TiO 2 nanotubes: Tailoring the geometry in H 3 PO 4 /HF electrolytes Sebastian Bauer, Sebastian Kleber, Patrik Schmuki * Department of Materials Science, WW4-LKO, University of Erlangen-Nuremberg, Martensstrasse 7, D-91058 Erlangen, Germany Received 5 May 2006; received in revised form 26 May 2006; accepted 31 May 2006 Available online 12 July 2006 Abstract In the present work, we investigate the formation of self-organized titanium oxide nanotube layers at different fluoride concentrations and potentials in H 3 PO 4 electrolytes. We demonstrate that in optimized phosphate/HF electrolytes, in contrast to other electrolytes the tube length and diameter can be controlled over a wide range by the applied potential. It was found that for potentials between 1 and 25 V tubes could be grown with any desired diameter ranging from 15 to 120 nm combined with tube length from 20 nm to 1 lm. The diameter and the length depend linearly on the voltage. These findings represent an unprecedented level of control in the geometry of anodic TiO 2 nanotubes. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Titanium oxide; Self-organization; Nanotubes; Anodic oxidation; Phosphate electrolytes 1. Introduction Titanium and its alloys are of high interest for techno- logical applications due to their excellent mechanical prop- erties, high corrosion resistance and good biocompatibility [1–4]. Furthermore, TiO 2 and oxide layers on Ti also have attracted much attention because TiO 2 possesses a variety of functional properties as, for example, for gas sensing [5,6], self-cleaning [7,8], solar energy conversion [9], wetta- bility [10] and photocatalysis [11] applications. The effi- ciency of many of these TiO 2 devices depends on the geometry and surface area of the TiO 2 layers – therefore the self-organized anodic growth of TiO 2 nanotubes on titanium has attracted significant interest. Several applica- tions of the tubes such as dye-sensitizing, wettability and photo response have been explored with considerable suc- cess [12–16]. Several approaches for the formation of self-organized TiO 2 nanotube layers in fluoride containing electrolytes have been published [17–20] over the past few years. Commonly, in most electrolytes, tubes can be grown with diameters of about 50–100 nm and with anodic volt- ages in the range of 10–20 V. Outside these limits typically only very irregular structures were observed. However it has been reported that the pore diameters can be varied by altering the growth conditions (the anodization electro- lyte) [21–24]. In these experiments, tubes with specific diameters of 10, 50, 70 and 100 nm were reported. In the present work, we investigated the possibility to influence tube diameter and length in a given background electrolyte, 1 M H 3 PO 4 , with an optimized fluoride concen- tration by varying the applied potential. H 3 PO 4 was selected as it is a pH buffering electrolyte – a factor that is known to affect the pore geometry substantially [21]. 2. Experimental For all experiments titanium foils (99.6% purity, Advent Ltd.) with a thickness of 0.1 mm were used. Before the elec- trochemical treatment the foils were successively sonicated in isopropanol, deionized (DI) water and ethanol followed by drying in a nitrogen stream. For the electrochemical experiments, an electrochemical cell with a three-electrode configuration was used. Samples were contacted with a 1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.05.030 * Corresponding author. Tel.: +49 9131 85 275 75; fax: +49 9131 85 275 82. E-mail address: schmuki@ww.uni-erlangen.de (P. Schmuki). www.elsevier.com/locate/elecom Electrochemistry Communications 8 (2006) 1321–1325