Magnesium Alloy Precursor Thin Films for Efficient, Practical Fabrication of Nanoporous Metals
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us (np) materials are the subject of increasing research interest, due to the high surface-area-tovolume ratios that np structures typically exhibit. Np metals show promise as catalysts,[1–3] sensors,[4] actuators,[5] and energy storage materials.[6] Several methods are available to fabricate np materials,[7,8] with dealloying being the most common approach for np metals. Dealloying refers to the selective dissolution of one or more elements from a precursor alloy system. A classic application of dealloying is synthesis of Raney nickel,[2] which is made by leaching (dealloying) of aluminum from nickel-aluminum alloys in an alkaline solution. This technique has been used for over 80 years, and Raney nickel is a commercial catalyst used in the hydrogenation of vegetable oils. However, not all alloy systems can yield np structures upon dealloying. Erlebacher[9] identified four characteristics that a precursor alloy must possess to become np during dealloying: (1) the potential difference DF required to dissolve the alloy’s pure constituent elements must be on the order of a few hundred millivolts. One element is thus more noble (MN), whereas the other is less noble (LN); (2) the alloy composition is usually richer in LN components; (3) the alloy must have no LEI WANG, NICOLAS J. BRIOT, and PHILLIP D. SWARTZENTRUBER, Graduate Students, and T. JOHN BALK, Associate Professor, are with the Department of Chemical and Materials Engineering, University of Kentucky, 177 F. Paul Anderson Tower, Lexington, KY 40506-0046. Contact e-mail: [email protected] Manuscript submitted October 3, 2013. Article published online November 27, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
phase separation prior to dealloying; (4) diffusion of MN atoms at the alloy/electrolyte interface must be sufficiently fast. These criteria have been widely accepted and are typically considered in designing precursor alloy systems for the synthesis of materials such as np gold (np-Au),[10–13] np palladium (np-Pd),[14] np platinum (np-Pt),[15] and np iridium (np-Ir).[16] Dealloying can be performed either by simply immersing an alloy in acid (generally referred to as free corrosion), or by applying a potential to the sample in an electrolyte (electrochemical dealloying), with the latter technique widely used in research. Dealloying rate can be controlled by monitoring and adjusting the dissolution current. However, free corrosion is easier to implement and is likely more favorable for industrial applications. This paper presents a novel dealloying method for fabricating np metals. Thin films of np-Au, np-Ni, np-Ir, and np-OsRu have been successfully fabricated by free corrosion of magnesium-based alloy precursors. The dealloyed films were characterized by scanning electron microscopy (SEM) and exhibited fully dealloyed np structures with no cracks. The precursor films were also characterized by X-ray diffraction (XRD) and electron diffraction in the transmission electron microscope (TEM). This study indicates that Mg alloy precursors have significantly different cryst
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