DOI: 10.1002/ijch.201000059 Self-organized TiO 2 Nanotube Arrays: Critical Effects on Morphology and Growth Sergiu P. Albu, [a] Poulomi Roy, [a] Sannakaisa Virtanen, [a] and Patrik Schmuki* [a] 1. Introduction TiO 2 has been, over the past 20 years, one of the most studied compounds in materials science due to some out- standing properties that give it a virtually unique position for applications in photocatalysis, [1] dye-sensitized solar cells, [2] or for biomedical purposes. [3] In 1999, a first report appeared on the feasibility to grow highly ordered arrays of TiO 2 nanotubes (NT) by a simple but optimized elec- trochemical anodization of titanium. [4] This stimulated in- tense research activities that focused on growth, modifica- tion, properties, and applications of these 1D nanostruc- tures. Meanwhile, such tubes can be grown with a very high degree of order, diameter, and length control; for an overview see ref [5]. The crystalline structure can be ad- justed by heat treatments from amorphous to anatase to rutile phases. [6] Various secondary morphologies have been obtained, such as bamboo type or branched tube structures; [7] entire layers have been converted to flow- through membranes. [8] Optical and electrical properties can be adjusted by suitable doping or bandgap engineer- ing approaches. [9] The tube surface and the interior can be modified by particle decoration or various filling ap- proaches. [3g, 10] Organic linkers allow functional entities to be attached to the tube walls, and they can be released on demand. [3g, 11] TiO 2 nanotubes have been explored for vir- tually all applications where previously TiO 2 nanoparti- cles were used (e.g., solar cells, photocatalysis, intercala- tion devices (electrochromic, Li-batteries); for reviews, see refs. [2c, 5, 12]). In many cases, the tubular structures demonstrated superior properties to TiO 2 nanoparticles. In other cases, the unique geometry of the nanotube layers allows entirely novel uses (such as self-cleaning nano test tubes, [13] drug release capsules, [3g] or for eluci- dating highly defined size-dependent interactions with living cells [3e, f, 14] ). While many of these application aspects were covered in a comprehensive way in recent reviews, [5] the present work aims to give a detailed and coherent overview of principles and critical parameters for tube growth, and tries to give a perspective for future direc- tions in the field. 2. General Aspects on Growth of TiO 2 Nanotubular Arrays Self-organized closed packed TiO 2 nanotubes can be grown electrochemically in a fluoride-containing electro- lyte that is either water-based or organic. [5] For this a tita- nium metal sheet is used as an anode in a two- or three- electrode set-up, and voltages in the range of a few up to several hundred volts are applied. In water-based electro- lytes, tubes typically can be grown to a length of some few micrometers and diameters between 10 and 150 nm using voltages of 1–20V. Typically, the tube walls are rough and a high degree of self-organization is not really achieved. While such tubes have certain advantages re- garding their mechanical stability [15] and are thus useful, for example, to coat biomedical (load-bearing) devices, Abstract : TiO 2 nanotube layers possess a wide range of ap- plications for energy conversion, environmental clean-up, and biomedical implant materials. The formation process of these tube layers is based on a simple but highly optimized anodization process of Ti in a suitable solution. The present work gives a coherent overview on the effect of the key parameters to grow these tube layers. The influence of fluoride content, anodization voltage, water content, and temperature on tube morphology is shown and discussed. The results thus provide the reader with “recipes” and un- derstanding to grow desired TiO 2 nanotube structures, as well as show current limits and open room for improvement for the tailored growth of these structures. Keywords: electrochemistry · organic electrolyte · nanostructures · TiO 2 nanotubes · self-organization [a] S. P. Albu, P. Roy, S. Virtanen, P. Schmuki Department of Materials Science, WW4-LKO, University of Er- langen–Nuremberg, Martensstraße 7, 91058 Erlangen, Germany phone: + 49 (0)9131 852-7575 fax: + 49 (0)9131 852-7582 e-mail: schmuki@ww.uni-erlangen.de Isr. J. Chem. 2010, 50, 453 – 467  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 453 Review