Through-Hole, Self-Ordered Nanoporous Oxide Layers on Titanium, Niobium and Titanium–Niobium Alloys in Aqueous and Organic Nitrate Electrolytes

In the past decades, a variety of self-aligned functional oxides have successfully been grown on a wide range of metals using electrochemical, self-organizing anodization. The earliest reports on highly ordered oxide structures included porous aluminum oxide layers grown by optimized anodization of Al in oxalic acid. These nanoporous oxides found a considerable number of direct applications, such as size-exclusion filters and waveguide structures, or sacrificial uses, such as templates for secondary material deposition for the production of nanowires and tubes. A very versatile self-organizing anodization approach was introduced in 1999 by Zwilling et al. , which used fluoride-containing electrolytes for the fabrication of ordered TiO2 nanotube arrays on Ti. These fluoride-based electrolytes were optimized over the last ten years to enable the growth of self-organized oxide layers on many metals and alloys, including Ti, Zr, 9] Hf, Nb, Ta, W, Ti–W, and Ti–Nb. A detailed overview can be found in Ref. [7] . In 2005, Masuda et al. , followed by others, showed that by using perchlorate or chloride electrolytes, another type of nanotubes, the so-called rapid-breakdown anodization (RBA) nanotubes could be grown on Ti and W surfaces. This process was later extended to Ti–Nb, Ti–Zr and Ti–Ta alloys to form mixed-oxide nanostructures. In this anodization approach, the formation of tubes occurs with a high current flow, and tubes grow as bundles from a specific surface site on the metal into the electrolyte. Due to the localized nature of the process and high current densities, mechanistically, the formation process was attributed to repeated anodic breakdown events of the surface oxide layer. Over the past ten years, considerable efforts have been directed toward the finding of other electrolyte types that would lead to the formation of self-organized nanostructured metal oxides. While nitrate-based electrolytes were used to etch Ti through a porous alumina template and, thus, can form etch channels, we recently showed that nitrate-based electrolytes also may be a promising new route to achieve truly self-organized oxide structures in the context of Ti and Ta anodization. In the present work, we explore the use of nitrate-containing electrolytes for the formation of self-organized (template-free) oxide structures on Ti, Nb and Ti–Nb alloys. Ordered TiO2-based nanoscale structures are particularly interesting in terms of applications in catalysis, solar cells, photolysis, sensing, and electrochromic devices. Nb is an important element in combination with Ti, since composite oxides can be formed, or TiO2 can be Nb-doped for an alteration of the electronic properties. For TiO2 nanotubes, it has been shown that in large concentrations the incorporation of Nb leads to lattice widening and is, therefore, beneficial in ion insertion devices (e.g. , electrochromic applications and ion intercalation batteries). In smaller concentrations, Nb acts as a donor species to enhance the performance of TiO2-based solar cells and water splitting reactions. Herein, we demonstrate that anodization in nitrate-based electrolytes can be tuned to form ordered, nanoporous oxide structures, not only on Ti, but also on Nb and Ti–Nb alloys. Moreover, in contrast to any other previously reported electrolyte types, this nitrate-based anodization leads directly to a through-hole morphology for all investigated structures, i.e. , where the pores are open at the top and bottom. A series of preliminary anodization experiments for all the metals in various aqueous and ethylene glycol-based nitrate electrolytes were carried out, screening parameters being concentration, pH, and water content. The results showed that on Ti, Nb and Ti45–Nb, ordered porous layers could be grown (Figure 1). In aqueous electrolytes, a sufficiently high anodization voltage had to be applied to initiate the growth of a porous layer with an aligned pore structure. For the three investigated materials, the conditions to achieve a defined layer growth are different in each case. For Ti, well-ordered pores could be observed for anodization in HNO3. The example shown in Figure 1a resulted in an oxide-layer thickness of approximately 10 mm. The inset pictures in Figure 1 show that regular pore channels with a diameter of 10–20 nm and a through-hole morphology could be obtained. Using Nb, a stable compact oxide film, instead of a porous oxide layer, was formed in all explored aqueous nitric acid electrolytes. However, when anodization was carried out in an organic nitrate electrolyte, well-defined porous layers could be grown. Figure 1b shows the cross section of a self-organized nanoporous Nb oxide layer formed in an ammonium nitrate electrolyte. The resulting layer thickness was approximately 4 mm, and the pore diameter of the through-hole morphology was approximately 10–15 nm. A defined and self-organized pore morphology was obtained for the Ti45–Nb alloy in both types of electrolytes— [a] R. Kirchgeorg, W. Wei, K. Lee, S. So, Prof. Dr. P. Schmuki Department of Material Science WW4-LKO, University of Erlangen-Nuremberg Martensstraße 7, 91058 Erlangen (Germany) Fax: (+49)9131-852-7582 E-mail : [email protected] 2012 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. 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