Do Binary Supracrystals Enhance the Crystal Stability?

We study the oxygen thermal stability of two binary systems. The larger particles are magnetic amorphous Co (7.2 nm) or Fe₃O₄ (7.5 nm) nanocrystals, whereas the smaller ones (3.7 nm) are Au nanocrystals. The nanocrystal ordering as well as the choice of the magnetic nanoparticles very much influence the stability of the binary system. A perfect crystalline structure is obtained with the Fe₃O₄/Au binary supracrystals. For the Co/Au binary system, oxidation of Co results in the chemical transformation from Co to CoO, where the size of the amorphous Co nanoparticles increases from 7.2 to 9.8 nm in diameter. During the volume expansion of the Co nanoparticles, Au nanoparticles within the binary assemblies coalesce and are at the origin of the instability of the binary nanoparticle supracrystals. On the other hand, for the Fe₃O₄/Au binary system, the oxidation of Fe₃O₄ to γ-Fe₂O₃ does not lead to a size change of the nanoparticles, which maintains the stability of the binary nanoparticle supracrystals. A similar behavior is observed for an AlB₂-type Co–Ag binary system: The crystalline structure is maintained, whereas in disordered assemblies, coalescence of Ag nanocrystals is observed