Self-Peeling of Porous Nickel Foam from the Electrochemically Etched Porous Silicon
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Self-Peeling of Porous Nickel Foam from the Electrochemically Etched Porous Silicon Xi Zhang and King-Ning Tu Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, CA 90095-1595, U.S.A. ABSTRACT A low-doped p-type silicon wafer was wet-etched to form macropores in a high-aspect-ratio, straight and parallel manner along the Si (100) direction. It was then plated in aqueous alkaline solution containing Ni2+. Metallic Ni was rapidly deposited in the macropores on the sidewall surface without using a reducing agent or activation treatment at slightly elevated temperature. After being immersed for certain duration, the single crystalline Si of sidewalls was replaced by polycrystalline Ni while the initial porous structure was still maintained. When Ni became dominant in the entire porous regime, the porous film more than 200 µm thick was discovered to be able to peel off very easily from the Si substrate beneath. In this way, a piece of nickel foam with straight pores of very high aspect ratio is self-formed and self-peeled.
INTRODUCTION Porous metals are known containing large internal surface areas and can be used in numerous applications. Considerable interests have been placed in fields of catalysis, batteries, fuel cells, capacitors, sensors and so on [1-4]. Moreover, metals with well-ordered porous networks would exhibit photonic properties [5], with potential for photonic crystal and optical applications. Porous structured metals may also have important technological applications in electronics and optoelectronics [6]. Template-directed synthesis based on template chemistry has been recently adapted to the fabrication of both meso- and macroporous metals with highly large and accessible area of metallic surfaces. Templating so far refers to a technology that involves a prior formation of a temporary medium whose interstitial empty space is then filled with another material and a porous material is consequently formed via replication after the template removal by certain means. Colloidal crystalline films are proved to be promising templates for fabricating macroporous metals with 3D ordered interconnected voids whose diameters ranges from the scale of 100 nm to the scale of 1 µm, due to the long-range close-packed order of spherical colloidal particles with controllable sizes [7]. Work has been reported for various metals and templating processes through such a colloidal crystal [8-10]. Lyotropic liquid crystalline and anodic alumina were earlier utilized as templates for making nanostructures in metals [11,12]. As a consequence, developed metal pores can be extended to the meso-scale in diameter while taking the form of being parallel and cylindrical, or 2D arrayed based on the template’s morphology. For example, Masuda reported that by a two-step replication, metal films of pore diameter of 200 nm ~ 15 nm could be obtained by selecting the mother structure with variable initial pore sizes [13]. In the present study, however, we are focusing on a novel ap
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