Template assisted assembly of cobalt nanobowl arrays

3-D interconnected networks of magnetic cobalt nanobowls with a wall size of <100 nm dimension have been fabricated. A novel technique was used to overcome the problem of volume shrinkage usually observed in such structures when formed by chemical conversion route. Cobalt particles were formed in the interstitial spaces of 3-D closed packed polystyrene (PS) colloidal template by borohydride reduction of a cobalt salt. Template removal was carried out by annealing either at 500°C for 3 h in nitrogen or at 400°C for 4 h in air, followed by toluene etching. The presence of an oxygen atmosphere during annealing was found to be imperative to obtain metallic cobalt. Magnetic measurements on cobalt nanobowl arrays showed enhanced coercivity in comparison to spherical particles due to shape effects.