Since the 1960s, when the method for the anodization of aluminum oxide was developed, we have gone far and beyond to apply and optimize this technique. We can now use anodization on alloys of aluminum, titanium, magnesium, zinc and a few other materials, to form an oxide coating over our metallic surfaces, and by doing so we can change the resistance to corrosion, the conductivity and the appearance of the material.
Figure 1. A circular section of anodized aluminum (1.3cm diameter) on a 2cm diameter aluminum slab.
Examples of anodized materials include the often shiny metallic tools, orthopedic implants and templates for the synthesis of nanostructures (Figure 1). Aluminum templates have been among the most cost-effective materials in recent years and are versatile enough to be used on small-scale synthesis of specific products as well as industrial scale applications. Over the last years I have been synthesizing templates for nanowire structures by taking advantage of the aluminum oxide’s hexagonal structure or honeycomb shape.
Using an anodizing cell we were able to form a layer of porous aluminum oxide over a 99.99% aluminum slab. By dividing the anodization process in two and then removing the excess materials (the metallic aluminum and barrier layer) at the bottom of our template, we gained highly ordered pores that range from 40 nm to 80 nm (Figure 2). By sealing one of the ends of the template with a conducting metal (let us say gold) (Figure 3), then the template is perfect for the synthesis of separated nanowires.
Figure 2. Diagram showing the anodization process.
Figure 3. Back of an anodized aluminum template covered with gold (left). Top of the anodized aluminum template with gold (right).
After the template is sealed, then all that is left is to invoke a preferred style of synthesis (precipitation, electrochemical, physical or chemical vapor deposition, etc). And voilá - enclosed nanowires in a compact yet fragile container.
My experience:
We have primarily used electrochemical deposition to develop nanowires within the anodized template for the purpose of integrating nanowire circuits in gas sensing devices. To begin we found the reduction and oxidation potentials of copper(II) oxide (CuO) using cyclic voltammetry. Then, once we set the parameters in our voltage source and prepared a solution with the necessary precursors, we modified the time of electrodeposition so that the nanowires would not overflow from the porous anodized aluminum oxide template.
Given the experimental nature, the most difficult part of the electrodeposition process was figuring out and producing the solution with the correct components and pH so that the solubility of each reagent would not affect the nanowire growth. Now, we have focused our efforts in extracting the nanowires in an efficient way so that we may integrate the nanostructures in gas sensing circuits.
Want to learn about how nanoparticles, nanowires, nanotubes, nanopores and nanocones can all be achieved using anodized aluminum? Then read the article by Zhao, H. et. al., “A mini review: Functional nanostructuring with perfectly-ordered anodic aluminum oxide template for energy conversion and storage.” Front. Chem. Sci. Eng. 12, 481–493 (2018). https://doi.org/10.1007/s11705-018-1707-x