity for the formic acid electro-oxidation, with a broad anodic peak centered at ∼0.36 V (vs RHE), which is much lower than those from the single crystalline palladium electrodes and commerical Pd/C catalyst.17,18 It should be emphasized that the dealloying method presented here is easily applicable to other systems, especially when one considers the fruitful members in the metallic glasses family. Figure 4 illustrates one more example. Using the same three electrode setup, multicomponent Au35Si20Cu28Ag7Pd5Co5 metallic glass can be fully dealloyed in 1 mol/L H2SO4 solution at the potential of ∼0.95 V. As a result, only gold atoms remain and form nanoporous gold while other less noble elements have been selectively removed. The EDX spectrum shows that the fully dealloyed sample is almost (17) Hoshi, N.; Kida, K.; Nakamura, M.; Nakada, M.; Osada, K. J. Phys. Chem. B 2006, 110, 12480. (18) Habas, S. E.; Lee, H.; Radmilovic, V.; Somorjai, G. A.; Yang, P. D. Nat. Mater. 2007, 6, 692.
pure gold (inset of Figure 4). In summary, nanoporous palladium is synthesized by electrochemically dealloying a ternary Pd30Ni50P20 metallic glass. The as-obtained bimodal nanoporous palladium with nanocrystalline ligaments is expected to have superior performance in functional applications because of its bicontinuous nanoporous channels for fast mass transport and chemical reactions. In addition, nanoporous gold is synthesized by electrochemically dealloying a multicomponent Au35Si20Cu28Ag7Pd5Co5 metallic glass. The present study paves a new way for the development of new type nanoporous metals by electrochemically dealloying multicomponent metallic glasses and has implications for synthesizing nanoscale architectures by alloying design and atomic-scale manipulation with selective chemical or electrochemical etching. Acknowledgment. This work was sponsored by the “Grantin-Aid for Exploratory Research”, “Global COE for Materials Science”, and “World Premier International Research Center (WPI) Initiative for Atoms, Molecules and Materials”, the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, and the National 863 (2006AA03Z222) and 973 (2007CB936602) Program Projects of China. Supporting Information Available: Detailed experimental procedure, XRD pattern, SEM images, estimation of the electrochemical active surface area by CO electro-oxidation, and Figures S1-S3 (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM8009644
