Synthesis of concentrated colloidal dispersion of zinc-based spinel nanoparticles via liquid-phase crystallization from "double-hydroxide-like" metal hydroxides

We demonstrate a liquid-phase crystallization of interdispersed metal hydroxides nanoparticles (NPs) (diameter < 10 nm), toward the versatile syntheses of highly concentrated colloidal dispersions of spinel oxides. ZnGa2O4 was chosen as a representative system to demonstrate our strategy. Heating of precursory wetgels of hydroxide NPs, interdispersed in a quasi-double-hydroxide-state, induced the crystallization at 60-80 degrees C into ZnGa2O4 NPs with a size of single nanometer. X-ray absorption fine structure (XAFS) analysis reveals that octahedrally coordinated Zn2+ in the hydroxide NPs decreased as the result of the crystallizations, and the amount of tetrahedrally coordinated Zn2+ increased. By the chelation of acetylacetone (acac) to tetrahedrally coordinated zinc species in crystalline oxides NPs, deflocculation of the wetgel was induced and then acac worked as a dispersion stabilizer. The present method can be also applied to ZnAl2O4 and ZnFe2O4 NPs, allowing to yield the colloidal dispersions of spinel oxides NPs with a size of single nanometers in diameter with high dispersion stability (over 1 week) under highly concentrated conditions (ca. 20-40 g/L).