Electrochimica Acta 52 (2006) 1799–1804 Nanostructures from directionally solidified NiAl–W eutectic alloys  Achim Walter Hassel ∗,1 , Andrew Jonathan Smith, Srdjan Milenkovic Department of Interface Chemistry and Surface Engineering, Max-Planck-Institut f¨ ur Eisenforschung GmbH, Max-Planck-Str. 1, 40237 D¨ usseldorf, Germany Received 6 November 2005; received in revised form 19 December 2005; accepted 22 December 2005 Available online 6 June 2006 Abstract A directionally solidified eutectic NiAl–W alloy was employed as a source for NiAl nanopore arrays, W-nanowire arrays and W-nanowires. The NiAl–W eutectic alloy containing 1.5 at.% W. A growth rate of 30 mm h -1 was used at a temperature gradient of 40 K cm -1 in a Bridgman-type directional solidification furnace. A combined stability diagram was derived from the Pourbaix diagrams of the three elements involved (Ni, Al, W). It allowed predicting proper conditions for the selective dissolution of either of the phases. Etching in a mixture of HCl:H 2 O 2 released parallel aligned W-nanowires with a wire diameter of ∼200 nm. The aspect ratio observed in these measurements was over 400. Selective electrodissolution of the W minor phase was possible in a neutral buffer at 0.5 V versus SHE. This procedure yielded regular nanopore arrays with pore diameters of ∼200 nm. The pore density obtained was 8 × 10 10 m -2 . The relative coverage of the wires or the corresponding pores in a cross section was 0.3%. For the material employed in this study a 12 m 2 area corresponds to a single nanowire electrode. The potential of this material and the processing developed is discussed in terms of the production of nanofilters, nanoelectrode sensors, nanowire arrays and quantitative amounts of single crystalline tungsten nanowires. © 2006 Elsevier Ltd. All rights reserved. Keywords: Eutectic alloy; Nanowire; Selective etching; Nanoelectrodes array 1. Introduction Nanotechnology is of interest as devices can show unusual properties due to quantum confinement if the feature size falls below a certain value. Metallic nanowires show this behaviour only in the lower nm range (below 10 nm). But also mesoscale structures from 100 to 1000 nm can have favourable properties. Their link to electrochemistry can be either through an electro- chemical step involved in the production method or through their potential use in electrochemistry. Metallic nanowires can be produced by template-directed synthesis, involving either chemical or electrochemical depo- sition of the wires [1–4]. Gold nanowires for example have been fabricated on polycarbonate templates by pulsed-laser depo- sition combined with electrochemical plating [1], generating nanowire arrays with a high aspect ratio and a diameter between 100 and 600 nm after dissolution of the carbonate template.  Presented in parts at the ISE meeting in Busan, Korea. ∗ Corresponding author. Fax: +49 211 6792 218. E-mail address: hassel@elchem.de (A.W. Hassel). 1 ISE active member. Arrays of Ni nanowires with a diameter of 30 nm have been electrodeposited in alumina membranes [2,3]. A large number of studies have been directed towards the use of anodic alumina oxide as a template. The procedures for preparing highly ordered alumina membranes were extensively described in the literature [5–8]. Both chemical and electrochemical deposition into the pores with a subsequent dissolution of the matrix and thus a release of the wires is possible [8,9]. These wires are usually polycrystalline, which can be a drawback for their application. The advantages of ultramicroelectrodes or nanoelectrodes (nanodes) have been demonstrated for example for SECM [10,11] and stripping voltammetry [12]. When organised in an array, the resulting electrodes could combine the advantages of both the nanoelectrode and the signal intensity from a macroscopic electrode. Directional solidification of a eutectic alloy is a method that can facilitate the nanostructuring of a material [13]. From here on this material will be referred to as DS–NiAl-X were DS indi- cates directionally processed material and NiAl-X indicates the quasibinary system consisting of the B2 NiAl phase and the minor phase X. This method has several advantage. It produces arrays of single crystalline wires embedded in a single crystalline 0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.12.061