Oleate-based hydrothermal preparation of CoFe2O4 nanoparticles, and their magnetic properties with respect to particle size and surface coating
Abstract
We present a facile and high yield synthesis of cobalt ferrite nanoparticles by hydrothermal hydrolysis of Co-Fe oleate in the presence of pentanol/octanol/toluene and water at 180 or 220 degrees C. The particle size (610 nm) was controlled by the composition of the organic solvent and temperature.
Magnetic properties were then investigated with respect to the particle size and surface modification with citric acid or titanium dioxide (leading to hydrophilic particles). The as prepared hydrophobic nanoparticles (coated by oleic acid) had a minimum inter-particle distance of 2.5 nm.
Their apparent blocking temperature (estimated as a maximum of the zero-field-cooled magnetization) was 180 K, 280 K and 330 K for the particles with size of 6,0 and 10.5 nm, respectively. Replacement of oleic acid on the surface by citric acid decreased inter particle distance to less than 1 nm, and increased blocking temperature by ca. 10 K.
On the other hand, coating with Uranium dioxide, supported by nitrilorri(methylphosphonic acid), caused increase of the particle spacing, and lowering of the blocking temperature by ca. 20 K. The CoFe2O4@TiO2 nanoparticles were sufficiently stable in wafer, methanol and ethanol.
The particles were also investigated by Mossbauer spectroscopy and alternating-current (AC) susceptibility measurements, and their analysis with Vogel-Fulcher and power law. Effect of different particle coating and dipolar interactions on the magnetic properties is discussed.