A Novel Approach To Fe₃O₄ Nanoparticles

Magnetic nanoparticles, also called zero-dimensional magnetic nanomaterials, have unique magnetic properties, such as superparamagnetism, low Cure Point, high coercivity and etc. Such size-depended properties are quite different from the bulk magnetic materials. The surface effects and quantum effects of nanoscaled materials bring such fantastic behaviors to magnetic nanoparticles. Fe₃O₄ is a typical ferrimagnetic material with inverted spinel crystalline structure, and its ultrafine powder had been widely applied because of industrial and military purposes. Nowadays, Fe₃O₄ nanoparticles are exploring new applications in biology and medicine. The high quality magnetic nanoparticles are the foundation of such potential researches. Therefore, a lot of preparation methods had been investigated. Actually, few of them are practical in the common lab conditions. It is without saying to apply conveniently after post processing.This thesis reports a new approach to Fe₃O₄ nanoparticles: Partially reduced and confined precipitation route. Firstly, microelectrode electrochemical method was used to monitor the partial reduction process of ferric ion to ferrous ion in FeCl3-Na2SO3 system. The stoichiometric ratio of coprecipitation reaction was achieved when the beginning molar ratio of FeCl3 and Na2SO3 was 3:1. The system needed about 10 minutes to finish such partial reduction. The rigid control of the reagent ratio ensured the synthesis of Fe₃O₄ and avoided the occurrence of side reactions. The purity and crystalline structure of Fe₃O₄ nanoparticles were guaranteed. Secondly, an O/W emulsion was chosen as the medium of the whole reaction. The stability and dispersity of oil phase in such macromolecular surfactant emulsion had been examined by the means of fluorescent microscopic observation. The optimal constitutes of emulsion was found: the mass ratio of aqueous phase, cyclohexane (oil phase) and emulsifier was 35:4:1. Thirdly, the electrical conductivity of reagent emulsions was measured to look for the maximum concentration range of ferric ion. The confinement effect could work when the concentration was controlled around 0.15 mol·L-1. This experiment insured the narrow size distribution of synthesized nanoparticles. Finally, Fe₃O₄ nanoparticles were synthesized according to the above-mentioned three experiment findings. The validity of technological parameters was verified. The size-controlled synthesis could be achieved by altering the reagent concentration at the presence of the confinement effect. The research work related to the knowledge of electrochemistry, coordination chemistry, colloid chemistry and etc.The synthesized Fe₃O₄ nanoparticles were almost monodispersed because the macromolecular surfactants in the emulsion confined the nucleation and the nucleus growth. Compared with microemulsion method, the much higher concentration of reactants increased the Fe₃O₄ nanoparticle yield. The intermediate of partial reduction reaction was complex which avoided the oxidation of ferrous ion, and this made the inert atmosphere unnecessary. The transmission electron microscope, X-ray powder diffractometer, dynamic light scattering particle size analyzer and vibrating sample magnetometer were used to characterize the nanoparticles. The results showed the synthesized nanoparticles were nearly pure Fe₃O₄ with inverted spinel crystalline structure, and the size distribution was narrow. The Fe₃O₄ nanoparticles with average diameter 15 nm were superparamagnetism, and the saturation magnetization exceeded 61.3 emu·g-1 at room temperature.The partially reduced and confined precipitation route is derived from the traditional coprecipitation. It integrates the advantages of chemical coprecipitation and microemulsion method, and dispelled the drawbacks of them. Such method makes it possible to synthesize high quality Fe₃O₄ nanoparticles in the common lab conditions.