Propriedades magneto-ópticas de colóides magnéticos á base de nanopartículas de magnetita recobertas com prata
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2010-05-17
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Universidade Federal de Goiás
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In this work we investigated, theoretically and experimentally, the magneto-optical properties
of a magnetic fluid consisting of core-shell nanoparticles, where the core is made of magnetite,
while the shell is silver.
The theoretical model used was based on Mie s theory, under the electrostatic approximation,
i.e. for nanoparticles with diameters much less than the incident wavelength (lambda).
A Clausius-Mosotti for a core-shell system was used to calculate the electrical susceptibility of
the core-shell nanoparticle for equals to 632 nm. The susceptibility was shown to be strongly
dependent on the core diameter and the shell thickness. Nevertheless, a maximum value of
7.20 (greater than isolated nanoparticles of silver, which has 0 = 4.30, or magnetite with
0 = 1.47) was obtained for a fraction f, defined as f = (Dcore/Dcore−shell)3, equal to 0.36. This
result suggest that there exist an ideal fraction f for nanocomposites with enhanced optical
properties.
In order to compare our theoretical results with experimental data a core-shell magnetic
fluid was synthesized on the Institute of Chemistry of UFG by the group of Dr. Em´ılia Celma
de Oliveira Lima. The nanoparticles were suspended in water at fisiological pH and recovered
by a double layer of lauric acid (dodecanoic acid). The nanoparticles were characterized by
X-ray diffraction, high resolution electron transmission, energy dispersive X-ray spectroscopy,
and vibrating sample magnetometer. The Sturges method was used to obtain the nanoparticle
diameter histogram. The data revealed the existence of a bimodal nanoparticle distribution.
Both distributions were curve fitted using a lognormal function. The modal diameter of one of
them was 9.24 ± 0.03 nm with a dispersity of 0.27 ± 0.02, while for the other one we found a modal diameter of 23.0 ± 0.2 nm with disperisty 0.2 ± 0.1. The energy dispersive X-ray
spectroscopy confirmed the existence of magnetite and silver only for larger particle diameters,
while the lower ones only magnetite was found. From the experimental analysis we confirmed
the synthesis of a magnetic fluid containing 10% of core-shell nanoparticles. Magnetization
data was used to estimate the magnetic particle volume fraction.
The magneto-optical properties were obtained using a magnetotransmissivity technique,
where the polarizer and analyser axis are positioned on the magnetic field direction.
The sample containing 10% of core-shell nanoparticles, with a total particle volume fraction
of 0.18%, had shown an extinction of light of 100% for a magnetic field of only 500 Oe, while
a magnetic fluid with 100% of core nanoparticles, at a similar particle concentration (0.15%),
had shown a 50% extinction of light at the same field range. The magnetotransmissivity data
were curve fitted with a theoretical model containing only two parameters, one related to the
electrical susceptibility and the other to the formation of self-organized nanostructures in the
colloid. The mean agglomerate size (nanoparticles forming linear chains) had changed from 2.09
to 3.36 for a particle volume fraction increasing from 0.06% to 0.18%. Using the estimative of
the double layer lenght of lauric acid, approximately 2 nm, and analyzing the magnetotransmissivity
data for several particle concentrations, we were able to obtain the fraction f of core-shell
nanoparticles of 0.17. This result, together with TEM data, allowed us to calculate the core
diameter of the core-shell nanoparticle as 13 nm. Indeed such result suggest that in order to be
suscessful in coating the nanoparticle with the shell element one might need monodisperse-like
nanoparticle systems.
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LOPES JUNIOR, José Carlos Campello. Propriedades magneto-ópticas de colóides magnéticos á base de nanopartículas de magnetita recobertas com prata. 2010. 93 f. Dissertação (Mestrado em Fisica) - Universidade Federal de Goiás, Goiânia, 2010.