Synthesis of composite nanoparticles using co-precipitation of a magnetic iron-oxide shell onto core nanoparticles

Composite nanoparticles can be synthesized by coating a shell made of one material onto core nanoparticles made of another material. Here we report on a novel method for coating a magnetic iron oxide onto the surface of core nanoparticles in an aqueous suspension. The method is based on the heterogeneous nucleation of an initial product of Fe³⁺/Fe²⁺ co-precipitation on the core nanoparticles. The close control of the supersaturation of the precipitating species required for an exclusively heterogeneous nucleation and the growth of the shell were achieved by immobilizing the reactive Fe³⁺ ions in a nitrate complex with urea ([Fe((CO(NH₂)₂)₆](NO₃)₃) and by using solid Mg(OH)₂ as the precipitating reagent. The slow thermal decomposition of the complex at 60 °C homogeneously releases the reactive Fe³⁺ ions into the suspension of the core nanoparticles. The key stage of the process is the thermal hydrolysis of the released Fe³⁺ ions prior to the addition of Mg(OH)₂. The thermal hydrolysis results in the formation of γ-FeOOH, exclusively at the surfaces of the core nanoparticles. After the addition of the solid hydroxide Mg(OH)₂, the pH increases and at pH ~ 5.7 the Fe²⁺ precipitates and reacts with the γ-FeOOH to form magnetic iron oxide with a spinel structure (spinel ferrite) at the surfaces of the core nanoparticles. The proposed low-temperature method for the synthesis of composite nanoparticles is capable of forming well-defined interfaces between the two components, important for the coupling of the different properties. The procedure is environmentally friendly, inexpensive, and appropriate for scaling up to mass production.