A new route for the formation of self-organized anodic porous alumina in neutral electrolytes Hiroaki Tsuchiya, Steffen Berger, Jan M. Macak, Andre ´s G. Mun ˜ oz, Patrik Schmuki * Department of Materials Science, Institute for Surface Science and Corrosion (LKO), University of Erlangen-Nu ¨ rnberg, Martensstrasse 7, D-91058 Erlangen, Germany Received 22 September 2006; received in revised form 17 October 2006; accepted 19 October 2006 Available online 17 November 2006 Abstract The present work reports the formation of regular porous aluminum oxide layers in neutral fluoride containing (NH 4 ) 2 SO 4 electro- lytes. For a fluoride free (NH 4 ) 2 SO 4 electrolyte only irregular and thin porous alumina layers can be grown under these conditions. Upon addition of small amounts of fluorides highly regular, smooth high aspect ratio pore arrays can be produced. Pores with a typical diam- eter of approximately 50 nm and a length of several micrometers are formed. This finding significantly widens the spectrum of synthesis routes for ordered porous alumina structures and templates. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Alumina; Self-organized; Nanoporous; Fluoride 1. Introduction Since the first reports of Masuda et al. [1,2], the forma- tion of highly ordered porous aluminum oxide layers with a hexagonal configuration has stimulated an intense research activity. A main interest in such structures is based on potential applications such as a host or template structure of nanometer dimensions. Channels of a high aspect ratio can be produced in a self-ordered manner. This offers con- siderable advantages in comparison with lithography tech- niques regarding the construction of nanostructures for magnetic recording media [3,4], optical devices [5] and functional electrodes [6,7]. Many of these applications are based on the filling of pores with metals, semiconductors or polymers. Alumina nanoporous films have been success- fully applied for the template synthesis of nanofibers and tubes of metals [8], semiconductors [9] and conducting polymers [10,11]. Anodization of Al under pore forming conditions leads to the growth of oxides in hexagonal cells with a center pore. Much effort has been directed towards the fabrication of cell diameters in a wide range of sizes. It has been established that the cell size is directly related to the anodization voltage, but the pore geometry is also strongly linked to the nature of the electrolyte, making it necessary to use different media for achieving specific pore diameters [12]. Typical electrolytes for the growth of high ordered por- ous oxides are sulfuric acid, oxalic acid and phosphoric acid, where self-ordered pore arrangements are obtained at very different potentials, typically at 25 V in sulfuric acid [2], 40 V in oxalic acid [1] and 195 V in phosphoric acid [13], leading to pore diameters of 65 nm, 100 nm and 400–500 nm, respectively. This is the result of the linear relationship between cell diameter and applied formation voltage as well as the electric field necessary for the pene- tration of ions through the oxide structure. Furthermore, it is important that Al 3+ ions are solvated once they reach the oxide/electrolyte interface. This is either achieved by the ‘‘acidity’’ in the electrolyte or by complexation such as in the case of an oxalate solution. 1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.10.037 * Corresponding author. Tel.: +49 9131 852 7575; fax: +49 9131 852 7582. E-mail address: schmuki@ww.uni-erlangen.de (P. Schmuki). www.elsevier.com/locate/elecom Electrochemistry Communications 9 (2007) 545–550