Synthesis of Hierarchical Nanoporous Microstructures via the Kirkendall Effect in Chemical Reduction Process

A series of novel hierarchical nanoporous microstructures have been synthesized through one-step chemical reduction of micron size Cu₂O and Co₃O₄ particles. By controlling the reduction time, non-porous Cu₂O microcubes sequentially transform to nanoporous Cu/Cu₂O/Cu dented cubic composites and hollow eightling-like Cu microparticles. The mechanism involved in the complex structural evolution is explained based on oxygen diffusion and Kirkendall effect. The nanoporous Cu/Cu₂O/Cu dented cubic composites exhibit superior electrochemical performance as compared to solid Cu₂O microcubes. The reduction of nonporous Co₃O₄ also exhibits a uniform sequential reduction process from nonporous Co₃O₄ to porous Co₃O₄/CoO composites, porous CoO, porous CoO/Co composites, and porous foam-like Co particles. Nanoscale channels originate from the particle surface and eventually develop inside the entire product, resulting in porous foam-like Co microparticles. The Kirkendall effect is believed to facilitate the formation of porous structures in both processes.