In-situ Observation And Self-assembly Study Of Magnetic Nanoparticles

In addition to the excellent physical and chemical properties of nanomaterials,magnetic nanomaterials also exhibit unique magnetic properties.Existing research methods cannot enable us to observe in real time during the reaction process,and transmission electron microscopy(TEM)in-situ technology can perform in-situ observation and structural characterization for understanding the relationship between the magnetic properties and the structure of magnetic nanoparticles.Fe₃O₄nanoparticles are the most studied materials among magnetic nanoparticles due to their unique chemical stability and non-toxicity.Self-assembly is a technique that can spontaneously assemble some disordered nano-components into an ordered structure,but the research on the self-assembly of triangular Fe₃O₄ nanoparticles remains to be explored.In this paper,Fe₃O₄-Ag and Fe₃O₄-Mn O_X heterojunction nanoparticles were synthesized by chemical synthesis.The in-situ observation of Fe₃O₄-Ag and Fe₃O₄-Mn O_X heterojunction nanoparticles using TEM was performed to explore the structural evolution and growth process of magnetic nanoparticles.The results obtained are as follows:(1)The transformation from Fe3O4-Ag heterodimers having loose interface configuration to a Janus structure at the atomic scale from room temperature to 600?.After a dwell time of 32 min at 600?,two scenarios can be co-existed,i.e.Fe3O4-Ag Janus structure having a clear interface and Fe3O4-Ag Janus structure having a semi-crescent-shaped interface.(2)The Fe₃O₄-Mn O_X heterodimers can maintain structural stability at 500°C.When the temperature rises from 500 to 700°C,the heterodimers structure inside the particles will change and move,and a certain melting phenomenon will occur.The addition of Mn element makes the heterojunction particles catalytically active,catalyzing the crystallization of amorphous carbon at 700-1000?,while Fe₃O₄ particles without Mn do not show obvious catalytic activity under the same conditions.The triangular Fe₃O₄ nanoparticles were prepared by hydrothermal method,and the preparation process and self-assembly process were explored.Using X-ray diffractometer(XRD),transmission electron microscope(TEM),magnetic measurement system(MPMS)and electron holography technology to test the structure and magnetic properties of the above samples,and explore the preparation process,structure and performance law.The results obtained are as follows:(1)The synthesized triangular plate has the best morphology and good crystallinity when adding surfactant oleylamine for 3ml and Heating at 200?for 24h.Observing microscopic morphology and magnetic properties,it was found that most of the particles are stacked face to face to form a particle string.At the same time,the coercivity H_C is up to 500Oe at 2K,and the Spin freezing temperature(T_b)reaches 300K.(2)The magnetic structure of triangular nanoparticles was explored using electronic holography.After applying an out-of-plane magnetic field of±1T,the magnetic signal of the triangular Fe₃O₄ nanoparticles obtained by photographing the electronic hologram changes with the applied external magnetic field,which proves that it has strong magnetism.At the same time,the triangular Fe₃O₄ nanoparticles were photographed on both sides,and the magnetic signals were observed.It was found that there was no obvious regular magnetic signal due to clutter interference due to excessive particle stacking.(3)Matrix-assisted pulsed laser deposition technology was used to prepare spherical and triangular Fe₃O₄ nanoparticle films.From this comparison,the spherical nanoparticle dominated by isotropic force has a two-dimensionally uniform particle film assembly structure.Triangular nanoparticles dominated by anisotropic forces have a one-dimensional particle string assembly structure due to their easy magnetization axis perpendicular to the triangular surface of the nanoplate.(4)The triangular Fe₃O₄ nanoparticles were induced by external magnetic fields to form some long and large chain-like structures similar to corn cobs.This is because the external magnetic field greatly enhances the magnetic dipole interaction between the nanoparticles,resulting in a more compact structure.The magnetic properties have been measured and found that the coercivity H_C is up to 590Oe at 2K and the spin freezing temperature(T_b)reaches 330K,which is greater than the magnetic properties of the unassembled particles.