Nanopillar array film of cellular cobalt by wet etching of the grain boundaries

• Straightforward process of surface nanostructuring by electrodeposition and wet etching.
• Effective grain boundary selective etching of columnar cobalt thin films.
• Fabrication of a nanopillar array film of potential interest as a supporting scaffold structure for further deposition or surface modification.

In this work, a straightforward strategy was developed for the fabrication of cobalt thin films made of a dense array of vertically aligned nano and / or meso-crystals. Briefly, the proposed process consists in the electrodeposition of columnar cobalt thin films followed by a selective wet-etch treatment. Based on a preliminary screening of either aqueous or methanol based acid etchants, either a nitric acid or methanesulfonate methanol solution was further considered. The degree of selectivity of the etching solutions was evaluated on the basis of the observation of film surface/cross section morphology before and after etching. Contrary to water-based solutions, methanol-based solutions showed definitely a degree of selectivity toward grain boundary preferential etching. In particular, with methanesulfonic acid (MSA)- methanol solution, saturation of the etchant with oxygen was found to have a profound effect in improving the control of the etching process. Accordingly, an 11.2% methanol-based MSA solution was able to selectively etch the Co layer at the grain boundaries, leaving Co individual crystals separated by nanochannels.