Embedded Palladium Activation as a Facile Method for TiO2-Nanotube Nanoparticle Decoration: Cu2O-Induced Visible-Light Photoactivity

In the present work, we introduce a facile and reliable approach to decorate TiO2 nanotubes homogenously with wellseparated copper oxide nanoparticles. For this, we use a palladium-containing titanium alloy (commercial Ti 0.2 atom% Pd) and grow anodic, self-organized oxide nanotubes. Palladium is embedded in the tube wall during growth and acts as activator for a subsequent electroless copper-nanoparticle deposition at distinct sites within the nanotubes. Using an appropriate heat treatment, the copper nanoclusters can be converted to Cu2O or CuO. Such decorated nanotubes show a significant enhancement of the water-splitting performance under artificial solar light irradiation. Over the past decade, TiO2 nanotube layers formed by a self-organizing anodization process have attracted tremendous interest in science and technology, as their geometry and properties have in the past demonstrated and continue to promise better performance in applications such as photocatalysis, solar cells, or biomedical coatings. One of the highly active research fields is their application for photocatalytic reactions including the decomposition of numerous pollutants (e.g. , CO2), [4–6] and the direct or photoelectrochemical splitting of water into H2 and O2. [9–12] For water-splitting applications, TiO2 is generally considered only to be efficient under photoelectrochemical conditions (applied bias) and, due to a band gap of 3.0–3.2 eV, under UV illumination. Efforts to overcome these drawbacks are mainly based on band-gap engineering of TiO2, or coupling TiO2 with a p-type narrow-gap semiconductor with suitable band edge positions. In this context, a main candidate is Cu2O, as it is potentially able to form a visible lightactive p–n junction. Therefore, considerable efforts are undertaken trying to optimally decorate TiO2 and TiO2 nanotubes with Cu2O particles or clusters, [13–16] with the target to obtain a homogeneous distribution of Cu2O particles over the TiO2 nanotubes in order to reach an efficient performance. When trying to achieve a homogeneous deposition with conventional electrodeposition processes, a frequent difficulty is that particle deposition takes place on the top of the TiO2 nanotubes rather than uniformly within the nanotube structure. In the present work, we report a novel approach to uniformly decorate TiO2 nanotube walls with copper oxide nanoparticles. The key is to grow oxide nanotubes from a palladiumcontaining titanium alloy (in our case, commercial Ti0.2Pd). This alloy was selected, as palladium is well-known for its activation of electroless deposition reactions, for example, of copper or nickel. After anodic tube formation from the alloy, palladium is embedded in the oxide of the nanotube wall and can be exploited as activation site for selective and defined electroless copper deposition. The copper particles can then be thermally converted to Cu2O or CuO, and the resulting structures show significantly enhanced activity for visible-light water-splitting reactions. Figure 1 shows the morphologies of the nanotube (NT) layers grown anodically on a plain titanium metal sheet (Ti-NT, see Figure 1A) and on the TiPd alloy (TiPd-NT, see Figure 1B). In both cases, the oxide nanotube layers were grown in the same glycerol/water electrolyte, and in both cases identical tube morphology with typical tube lengths of approximately 850 nm and tube diameters of about 80 nm is obtained. These layers were then annealed and immersed in a copper plating solution; Figures 1C, E and 1D,F show scanning electron microscope (SEM) images of the tube layers on titanium and TiPd, respectively, after the deposition process. Clearly, in the case of the palladium-containing alloy, copper deposits can be formed regularly over and within the tube layers (Figure 1D), whereas for the reference TiO2 nanotubes obtained on titanium no trace of copper deposits could be observed (Figure 1C). The transmission electron microscope (TEM) image in Figure 1G for the tubes in Figure 1D shows that copper was deposited in the tubes, and that after annealing Cu2O particles had a size of approximately 25 nm. After annealing the copper-decorated tubes in air at 140 8C, TEM selected area diffraction (TEM-SAD) patterns (Figure 1G) are in line with the formation of Cu2O. Experiments with or without annealing treatments and with different copper deposition conditions show that for non-annealed TiPd samples, copper deposition occurs in a much less defined manner, and the timing of the electroless process needs to be optimized to avoid an overgrowth of the copper deposit. However, it should be noted that the argon-annealing step was extremely crucial for obtaining a good distribution of copper deposits on and into the tubes, thus being very important for a reproducible and reliable electroless deposition. From the X-ray diffraction (XRD) patterns in Figure 2, one can deduce that for conditions as in Figure 1D, metallic copper is deposited. For reference, XRD measurements were also carried out on Ti-NT (Figure 2A) and TiPd-NT (Figure 2B) after argon annealing. From the corresponding patterns, it is obvious that after the argon annealing, for both substrates, TiO2 is present in the anatase form. XRD patterns for copperdeposited samples after thermal oxidation at 140 or 300 8C in [a] Dr. A. Mazare, Dr. N. Liu, K. Lee, M. S. Killian, Prof. Dr. P. Schmuki Department of Materials Science and Engineering, WW4-LKO University of Erlangen-Nuremberg Martensstrasse 7, 91058 Erlangen (Germany) E-mail : [email protected] 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This is an open access article under the terms of the Creative Commons Attribution Non-Commercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.