Lattice Expansion in Metal Oxide Nanoparticles: MgO, Co₃O₄, & Fe₃O₄

Uniform sets of mono-crystalline nanoparticles ranging from 6 nm to over 100 nm were prepared for the MgO, Co₃O₄, and Fe₃O₄ oxide systems. The nanoparticles were characterized by transmission electron microscopy (TEM) and x-ray diffraction (XRD). A careful analysis shows increased lattice parameter for smaller nanoparticles of each oxide system: 0.47% expansion from bulk for 7 nm MgO crystallites, 0.15% expansion from bulk for 9 nm Co₃O₄ crystallites, and 0.13% expansion from bulk for 6 nm Fe₃O₄ crystallites. The compressive surface stresses and expansion energies against hydrostatic pressure for each oxide system were calculated, respectively, to be 4.13 N/m and 1.8 meV/formula unit for MgO, 3.09 N/m and 0.87 meV/formula unit for Co₃O₄, and 1.26 N/m and 0.67 meV/formula unit for Fe₃O₄. The fundamental understanding of oxide nanoparticle mechanics as presented here will facilitate integration of these materials into technological applications in a rationally designed manner.