Colloidal hollow nanoparticles (NPs) of Ni2P have been prepared by a one-pot reaction from a mixture of nickel acetate, oleylamine, trioctylphosphine (TOP), and 1-octadecene. The mechanism to the hollow structure is related to the nanoscale Kirkendall effect. The process contains two important steps. First, oleylamine-stabilized Ni NPs were formed, which can protect them from TOP etching and slow down the inward diffusion of P atoms. Second, the solid-state reaction between the Ni NPs occurred when the TOP concentration and the reaction temperature were correctly adjusted.
Monodisperse Fe1−x
O nanoparticles (NPs) with a mean size of 21.7 ± 2.1 nm were prepared by the thermal decomposition of iron(III) oleate complex at 380 °C using oleic acid as the solvent. Variation in their composition was monitored using XRD for a period of 120 days under ambient conditions, under which the dominant phase changed from wüstite to a spinel-type iron oxide phase. HR-TEM images and absorption spectra of the 10-day sample further revealed an FeO/spinel-type phase core−shell structure. Exchange-bias coupling on the interfaces between the wüstite and the spinel-type phase accompanied the variation in composition. The dependence of H
E on temperature demonstrates that the H
E onset temperature is approximately 200 K, which correlates with the T
N of bulk FeO.
A thermal reduction method has been developed to prepare magnetite/hematite nanocomposites and pure magnetite nanoparticles targeted for specific applications. The relative content of hematite α-Fe2O3 and magnetite Fe3O4 nanoparticles in the product was ensured by maintaining proper conditions in the thermal reduction of α-Fe2O3 powder in the presence of a high boiling point solvent. The structural, electronic, and magnetic properties of the nanocomposites were investigated by F57e-Mössbauer spectroscopy, x-ray diffraction, and magnetic measurements. The content of hematite and magnetite phases was evaluated at every step of the chemical and thermal treatment. It is established that not all iron ions in the octahedral B-sites of magnetite nanoparticles participate in the electron hopping Fe2+⇄Fe3+ above the Verwey temperature TV, and that the charge distribution can be expressed as (Fe3+)tet[Fe1.852.5+Fe0.153+]octO4. It is shown that surface effects, influencing the electronic states of iron ions, dominate the vacancy effect, and thus govern the observed specific features of the Verwey transition and magnetic properties. The sharp increase in coercivity observed in magnetite nanoparticles below TV is much stronger than for bulk magnetite.
We have synthesized a set of monodisperse iron oxide nanoparticles ranging from 7.8to17.9nm by thermal decomposition methods. Based on the evidence of high-resolution transmission electron microscopy, the iron oxide nanoparticles appear as spherical dots with size standard deviations of less than 5%. Blocking temperatures of the set of nanoparticles were measured by the zero-field-cooled magnetization measurements. The anisotropy energy constants are estimated from the measured blocking temperatures. The contribution from the surface anisotropy is the dominant factor of the higher anisotropy energy found. The saturation magnetization and coercive force HC (77K) are functions of the particle size and increase with the particle size.
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