DOI: 10.1002/chem.201000397 WO 3 /TiO 2 Nanotubes with Strongly Enhanced Photocatalytic Activity Indhumati Paramasivam, Yoon-Chae Nah, Chittaranjan Das, Nabeen K. Shrestha, and Patrik Schmuki* [a] Initiated by the work of Fujishima and Honda in 1972, over the past decades, the use of particulate or colloidal semiconductors in solutions has extensively been studied for the photocatalytic oxidation of organic waste and pollutants in water. [1–3] By far the most studied material is TiO 2 , as it is considered to represent the most suitable photocatalyst, in view of effectiveness and stability against photodecomposi- tion. [4–7] Typically these photocatalytic systems are used in form of nanoparticles, either freely suspended in solution or compacted to a robust photoelectrode. Earlier studies mainly focused on geometric parameters of the particles that influence the photocatalytic activity, such as surface area, size distribution in solution, as well as the TiO 2 crystal structure, which was found to play a crucial role. [8–10] Later, various approaches were reported to enhance the photocata- lytic activity of TiO 2 , for example, by decorating the surface of TiO 2 nanoparticles with noble metals such as Pt, Pd, Ag, Au, and so forth. [11–15] More recently, improved photocatalyt- ic activity was reached by modifying TiO 2 particles with other oxides (such as Cr x O y , Fe x O y ,V x O y , MoO x , WO x , etc.). [16–22] The effect of noble particle decoration was mainly interpreted in terms of a facilitated contribution of the pho- toexcited electrons in the photocatalytic reaction producing, for example, superoxide from O 2 dissolved in aqueous elec- trolytes, while decoration with other oxides particles may in- fluence the rate of charge transfer to the environment via surface states or junction formation. In general, two main reactions are considered to be relevant for the photocatalyt- ic activity of TiO 2 : 1) the generation of valence-band holes that upon ejection to the environment (electrolyte) have an oxidative power sufficient to oxidize almost any organic ma- terial and 2) conduction-band electrons ejected to the elec- trolyte that may form reactive superoxides. [7] Most recently, advanced geometries of TiO 2 have been in- creasingly explored, in particular self-ordered TiO 2 nano- tubes (TiNT) have attracted wide attention due to the high level of geometrical definition combined with a high surface area (for an overview see references [23–26]). Such self-or- dered TiO 2 nanotubular layers can easily be grown on Ti metal sheets by a simple but optimized electrochemical anodization in F  containing electrolytes. [23, 24, 27, 28] Investiga- tions of their photocatalytic properties have shown that these tubular layers can be more efficient than classical nanoparticulate layers of a comparable thickness. [29, 39–41] Self-ordered oxide nanotubes cannot only be grown on pure Ti, but also on other transition metals such as Mo, W, Ta, Nb, and so forth, and a full range of Ti alloys including TiW, TiNb, TiAl, TiMo, TiTa. [30–34] In the present work, we dem- onstrate a very strong effect of tungsten addition to the TiO 2 nanotubes in terms of their photocatalytic activity. For this, different TiW alloys (Ti 0.2 at % W (Ti 0.2 W) and Ti 9 at%W (Ti 9 W)) as well as pure Ti were anodized to form  2 mm-long self-organized tube layers as shown in Figure 1. To achieve these self-organized layers different anodization conditions had to be applied as outlined in the Supporting Information. For all cases, comparable dimen- sions of nanotubular layers with a tube length between  2.2 mm to 2.6 mm and a diameter (obtained from SEM cross sections) between  85 nm to 100 nm were used. For the TiW alloys (Figure 1a–d), a thin porous initiation layer is present on the top of the highly ordered nanotubes, as visi- ble in the cross-sectional images of Figure 1b and d. Fig- ure 1e and f show for comparison, the top view and cross section of pure TiO 2 nanotube layers. The top layer can be removed, [35] but this was found to not affect the results strongly. [a] I. Paramasivam, Y.-C. Nah, C. Das, N.K. Shrestha, Prof. Dr. P. Schmuki Department of Materials Science WW-4 Institute of Corrosion and Surface Science (LKO) University of Erlangen—Nürnberg Martensstr.7, 91058 Erlangen (Germany) Fax: (+ 49) 9131-852-7575 E-mail: schmuki@ww.uni-erlangen.de Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000397. It contains experi- mental descriptions, XRD patterns of TiO 2 -WO 3 nanotubes at 350 8C and 550 8C, and EDX of elemental compositions for Ti 9W and Ti 0.2 W. Chem. Eur. J. 2010, 16, 8993 – 8997  2010 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 8993 COMMUNICATION