Templating Using Self‐Aligned TiO2 Nanotube Stumps: Highly Ordered Metal and Polymer Bumped Arrays

Abstract

Over the past decades, self-ordered nanostructured oxides, prepared by simple but optimized anodization of a metal electrode, have attracted great interest. In 1995, Masuda et al. reported first on the growth of highly ordered alumina structures by anodizing Al under a set of optimized electrochemical conditions. Meanwhile, a wide range of metals and their alloys have been found to form highly ordered porous or tubular oxide structures. Key for this was the introduction of fluoridebased electrolytes that allow anodic oxide formation, and at the same time mild permanent oxide etching by fluoride complex formation. As a result, not only typical valve metals such as Ti, Nb, and Ta but also Fe, V and Co were over the years anodized to self-organized porous or tubular structures. Particularly the formation of highly ordered TiO2 nanotube layers found wide resonance, as TiO2’s functional features can find broad applications, for example, in dye-sensitized solar cells (DSSCs), 9] photocatalysis, 11] memristors, biomedicine, and sensors. Depending on the different application requirements, the oxide nanotube layers can be prepared with different thicknesses, individual tube diameters, with various morphology variations, and crystallinity. The nanotube layers usually show a high degree of lateral ordering for longer anodization times, that is, that lead to several mm to several 10 mm tube lengths. For many applications, for example, solar cells or photocatalysis, these tubes suit their purpose well, as several mm in length typically correspond to an optimum performance. Even though high-aspect-ratio tubes are occasionally reported to be of use for templating (e.g. polymers or metals), for these short-circuiting problems may occur for cathodic electrodeposition, and the deposit filling is still far from being ideally conformal. However, recently we reported on the formation of extremely well-ordered arrays of short TiO2 nanotubes. These TiO2 nanotube stumps (TiNTS) form when Ti is anodized in a HF containing concentrated phosphoric acid electrolyte and have a short aspect ratio (<5) with layer thicknesses of ~200 nm and individual tube diameters of ~80 nm. In the present work, we show that these TiO2 nanotube stumps with their open and short aspect morphology are ideal for conformal filling of the tubes by sputter deposition, and thus ideally serve as a template. We demonstrate this with a range of noble metals and even more using cast filling with a molten polymer. As a result, in other words, these structures can be exploited for the fabrication of highly ordered replica structures—which can be used, for example, to create highly defined corrugated surfaces with a virtually ideal hexagonal ordering of metal (or polymer) bumps. Figure 1 shows SEM images of the titania nanotube layers (TiTNS) used in this work, that is, layers that were formed by anodizing Ti in a HF/H3PO4 electrolyte for 2 h. From the SEM top views (in Figures 1a and c) it is clear that not only do