MAGNETIC OXIDE NANOPARTICLES WITH ANISOTROPIC SHAPE OR HETEROGENEOUS STRUCTURE

To briefly summarize the work reported in this PhD thesis, we can say that the study of the solvothermal synthesis of MnO NPs led to procedures to obtain anisotropic MnO NPs starting from manganese(II) oleate and stearate. A detailed investigation on the influence of the reaction conditions on the size, shape, crystal structure and magnetic properties of the obtained nanoparticles was carried out, including a detailed comparison between the two precursors (manganese oleate and manganese stearate) and surfactants (oleic acid and stearic acid) and a thorough investigation of the influence of the precursor : surfactant molar ratio. Having used MnO as antiferromagnetic material for the core-shell structure, we were prompted to further consider the use of MnS, an antiferromagnetic sulfide with the Néel temperature  160 K (higher than the MnO TN = 116 K). The higher Néel temperature makes MnS a good candidate for the building of an exchange-bias coupling. MnS, unlike MnO, presents three different polymorphs: cubic α-MnS (rock-salt), cubic β-MnS (zinc-blende), and hexagonal γ-MnS (wurtzite). Thus, synthetic investigation about MnS NPs was mainly focused on the control of the nanoparticle crystal phase that, in our case, could be achieve through the use of different surfactants. Polymorphism control is a crucial point because different polymorphs exhibit different physical properties, among which, the magnetic behavior. Next, we focused on the synthetic strategy to coat anisotropic MnO NPs with a FeOx coating (FeOx stands for Fe3-xO4-x, 0 ≤ x ≤ 1). We conceived to approach this problem by a two step strategy. First, we set out to develop a procedure to grow a FeOx shell (several nanometers thick) onto large (20-30 nm) isotropic MnO cores; once obtained such procedure, we will optimize it to uniformly coat anisotropic NPs. Using isotropic MnO NPs as cores, many synthetic strategies were devised and assessed with respect to the achievement of growing a significantly thick and uniform iron oxide shell simultaneously preventing the formation of undesired homogeneous iron oxide nanoparticles. We finally developed a procedure able to grow a FeOx shell of up to 6 nm on the MnO core. We are at present working on the development of a multi-step procedure to achieve a thicker and more compact FeOx shell. The synthesis of core-shell MnO@FeOx NPs and their characterization by electron microscopy (C-TEM, electron diffraction, HRTEM, Analytical TEM) are described in detail in the Thesis, while the magnetic characterization are in progress in these days. Besides the main aim of my Thesis research, we decided to explore the feasibility to use MnO nanoparticles having different crystallographic faces as a catalysts in the water splitting reaction. We just started a collaboration with Dott. A. Minguzzi, O. Lugaresi and A. Visibile at the University of Milan to carry out an electrochemical study to investigate whether nanoparticle with different shape, the surface of which are different crystal faces, have unequal catalytic activity in the water splitting reaction. Since the work is at an early stage, here we reported only samples treatment and characterization before the electrochemical tests that are currently in progress. Finally, a complete magnetic characterization of thin-film assemblies of Ni@NiO core-shell nanoparticles, was performed and here reported thanks to a research project carried out in collaboration with the group of Professor S. D’Addato, Dr. P. Luches, and Prof. S. Valeri at CNR NANO S3 and University of Modena and Reggio Emilia. Nanoparticles were synthesized by metal vapor deposition in Modena and their magnetic behavior was investigated in our laboratory by SQUID magnetometry. The Ni@NiO core-shell assemblies prepared by a three-layer procedure (NiO layer – Ni NPs – NiO layer) turned out to display a large exchange bias that could be accurately tuned by varying the thickness of the top NiO layer.