The work focuses on synthesis of magneto-optical composite particles combining gold microplatelets and barium-hexaferrite magnetic nanoplatelets.
First, both types of particles were synthesized and subsequently assembled into the composite particles in ethanol suspensions. The particles were characterized by various methods of electron microscopy and with magnetic measurements. The colloidal properties of particles suspensions (e.g., zeta potential) were also evaluated.
The barium-hexaferrite nanoplatelets were synthesized using a hydrothermal method. The Ba2+, Fe3+ and Sc3+ ions in the aqueous suspension were precipitated with a large excess of hydroxyl ions and heated to 240 °C in a closed autoclave. The synthesized nanoplatelets (approximately 50 nm wide and 3 nm thick) were dispersed in water using adsorbed citric acid and coated with a thin (approximately 3 nm thick) silica layer using hydrolysis and polycondensation of tetraethoxysilane (TEOS) in the suspension. A mixture of 3-mercaptopropyltriethoxysilane and the TEOS was used to introduce thiol groups on the surfaces of nanoplatelets. The thiol groups exhibit a high affinity towards a gold surface.
Gold microplatelets were synthesized by reduction of tetrachloroauric acid with aniline in ethylene glycol. The product contained a mixture of microplatelets and isotropic particles. The microplatelets were several µm wide, but only a few tens of nm thick. The isotropic particles, approximately 1 μm in size, were of a pentagonal-bipyramidal shape. The size of both types of particles was measured from images acquired with a scanning electron microscope (SEM). A new method was developed for estimation of a volume ratio between the microplatelets and the isotropic particles based on digital processing of the SEM images. The influence of various parameters, e.g., the temperature, the concentrations of reagents and the aniline–to-Au3+ ratio, on the morphology of the particles was systematically evaluated. The analysis showed that both, the microplatelets and the isotropic particles were formed by a mechanism of exaggerated growth of primary gold nanoparticles, which were formed by rapid reduction of Au3+ at the elevated temperature. During the exaggerated growth the nanoparticles, which contained planar defects - (111) twin boundaries, grew on the expense of other nanoparticles.
Composite nanoparticles were synthesized by mixing a suspension of the gold microplatelets into a colloidal suspension of the barium-hexaferrite nanoplatelets in ethanol. The nanoplatelets bonded homogeneously onto the basal surfaces of microplatelets, presumably with chemical interactions between the surface thiol groups on the nanoplatelets and the surfaces of gold microplatelets. The surface concentration of the nanoplatelets bonded on the microplatelets was controlled by changing the concentration of particles in the suspensions. Bonding between the two types of particles can also occur as the result of electrostatic interactions, i.e. without prior functionalization of the silica on the nanoplatelets with the thiol groups. However, the layer of nanoplatelets was less homogeneous in this case. The suspended composite microplatelets efficiently responded to the external magnetic field. Due to permanent magnetic moments of the hexaferrite nanoplatelets, which points perpendicular to the plate, the suspended composite platelets orient perpendicular to the direction of an applied magnetic field resulting in a strong magneto-optical effect. A golden glare of the suspension of composite microplatelets depends strongly on the strength and the orientation of magnetic field.
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