Templated fabrication of nanostructured Ni brush for hydrogen evolution reaction
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We fabricated a nanostructured brush by carrying out Ni deposition on a through-channel anodic aluminum oxide (AAO) template, followed by removal of the AAO skeleton. The AAO was prepared by a two-step anodization process resulting in pore diameter and thickness of 350 nm and 40 mm, respectively. Subsequently, the AAO underwent an electroless deposition involving sensitization, activation, and Ni plating, in conjunction with polyethylene glycol used as the inhibitor to prevent premature closing of pore opening. After deliberate control in relevant parameters, we obtained a conformal Ni overcoat along every pore channel leading to a reduced average pore diameter of 78 nm. Afterward, the sample was immersed in a KOH solution to remove the AAO structure, forming freestanding Ni tubules in a brush configuration. The nanostructured brush revealed considerable enhancement for hydrogen evolution reaction in both current-potential polarization and galvanostatic measurements, which were attributed to the increment in apparent surface area.
I. INTRODUCTION
Preparation of nanostructured materials using an anodic aluminum oxide (AAO) template has received considerable attention recently for possible applications in optics, electronics, batteries, and biosensing.1–4 For example, metals, oxides, and polymers in nanowires or nanodots have been successfully fabricated with impressive results.5–8 Formed by anodization of Al in an acidic electrolyte, the AAO revealed perpendicular pore channels in adjustable pore diameters and densities contingent on the type of acid, temperature, and voltage involved during the anodization process.9,10 Unfortunately, due to its insulating nature, templated synthesis using the AAO structure is limited to electroless approaches such as sol-gel, chemical conversion, electroless plating, and physical vapor deposition.11–14 Among them, the electroless plating route is considered to be the most versatile one because it entails a relatively simple setup to produce a rich variety of deposits.15,16 Metallization of AAO is a field of few studies but its successful demonstration is expected to enable the synthesis of new materials for novel applications. To fabricate a conductive AAO, a conformal metal deposition is necessary on every pore channel without clogging the pore opening prematurely. This is especially challenging as the aspect ratio for these pore channels is typically in a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0248 J. Mater. Res., Vol. 25, No. 10, Oct 2010
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the range of 50 to 200. Therefore, a practical solution is to adopt an electroless formulation in conjunction with suitable inhibitors. For the past 10 years, extensive efforts have been devoted to electroplate Cu in submicronmeter trenches for damascence structure.17–19 Among many additives explored, polyethylene glycol (PEG) is a popular inhibitor to retard Cu deposition because it easily adsorbs on the active sites of the subs
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