Three-dimensionally ordered macroporous (3DOM) materials of spinel-type mixed iron oxides. Synthesis, structural characterization, and formation mechanism of inverse opals with a skeleton structure

Three-dimensionally ordered macroporous (3DOM) materials of spinel-type MFe2O4 (M = Zn, Ni, ZnxNi1-x (x = 0.2-0.8), and Co) mixed iron oxides were prepared in excellent yields by using a colloidal crystal templating method. Mixed metal nitrates were dissolved in an ethylene glycol (EG)-methanol mixed solvent and penetrated into the void of the colloidal crystal template of poly(methyl methacrylate) (PMMA) spheres. During the calcination process, the mixed metal nitrates reacted with ethylene glycol and converted into mixed metal glyoxylate derivatives in the voids of the colloidal crystals before the polymer sphere was removed. After removing the sphere template and converting the mixed metal glyoxylates into the mixed metal oxides, the well-ordered 3DOM materials of the desired spinel-type mixed metal oxides with a skeleton structure were obtained. The preparation procedure was analyzed on the basis of TGDTA, XRD, SEM, TEM, elemental analysis, and BET surface area determination, and the formation mechanism was clarified. After the PMMA templates were removed, an amorphous 3DOM material with a shell structure was obtained. Crystallization of the amorphous material to the spinel-type mixed metal oxides by further calcination changed the morphology of the 3DOM material from the shell structure to a skeleton structure.