Photocatalysis DOI: 10.1002/smll.200600426 Self-Organized TiO 2 Nanotube Layers as Highly Efficient Photocatalysts** Jan M. Macak, Martin Zlamal, Josef Krysa, and Patrik Schmuki* In 1972, Fujishima and Honda reported for the first time on light-induced water splitting on TiO 2 surfaces opening novel approaches in heterogeneous catalysis. [1] Since then, TiO 2 has shown to be an excellent photocatalyst [2,3] with a long- term stability, low-cost preparation, and a strong enough ox- idizing power to be useful for the decomposition of unwant- ed organic compounds. [4–6] For these applications in pollu- tion control, mainly compacted nanoparticulate films or sus- pensions are used, based mainly on the commercial Degussa P25 TiO 2 nanopowder. In 1999, electrochemical formation of self-organized arrays of TiO 2 nanotubes by anodization of Ti was reported by Zwilling et al. [7] However, the thick- ness of these layers did not exceed 500 nm and the aspect ratio of these nanotubes was approximately five. Very re- cently, revised approaches were reported to grow high- aspect-ratio nanotubes with lengths of up to several micro- meters and aspect ratios up to 150. [8–12] In the present work we demonstrate that these high- aspect-ratio TiO 2 nanotubular layers possess excellent pho- tocatalytic properties that are significantly enhanced com- pared to nanoparticulate layers. Figure 1 shows the principle of the photocatalytic de- composition on a semiconductor electrode. Photons from a light source that have sufficient energy, that is, higher than the bandgap energy, E g , of the semiconductor, excite elec- trons from the valence band to the conduction band of the semiconductor and charge-carrier pairs (consisting of a hole h + and electron e À ) are formed. [13] These charge carriers either recombine inside the particle, or migrate to its sur- face, where they can react with adsorbed molecules. In aqueous solutions positively charged valence-band holes typically form hydroxyl radicals, HOC , while electrons in the conduction band mainly reduce dissolved molecular oxygen to superoxide CO 2 À radical anions. Organic molecules pres- ent in the solution may react with these oxidizing agents in- ducing their oxidative degradation to inorganic compounds including carbon dioxide and water. [14] In order to achieve a maximum decomposition efficiency, except for adequate band-edge positions, rapid charge separation, and high quantum yields, a large catalyst area is desired. Figure 2 shows scanning electron microscopy (SEM) images of different types of tubular TiO 2 layers investigated in this work, with lengths of 500 nm (short tubes [15] ), 2.5 mm (medium tubes [8] ), and 4.5 mm (long tubes [12] ). For the dif- ferent types of tube the inner nanotube diameter is approxi- mately 100 nm for the short and medium tubes (Figure 2a, b); the longest tubes (Figure 2c) have a diameter of about 45 nm. This third type of tube not only has a smaller diame- ter, but also shows extremely smooth tube walls due to its production in a highly viscous electrolyte. [12] In order to con- vert the tubes to a defined anatase structure, which typically shows the highest catalytic activity of the TiO 2 crystal struc- tures, [2,3] these samples were annealed at 450 8C over 3 h. For comparison we produced a 2.5-mm-thick P25 film with a particle size of about 30–40 nm, as shown in Figure 2d. To evaluate the photocatalytic activity of the nanotube layers and the compacted nanopowder, the substrates were irradiated by UV light in the presence of organic azo dyes, namely, Acid Orange 7 (AO7) and Methylene Blue (MB), which are non-biodegradable dyes used in the textile indus- try and often considered as standard dyes for testing photo- catalytic activity. It has been shown that organic molecules can be decomposed by holes essentially via two closely re- lated paths: by direct oxidation of the holes [16] or by HOC radicals [17] formed by reaction of the holes with water. In case of MB, its adsorbed molecules are directly oxidized by photoinduced holes to a radical CMB + , which reacts further with O 2 to form [MBOOC] + and afterwards its heteropolyar- omatic ring is broken and further decomposition occurs. [18] In the case of AO7, the specific decomposition mechanism is more complicated because its adsorption on the TiO 2 sur- face is strongly pH dependent. [4] At a pH value higher than 6 (in the present case pH (AO7) = 6.5) there is only very weak adsorption, and decomposition is induced by photo- [*] J.M. Macak, Prof. P. Schmuki Department of Materials Science, WW4-LKO University of Erlangen-Nuremberg Martensstrasse 7, 91058 Erlangen (Germany) Fax:(+ 49)9131-852-7582 E-mail: schmuki@ww.uni-erlangen.de M. Zlamal, Prof. Dr. J. Krysa Department of Inorganic Technology Institute of Chemical Technology Prague Technicka 5, 16628 Praha 6-Dejvice (Czech Republic) [**] The authors acknowledge Robert Hahn, Hans Rollig, and Martin Kolacyak for valuable technical help. Figure 1. Principles of photocatalytic decomposition on semiconduc- tor TiO 2 surfaces. Photons excite electrons from the valence band to a conduction band forming an electron–hole (e À –h + ) pair. This charge carrier can recombine in bulk or migrate to the surface and react with the adsorbed species, which leads to their decomposition by direct oxidation on the holes, or by HOC and C O 2 À radicals. 300 # 2007 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim small 2007 , 3,No.2,300–304 communications