Design of Multiphase Metal Balls via Maskless Localized Anodization Based on Bipolar Electrochemistry

Abstract Nanoporous metal‐oxide films formed on some metals and their alloys during anodization are crucial for both basic research and commercial applications. They are used as important materials for various types of device fabrication due to their distinctive solid‐state shapes, such as nanopores and nanotubes. However, preparing nanoporous films using anodization, which is a surface treatment for plate‐like specimens, requires a direct electrical connection to the workpiece. Developing new materials independent of the specimen's shape and size will vary if the need for direct energization is eliminated. Here, bipolar electrochemistry is employed to fabricate two‐phase (insulator–conductor) and three‐phase (insulator–conductor–insulator) aluminum balls by controlling the redox reaction region in an electric field. The aluminum balls are site‐selectively anodized employing a wireless and maskless operation. The multiphase aluminum balls’ versatility is demonstrated in the site‐selective electroless deposition of copper on their exposed regions. Additionally, it is demonstrated that various types of materials, including aluminum and titanium, can be oxidized simultaneously in the same cell.