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.