Fabrication of Co₃O₄ hierarchically superhydrophobic boat-like hollow cages at the silicon surface

By employing a simple bottom-up method for the first time and using ideas learned from nature, especially from the lotus leaf, we fabricated unique Co₃O₄ hierarchical boat-like hollow cages about 30 μm in length and 8 μm in width at the middle constituted of many nanorods and nanoparticles. The various morphologies of the products can be tailored from nanoparticles to nanorods and to microcages by tuning the concentration of Co(NO₃)₂·6H₂O. We believe that ammonia employed in this method plays not only the role of ligand to ensure the oxidation of Co²⁺ to Co³⁺ but also helps with the variation of concentration for the final morphologies. The products, which might be used as catalysts and microreactors, show superhydrophobic properties after chemical modification with poly(dimethylsiloxane) vinyl-terminated (PDMSVT) compound. It is confirmed that the synergic effect of the surface morphology and the surface free energy contribute to this unique surface water repellence.