REPOSITORY > RESULTS

Abstract

Composite nanoparticles represent novel materials where coupling between properties of different materials inside a single nanoparticle could lead to improved or even entirely new properties. One of the possible approaches for the synthesis of composite nanoparticles is the deposition of a shell made of one material onto the surfaces of core nanoparticles made of another material. A magnetic shell of spinel ferrite can be deposited onto the core nanoparticles using a simple, controlled coprecipitation method. The method is based on heterogeneous nucleation of a product of coprecipitation of Fe³⁺/Fe²⁺ ions on the surfaces of the core nanoparticles in an aqueous suspension. The supersaturation of the precipitating product is closely controlled by the controlled release of Fe³⁺ ions through the thermal decomposition of a coordination compound, Fe((H₂N)₂C=O)₆₃ (Fe–urea complex).
This method was already successfully used for the deposition of soft magnetic maghemite layers on the surfaces of hard magnetic hexaferrite core nanoplatelets. In my thesis, the method was modified to allow close control over the thickness of the deposited maghemite layers on the cores. The thickness of the maghemite layers was controlled by varying the ratio between the iron precursors and the hexaferrite cores or by depositing the layers in two subsequent steps. Morphostructural characterization using a combination of X-ray diffractometry (XRD), Raman spectroscopy, and atomic-resolution transmission and scanning transmission high-angle annular dark-field (HAADF-STEM) microscopy revealed that the bim agnetic composite nanoplatelets displayed an exceptionally uniform maghemite–hexaferrite–maghemite sandwich-type structure. The layers of uniform thickness grew epitaxially exclusively on the basal surfaces of the hexaferrite nanoplatelets.
Varying the layer thickness enabled the tuning of the magnetic properties of the bim agnetic nanoplatelets. Magnetic measurements showed that the bim agnetic nanoplatelets displayed a single-phase hysteresis loop, indicating that the two magnetically different phases were rigidly exchange-coupled. The nanoplatelets retained the desired out-of-plane magnetization of the hexaferrite cores, while the saturation magnetization (Mₛ) was significantly improved by maghemite deposition. Mₛ increased proportionally with increasing layer thickness. However, the increase in Mₛ was accompanied by a decrease in coercivity (H꜀), which was related to exchange coupling between the two magnetically different phases and to structural strains induced in the hexaferrite core by the epitaxial layers.
In the second part of my thesis, I attempted to implement the controlled coprecipitation method to coat the surfaces of ferroelectric bismuth titanate (BIT) core nanoplatelets with a spinel ferrite shell. Combining the magnetic shell with the ferroelectric core would be beneficial for controlling the nanoplatelets in suspension using an applied magnetic field. Alternatively, the composite nanoparticles could be used as building blocks for magnetoelectric composites. However, the method could not be directly transferred to the deposition of magnetic layers onto the surfaces of BIT cores. Fe-rich nanoparticles deposited onto the core surfaces remained amorphous. Moreover, the nanoparticles remained amorphous even after hydrothermal treatment or annealing above 500 °C, whereas at 700 °C, the iron-rich phase reacted with the BIT core nanoplatelets. The observed differences between the deposition of spinel ferrite shells onto the different core nanoparticles were attributed to the partial dissolution of the BIT cores in the presence of urea released during the decomposition of the Fe–urea complex.