Anodization of Au thin film on Al in oxalic acid

- A spongelike nanoporous Au film was formed on Al by anodization in oxalic acid.
- When nanopores penetrated the Au, the current significantly decreased.
- A fine nanoporous alumina film was also formed under the nanoporous Au.
- Anodization current of Al was less than 1% of that of Au.
- Growth rate of the nanoporous alumina was less than 3% of that of nanoporous Au.

An Au thin film, which was sputter-deposited on an Al substrate, was potentiostatically anodized in oxalic acid. The Au film was first anodized and a spongelike nanoporous film grew down to the interface between Au and Al. Then, the Al was anodized and a very thin and fine nanoporous alumina film was formed underneath the nanoporous Au. Under the same anodization conditions, the current density for Al was ~ 40 μA cm− 2, less than 1% of that for Au (~ 30 mA cm− 2). The growth rates of the nanoporous films were ~ 0.7 nm/min for Al and 26 nm/min for Au, indicating that the growth rate of nanoporous alumina was less than 3% of that of nanoporous Au. Al is suitable as the substrate for preparing nanoporous Au films because the electrochemical reactions of both the electrolyte and the substrate are significantly suppressed when the nanopores penetrate Au and the electrolyte reaches the substrate.

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