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 Cu(2)O and Co(3)O(4) particles. By controlling the reduction time, non-porous Cu(2)O microcubes sequentially transform to nanoporous Cu/Cu(2)O/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/Cu(2)O/Cu dented cubic composites exhibit superior electrochemical performance as compared to solid Cu(2)O microcubes. The reduction of nonporous Co(3)O(4) also exhibits a uniform sequential reduction process from nonporous Co(3)O(4) to porous Co(3)O(4)/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.