Monodisperse nanoparticles (NPs) below 20nm are of intense current interests, not only for the general miniaturization of the size, but also for their novel properties. The magnetic properties of magnetic NPs are dramatically size-dependent when they get into nanoscale, and the transition of these NPs from ferromagnetic into superparamagnetic occurs within tens of nanometers. Recent advances in chemical synthesis have led to various monodisperse NPs with controlled size, shape, composition and crystallinity. In this dissertation, monodisperse NPs of magnetic Fe3O4, Fe, Co, FeCo and SmCo5, as well as noble metallic Au, Ag and AuAg NPs will be discussed.
Monodisperse magnetic ferrite NPs, MFe2O 4 (M = Fe, Co, Mn, etc) with controllable size of 5–20nm have been synthesized. The NPs are superparamagnetic with moderate magnetic moments up to 80emu/g at room temperature.
Metallic iron (Fe) and cobalt (Co) NPs with high initial moments (>120emu/g) have been produced via thermolysis of organometallic precursors. However, due to the chemical instability, the NPs are quickly oxidized in air, leading to drastic moment drop and rapid aggregation. Crystalline iron oxide (Fe 3O4) shell has been employed to protect the NPs. The resulted bimagnetic core/shell NPs prove to possess greatly enhanced chemical and magnetic stabilities. FeCo NPs were made by coating Co seeds with Fe followed by interfacial diffusions between the two components.
The synthesis of hollow Fe3O4 NPs has been achieved by controlled oxidation of Fe NPs, utilizing Kirkendall diffusions. The crystallinity and the surface pores of the hollow NPs can be readily tuned by controlling the reaction temperature.
Nanocrystalline SmCo-based hard magnets have been fabricated via solid state reduction of SmCoO-based NPs with calcium (Ca) granules. Hard magnetic properties have been revealed from the annealed products of SmCo5 or Sm2Co17.
Monodisperse non-thiolated Au and Ag NPs have been prepared. The resulted Au NPs reveal high catalytic activity for carbon monoxide (CO) oxidation. AuAg alloy NPs have been made by interfacial diffusion of core/shell Ag/Au.
Such NPs with promising properties are thus interesting for various bio-medical, magnetic and catalytic applications. Magnetic Fe3O4, Fe, Co, FeCo and hollow Fe3O4 NPs are promising for drug delivery, imaging and hyperthermia. Nanocrystalline SmCo is an excellent candidate of superior permanent magnets.