Self‐Organized High‐Aspect‐Ratio Nanoporous Zirconium Oxides Prepared by Electrochemical Anodization

Abstract

The electrochemical formation of self-organized porous structures has recently attracted much attention due to the broad range of nanoscale applications that become accessible through nanopore arrays made of materials such as Al2O3, [1–12] Si, and InP. Particularly, the anodic oxidation of Al has been studied in detail and a variety of highly ordered porous oxide structures with a high-aspectratio pore geometry have been successfully prepared. In addition to direct applications, the nanostructures have also been employed as a template for the deposition and fabrication of secondary nanostructures. Applications include magnetic recording media, optical devices, functional electrodes, and waveguides. 7, 8] Recently we reported that under optimized electrochemical conditions, self-organized pore formation on Ti can be achieved to grow TiO2 nanotubes with aspect ratios of more than 20. Herein, we report the electrochemical fabrication of highly ordered porous zirconium oxide layers of nanotubular shape with very high aspect ratios (>100) by anodization of Zr. This is a clear improvement in comparison with other ongoing attempts. Key to achieving the high aspect ratio is an adjustment of the electrochemical parameters to an optimized competition between dissolution and formation of the oxide. Zirconium oxide is an important functional material that plays a key role as an industrial catalyst and catalyst support with particular acid catalysis properties. 22] Furthermore, zirconium oxide is known to have excellent ion-exchange properties that are exploited in various applications. Highsurface-area structures are often required to provide an enhanced performance. These structures are typically synthesized by sol–gel methods or colloid electrodeposition. A well-established fact in the literature is that when Zr or other so-called “valve metals” are anodized in most electrolytes, a compact oxide layer of up to several hundred nanometers thickness is formed. A unique feature in comparison to other valve metals is that the growth of these compact ZrO2 layers at room temperature in most electrolytes leads to a crystalline film. However, in electrolytes containing fluoride ions, the oxide formation process is fundamentally changed. Figure 1 shows SEM images of porous zirconium oxide layers observed after anodization in fluoride-containing electrolytes. Figure 1a shows the surface morphology, and