IOP PUBLISHING NANOTECHNOLOGY Nanotechnology 20 (2009) 045603 (9pp) doi:10.1088/0957-4484/20/4/045603 Photo-induced effects on self-organized TiO 2 nanotube arrays: the influence of surface morphology A G Kontos 1 , A I Kontos 1 , D S Tsoukleris 1 , V Likodimos 1 , J Kunze 2 , P Schmuki 2 and P Falaras 1 1 Institute of Physical Chemistry, NCSR ‘Demokritos’, 15310 Aghia Paraskevi Attikis, Athens, Greece 2 Department of Materials Science and Engineering, WW4-LKO, University of Erlangen-Nuremberg, Martensstrasse 7 D-91058 Erlangen, Germany E-mail: akontos@chem.demokritos.gr and papi@chem.demokritos.gr Received 21 August 2008, in final form 20 November 2008 Published 19 December 2008 Online at stacks.iop.org/Nano/20/045603 Abstract Self-organized TiO 2 nanotubes with packed, vertically aligned morphology and different lateral characteristics were grown on Ti metal substrates by controlled electrochemical anodization in phosphate/HF and ethylene glycol/HF electrolytes. The wetting, photo-induced superhydrophilicity, and photocatalytic activity of the nanotubular materials were investigated under ultraviolet irradiation. The photoactivity of the TiO 2 nanotube arrays was analysed in terms of their morphological characteristics that were determined by means of scanning electron microscopy and atomic force microscopy in conjunction with geometrical modelling. The wetting and the UV-induced superhydrophilicity could be accordingly modelled by the Cassie–Baxter mode arising from the large scale roughness of the nanotubular arrays in combination with the Wenzel mode due to the small scale roughness induced by ridges at the outer tube surface. The photocatalytic activity of the TiO 2 nanotube arrays was further found to correlate quantitatively with the variation of the geometric roughness factor, verifying the strong impact of morphology on the photo-induced properties of the vertically oriented TiO 2 tubular architecture. 1. Introduction Synthesis of low dimensional nanostructures based on elements other than carbon and tuning of their structure and morphology has attracted much scientific interest stemming from their unique size and shape dependent properties that can be exploited in numerous technological applications and nanodevice engineering [1, 2]. Anodic porosification is a very promising versatile fabrication route, successfully applied in the electrochemical growth of tailored porous structures of Al 2 O 3 and Si and more recently of self-organized, vertically aligned TiO 2 nanotube arrays [3]. The unique photo-induced activity of TiO 2 nanomaterials renders them ideal for use in key sustainable technologies, including environmental protection, health, and energy generation [4]. The majority of TiO 2 based applications such as solar cells [5] and self-cleaning surfaces [6], as well as air and water purification [7], rely primarily on the material’s interaction with light [8]. The efficiencies of the corresponding photo-induced processes strongly depend on the nanoparticulate size and require judicious control of the material’s structure and morphology. Production of titania nanotubes by electrochemical anodization provides a relatively simple synthetic approach to a robust, vertically oriented architecture that offers a large internal surface area together with the possibility of vectorial charge transfer with significantly diminished recombination rates, less affected by the inherent disorder that hampers electron transport in the random polycrystalline network of nanoparticulate TiO 2 films [9]. A major advantage of anodic porosification is the feasibility to tune the size and shape of the nanotubular arrays to the desired length scale meeting the demands of a specific application target by means of controlled anodic oxidation of the metal substrate [3]. Thorough research efforts have accordingly shown that the 0957-4484/09/045603+09$30.00 © 2009 IOP Publishing Ltd Printed in the UK 1