© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim phys. stat. sol. (RRL) 1, No. 5, 181– 183 (2007) / DOI 10.1002/pssr.200701148 www.pss-rapid.com pss Towards ideal hexagonal self-ordering of TiO 2 nanotubes Jan M. Macak, Sergiu P. Albu, and Patrik Schmuki * University of Erlangen-Nuremberg, Department of Materials Science, WW4-LKO, 91058 Erlangen, Germany Received 23 July 2007, revised 1 August 2007, accepted 1 August 2007 Published online 7 August 2007 PACS 61.46.Fg, 81.07.De, 82.45.Yz * Corresponding author: e-mail schmuki@ww.uni-erlangen.de, Phone: +4991318527575, Fax: +4991318527582 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim In the past several decades, formation of nanoporous alumina has attracted great attention due to a wide range of potential applications [1–8]. The material allows to tailor the porous morphology over a comparably wide range. However, it was only a decade ago that highly ordered self-organized structures were for the first time reported by Masuda and Fukuda [3]. Key factors that were found to be crucial for the per- fectness of the arrangement and the ideality of self- ordering process were the anodization voltage for a given electrolyte, the purity of the material as well as the possi- bility to operate with self-induced or external imprint moulds that serve as initiation sites for a secondary pore growth [4, 5]. A few years ago, another valve metal – tita- nium – has become a focus of research activities that target the growth of self-organized TiO 2 nanostructures. It was demonstrated that by anodization in fluoride containing electrolytes, arrays of self-organized TiO 2 nanotubes can be grown on Ti [9]. Over the past years we showed how different morphologies and dimensions under various ex- perimental conditions can be achieved [10–13]. These nanotubular structures, due to their highly defined geome- try combined with a large surface area, have shown for ex- ample to be highly effective for a range of applications including photocatalysis [14, 15], catalysis [16, 17], and biological interaction with cells [18]. In recent work, in HF-containing ethylene glycol electrolytes and using com- parably high anodic voltages it was found that the oxide morphology at the tube bottom shows an almost coherent comparably ordered hexagonal packing of the tubes [19], i.e. a morphology that resembles ordered porous Al 2 O 3 layers. Nevertheless, hexagonal ordering with a similar de- gree of order achieved for alumina was not reported so far for these anodic titania nanotubes. In the present work we explore some factors (some of which were successful in case of Al) that can improve or- dering in the hexagonal bottom structure of TiO 2 nanotube arrays. In particular we investigate anodization of Ti with two different purities (99.6% from Goodfellow and 99.99% from Alfa Aeaser) in ethylene glycol electrolytes containing NH 4 F. We investigate the effect of anodization voltage and repeated anodization. In the latter approach, the Ti surface is anodized for a first time, then the oxide is removed, which leads to a surface on the remaining Ti that is covered by comparably ordered dimples. These dimples act as initiation sites for the tube growth in a second anodi- zation step – thus they may pre-condition the ordering for the second layer. All anodization experiments were carried out in ethyl- ene glycol electrolytes containing 0.27 M NH 4 F at differ- ent potentials. The first anodization steps were performed with freshly prepared electrolytes for 12 hours (sweep rate 1 V/s). The second anodization steps were performed (with the electrolytes used in the first step) during 6 hours (sweep rate 5 V/s). The electrolytes are made from reagent grade chemicals purchased from Sigma–Aldrich, contain- ing less than 0.1 wt% water from production. The electro- chemical set-up consisted of a high-voltage potentiostat The present work reports on key factors that influence the de- gree of order in anodic TiO 2 nanotube layers. We show that the anodization voltage and the Ti purity are of crucial impor- tance for the ideality of self-organization within the nanotube layers and that repeated anodization can significantly improve hexagonal ordering. Optimizing each factor significantly re- duces the variation in the average pore diameter and strongly reduces the areal density of polygon ordering/packing errors.