Journal of Magnetism and Magnetic Materials, 2018, vol 462pp. 13-21
A co-precipitation synthesis method was used to obtain iron oxide nanoparticles coated with Yerba Mate (Ilex paraguariensis) extract. These particles have a core–shell structure with the iron oxide phase surrounded by an organic shell provided by an organic component which come from the Yerba Mate extract. Obtained nanoparticles were exhaustively characterized as powders and in aqueous colloidal suspensions using several techniques such as TEM, XRD, SAXS, TGA, ATR-FTIR, Raman and XPS spectroscopy, magnetic moment measurement and specimen ac susceptibility. All results together show that the obtained particles are single-crystal iron oxide particles with magnetite as the most probable phase. Yerba Mate extract shell mass to iron oxide core mass, mS/mc, could be increased up to 6.2 × 10−2, depending on the synthesis conditions. As a function of mS/mc the crystallite size of the nanoparticles decreased from about 15 nm to 11 nm, while saturation magnetization Ms and coercive field Hc of powders decreased. Ms. diminution was associated to increasing modification of core surface electronic states due to chemical bond of iron in iron oxide to Yerba Mate extract components; on the other hand, coercivity reduction was modelled on the basis of the increasing interparticle separation and dipolar interaction weakening, which occurs as shell thickness grows. Differences between the particle core mean size obtained with TEM, SAXS, XRD and magnetic measurements are observed. The fact that magnetic size was smaller than particle and crystallite sizes was attributed to the existence of intense dipolar interactions. It was found that low field susceptibility in a colloid sample was about 2.5 times that of the powder specimen, a result that shows that demagnetizing effects prevail in the powder specimen, while they may be absent in the colloid due to the larger interparticle mean separation expected in the latter. Analysis of powder and colloid susceptibility was done based on a recently developed model was in agreement with the one performed for the coercive field behaviour.