Mechanism and controlled growth of shape and size variant core/shell FeO/Fe3O4 nanoparticles

Abstract

We report a novel synthesis approach for the growth of core/shell FeO/Fe3O4 nanoparticles with controlled shape and size. FeO particles were partially oxidized to form core/shell FeO/Fe3O4 structures, as evidenced from transmission electron microscopy, X-ray diffraction, and magnetometry analysis. We find that the molar ratios and concentrations of surfactants are the key parameters in controlling the particle size. The particles can grow in either isotropic or anisotropic shapes, depending upon a chemical reaction scheme that is controlled kinetically or thermodynamically. The competitive growth rates of {111} and {100} facets can be used to tune the final shape of nanoparticles to spherical, cubic, octahedral, octopod, and cuboctahedral geometries. FeO particles can also be oxidized chemically or thermally to form Fe3O4 nanoparticles. By following the same synthesis technique, it is possible to synthesize rods and triangles of Fe3O4 by introducing twinnings and defects into the crystal structure of the seed. The thermally activated first-order Verwey transition at ~120 K has been observed in all the synthesized FeO/Fe3O4 nanoparticles, indicating its independence from the particle shape. These core/shell nanoparticles exhibit a strong shift in field-cooled hysteresis loops accompanied by an increase in coercivity (the so-called exchange bias effect), but the low field-switching behavior appears to vary with the particle shape.

The influence of oxidation process on exchange bias in egg-shaped FeO/Fe3O4 core/shell nanoparticles

“…The hysteresis loops recorded at 2 K are presented on Fig.5c [19], H EB = 0.6 and 0.5 kOe (10 K, particle size 9 nm-spherical and 45 nm-cubic) [7], H EB = 3.1 kOe (10 kOe cooling field, 2 K, 32 nm cubes) [32], H EB = 0.79, 2.97 and 5.1 kOe (40 kOe cooling field, T= 5K, corresponding particle size 9, 16 and 22 nm, cubes) [33], H EB = 1.6 kOe (50 kOe cooling field, T= 10 K, particle size 23 nm, cubes) [27].…”

“…It affects the thermal [15], magnetic [16] and electrical [17] [18] properties of the sample. The Verwey transition undergoes size effect and usually disappears in nanoparticles smaller than 30 nm; however, it also depends on the crystallinity of investigated nanoparticles [19].…”

Мессбауэровские исследования наночастиц FeO/Fe-=SUB=-3-=/SUB=-O-=SUB=-4-=/SUB=- типа ядро/оболочка

“…МНЧ FeO/Fe 3 O 4 типа Я/О получены методом негидролитического термического разложения ацетил-ацетоната железа (III) [26]. Термически неустойчивый ацетилацетонат железа (III) начинает разлагаться при температурах около 100−105 • C с образованием двуоки-си углерода и ацетона [27].…”

Magnetism in Granular Systems II

“…Drawer 7007, Greenville, NC 27835-7007, USA important for their wide range of applications, including their use to obtain hard or soft magnetic materials, magnetic microsensors, magnetic materials for recording, pigments for paints and ceramics, and in medical and pharmaceutical applications, among others [2][3][4][5][6]. More recently, studies have been centered on core-shell-type nanoparticles for their use in medical and pharmaceutical applications due to having better functionalization than other types of nanoparticles [7][8][9][10][11][12] A system composed of single-domain, magnetic, uniaxial, and non-interacting nanoparticles is superparamagnetic (SP) and can be described by the Stoner-Wohlfarth model [13]. Several reports of this type of system can be found in the literature, and some are included in Refs.…”